«J*i iw jo Aavaan tvnouvn snidiosw do Aavaan ivnouvn 3noiq3w do Aavaan tvnouvn a. Hi LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDICINE 2 5 "ISX >t*f Ml LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDICINE o Q. f — *^-^^ T? &^J — ^-S^ ri "~\ O *~^ o ~* iom jo Aavaan tvnouvn snoichw do Aavaan tvnouvn 3Nma3w do Aavaan tvnouvn NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDICIr- NATlONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDICINE NATIONAL LIBRARY OF MEDICU AJ Descriptive Anatomy OF THE Human Teeth. SECOND EDITION. BY G. V. BLACK, M.D., D.D.S. PUBLISHED BY The Wilmington Dental Manufacturing Co., 1413 Filbert Street, PHILADELPHIA, PA. Copyright, 1892, by The Wilmington Dental M'f'g Co., Phila. pirn. A/0. 6*'/■J, VK? PREFACE TO THE FIRST EDITION. BY my experience as a practitioner, as a teacher, and in my intercourse with fellow practitioners, I have be- come convinced of a serious defect in the teaching of the details of the anatomy of the teeth, and in the systemization of the terms used in their description. This defect has been a constant drawback at the chair, in the laboratory, and most of all, in the college. The object of the present volume is to remedy, in a measure, this defect. To this end I have had constantly in view the needs of the dental student and practitioner. "We have heretofore had excellent general descriptions in human and comparative dental anatomy; but these have dealt principally with the general forms of the dentitions of the mammalia and other orders of animate beings, rather than with specific descriptions of the forms of the various surfaces and surface markings, making up the sum of the forms of the individual teeth of-n^am Valuable as these works have been, they have left the acquirement of a knowl- edge of the details of the specific forms of the human teeth mostly to individual observation. By this means, many have attained to an excellent perception of the various forms of the human teeth ; but it is not reasonable to suppose the profession generally will do this without some fixed guide. What the dental student wants most in the college, and in the office, is a systematized nomenclature of the several parts of the teeth in detail; and such a description as will call his attention successively to every part of each tooth, as Gray, in his Anatomy, has called attention to every part of each bone, however apparently unimportant. It should be re- membered that anatomy is not to be learned from books II PREFACE. alone, but also by bringing the parts to be studied into view, and closely examining them in connection with the descrip- tions given. Any one who may read the present volume without a reasonable number of human teeth of each denomi- nation before him for examination and comparison, will be but partially benefited. It has been my object to systematize the nomenclature most in vogue with the profession, whenever practical, rather than to introduce new terms. However, the reader will find a few new terms, and possibly a few old ones, that are used differently from the former custom. The -terms up and down, to indicate direction or parts of teeth, are abandoned, because of their ambiguity. In a few instances, new forms of old words have been used, especially to avoid the terms inner, outer, backward, forward, etc., which are so often mis- leading. The words mesial, distal, labial, buccal, lingual, etc., are used as adverbs of direction by adding (y, or the same thing is accomplished by the use of the preposition to. It is as easy to say of a cavity that it extends far beyond, beyond, to, or nearly to, to the gingival line, as to say it ex- tends up or down, etc., and the meaning will not be mistaken; or to say that a cavity extends distally, or to the distal, or lingually, or to the lingual, instead of backward, or inward, either of which has different meanings in different situa- tions. The best rule is to use no extraneous object in the designation of the parts of, or direction on, the surface of a tooth; but to confine the phraseology to the specific and well defined terms applied to its several parts. The back part of a molar would not mean the same relative part as the back part of an incisor. In many such ways the author has endeavored to systematize, and make more definite, the phraseology applied to the teeth without going to extremes, knowing well that forms of language once in use can be im- proved more easily than they can be displaced by new terms, though more exact. PREFACE. Ill The absence of a Bibliography may be noted. The plan and object of this work has not seemed to call for many references to authorities. This does not imply, however, that authors who have preceded me, as Fox, Carabelli, Tomes, Wedl, Judd, Wortman, and many others, to whom we are greatly indebted, have been either overlooked or ignored. The illustrations have all been made by the author for the purpose of illustration, rather than as works of art. After experimenting with the various plans of the manage- ment of light and shade, diffused light has been used because more detail could be shown, especially in the difficult task of illustrating the occluding surfaces of the teeth. Each picture of a tooth, in all its details, is drawn from accurate measurements of the particular tooth in hand. Much of dental histology might properly find place in this book; but that subject is well represented by others. Malforms of the teeth, supernumerary teeth, and variations of arrangement, belong to the subject of irregularities, which is amply treated by several authors. My aim has been to confine the book strictly to normal macroscopic anatomy. However, a very serious difliculty, which has always met the dental anatomist, has been the variations of form in teeth of the same denomination. The endeavor has been to sys- tematize these under one, two, or more typical forms of each tooth, or its lobes, and point to the character of the changes which occur. This has occasionally led to the mention of abnormal forms. The reader will find scattered through the work some hints with regard to the practical bearing of anatomical points on operative procedures, which it is hoped will be of value. PREFACE TO THE SECOND FDITION. THE rapid sale of the first edition of this work has shown an appreciation of a need for a more com- plete anatomical description of the human teeth. In preparing the work for a second edition, the aim has been to render the original design more complete in its details, and to make such verbal alterations as would better fit it for the text-book for the college, and for reference by the practitioner. The most notable changes are in the sec- tion on nomenclature ; and these are confined to additional explanations of words and phrases. For the convenience of students a brief glossary has been added. A number of the illustrations have been re-drawn, and one has been added. Some of the numbers of the illustra- tions have been changed to render them consecutive; but the numbering of the paragraphs remains unchanged. Im- portant words, especially the subjects of paragraphs and technical words the first time used, have been printed in bold-faced type. This, it is hoped, will be of advantage to the student, and especially convenient for reference. Valuable aid in the preparation of the work for reissue has been rendered by Air. I. W. Davenport, Drs. D. M. Cat- tell, Edmund Noyes, and C. X. Johnson. G. V. B. Jacksonville, III., July IS, 1891. IV GLOSSARY. Alveolus. (PI. Alveoli.) A socket. The cavity in the process of the maxillary bone in which the root of a tooth is fixed. Alveolar process. The projection of the maxillary bones which envelops the roots of the teeth and forms their alveoli. Angle. The line, or point, where two or more surfaces of the teeth join. The mesial and buccal surfaces join in the formation of the mesio-buccal angle. (See Par. 0.) Apex. The terminal end of the root of a tooth. Apical foramen. The minute opening of the pulp canal at the apex of the root of a tooth. (See Par. 153.) Apical space. The space between the bone, or wall of the alveolus, and the apex of the root of a tooth. This space is filled with the soft tissues of the peridental mem- brane. Bell crowned. A tooth in which the mesio-distal di- ameter of the crown is much greater than that of the neck. Bicuspid. A tooth with two cusps. They are also called pre-molars. There are eight bicuspids ; two on each side of the upper jaw, and two on either side of the lower jaw. They are named right and left upper first and second, and right and left lower first and second bicuspids. They are situated between the cuspids and molars. Border of the alveolar process. The thin edge of the alveolar process surrounding the necks of the teeth. Buccal. Pertaining to the cheek. Toward the cheek. Buccal surface. The surface of a tooth next to the cheek. v VI GLOSSARY. Bucco-gingival ridge. A prominent ridge near the gingival line on the buccal surface of the deciduous molars. It is especially prominent on the first deciduous molars. Bucco-lingual. From the cheek toward the tongue; as the bucco-ling-ual diameter of the crown of a lower first molar. Canal. See Root Canal. Cementum. A tissue resembling bone which forms the outer surface of the roots of the teeth. Contact point. The point on the proximate surface of a tooth which touches a neighboring tooth. Crown. That portion of a tooth which is covered with enamel, and which projects from the tissues in which the root is fixed. Cutting edge. The edge formed by the junction of the labial and lingual surfaces of the incisor and cuspid teeth. In the cuspids the edge is raised to a point near its center. Cusp. A pronounced elevation, or point, on the surface of a tooth, more especially on the occluding surface. Cuspid. A tooth with one point, or cusp. There are four cuspids; one on either side in each jaw, situated at the corners of the mouth. Deciduous teeth. The teeth of the child which are shed to give place to the permanent teeth. They are also called temporary teeth. Dentine. The tissue of which the main body of a tooth is formed. Developmental grooves. Fine depressed lines in the enamel of a tooth which mark the junction of its lobes. (See Pars. 21 and 69.) Developmental lines. See developmental grooves. Distal. Away from the median line of the face follow- ing the curve of the dental arch. The surface of a tooth most distant from the median line. (See Par. 5.) Distally. Away from the median line of the face fol- lowing the curve of the dental arch. GLOSSARY. VII Distal angle. The corner formed by the junction of the cutting edge and distal surface of the incisors and cuspids. Also, the corner formed by the union of the distal, buccal, and occluding surfaces of the bicuspids and molars. Disto-buccal angle. The angle formed by the union of the distal and buccal surfaces of the bicuspids and molars. Disto-labial angle. The angle formed by the union of the distal and labial surfaces of the incisors and cuspids. Disto-lingual angle. The angle formed by the union of the distal and lingual surfaces of any of the teeth. Disto-occluding angle. The angle formed by the union of the distal and occluding surfaces of the bicuspids and molars. Enamel. A very hard tissue covering the crown of a tooth. Fossa. (PI. Fossse.) A round, or angular depression in the surface of a tooth. Fossse occur mostly in the occlud- ing surfaces of the molars, and in the lingual surfaces of the incisors. Fissure. A fault in the surface of a tooth caused by the imperfect joining of the enamel of the different lobes. Fissures occur along the lines of the developmental grooves. Gingival curvature. The deviation of the gingival line from the horizontal in its course around the neck of a tooth. Gingival line. The line around the neck of a tooth at which the gingiva is attached. The line of junction of the enamel and cementum. Gingival margin. The portion of the crown of a tooth next to the gingival line. Gingiva. (PI. Gingiva?.) The portion of gum tissue enveloping the necks of the teeth crown-wise from the attach- ment at the gingival line. The free margin of the gum. Groove. A long-shaped depression in the surface of a tooth. (See Par. 11.) Horn. A slender, or blunt pointed process extending from the pulp of a tooth toward the point of a cusp. VI [I GLOSSARY. Incisor. A tooth with a cutting edge. There are four incisors in the upper jaw, and four in the lower jaw. They are called the upper and lower right and left central, and the upper and lower right and left lateral, incisors. Inclination. Of a tooth ; the deviation of the long axis of a tooth from the perpendicular line; as the mesial incli- nation of the incisors. (See Par. 191.) Of a surface: the deviation of a portion of the surface of a tooth from the general plane of that surface. Incline. See Inclination. Interproximate space. The V-shaped space bounded by the proximate surfaces of adjoining teeth, and the border of the septum of the alveolar process between their necks. Xormally this space is filled with gum tissue. (See Par. 193.) Labial. Pertaining to the lips. Toward the lips. Labial surface. The surface of a tooth next to the lips. The incisors and cuspids have labial surfaces. Labially. Toward the lips. Labio-lingual. From the lips toward the tongue; as the labio-lingual diameter of the central incisor. Lingual. Xext to, or toward the tongue; as lingual surface. Lingually. Toward the tongue. Lingual surface. A surface of a tooth next to the tongue. All of the teeth have lingual surfaces. i^inguo-gingival fissure. A fissure occurring occasion- ally in the lingual surface of the upper incisors. It usually separates the lingual lobe from one of the marginal ridges and extends into the cementum. (See Fig. 10.) Linguo-gingival ridge. A ridge near the gum on the lingual surface of the incisors and cuspids. It is on the lin- gual lobe. (See Par. 21.) Marginal ridge. The ridges, or elevations of enamel on the margins of the occluding surface of the bicuspids and GLOSSARY. IX molars, and the mesial and distal margins of the lingual sur- face of the incisors and cuspids. Median line. The anterio-posterior perpendicular cen- tral line of the body'. Mesial. Toward the median line. Those surfaces of the teeth which, as they stand in the arch, and following its curve, are toward the median line, are called mesial surfaces. Mesial angle. The corner formed by the junction of the cutting edge and mesial surface of the incisors and cuspids. Also, the corner formed by the junction of the mesial, bucciil, and occluding surfaces of the bicuspids and molars. Mesially. Toward the median line. Mesio-buccal angle. The angle formed by the union of the mesial and buccal surfaces of the bicuspids and molars. Mesio-distal. From mesial to distal; as, the mesio-dis- tal diameter of the lower first molar. Mesio-labial angle. The angle formed by the union of the mesial and labial surfaces of the incisors and cuspids. Mesio-lingual angle. The angle formed by the union of the mesial and lingual surfaces of the teeth. Mesio-lingual groove. A developmental groove running from the mesial surface diagonally to the lingual surface in upper first molars that have the fifth cusp. (See Par. 69.) Mesio-occluding angle. The angle formed by the union of the mesial and occluding surfaces of the bicuspids and molars. Neck. That portion of the tooth which forms the junc- tion of the crown and root. Oblique ridge. A ridge running obliquely across the occluding surface of the upper molars. It is formed by the union of the triangular ridge of the disto-buccal cusp with the distal portion of the ridge forming 1he mesio-lingual cusp. Permanent teeth. The teeth of adult age as dis- tinguished from the temporary, or deciduous teeth. X GLOSSARY. Pit. A sharp, pointed depression in the enamel. Pits occur mostly where several developmental grooves join; as in the occluding surfaces of the molars. Point of proximate contact. The point at which the proximate surface of a tooth touches the proximate surface of a neighboring tooth. Proximate contact. The contact, or touching, of the proximate surfaces of neighboring teeth. Proximation. The near approach, or contact, of the proximate surfaces of the teeth. Proximate surface. The surface of a tooth which lies next to another tooth. (See Par. 5.) Pulp. The soft tissue that fills the pulp chamber and root canals of the teeth. Ridge. A long-shaped elevation on the surface of a tooth. Root. That portion of the tooth that is fixed in the bony alveolus, or socket; and is covered with cementum. Root canal. The opening through the center of the long axis of the root of a tooth from the crown to the apex. Rugae. A series of irregular ridges in the roof of the mouth. Septum. (PI. Septa.) A partition. That portion of the alveolar process which lies between the roots of the teeth separating their alveoli. Succedaneous teeth. Those of the permanent teeth which succeed, or take the places of, the temporary teeth. Sulcate groove. A groove following the bottom of a sulcus. Sulcus. (PI. Sulci.) A notable long-shaped depression in the surface of a tooth the inclines of which meet at an angle. A sulcus has a developmental groove at the junction of its inclines. Supplemental groove. A shallow long shaped depres- sion in the surface of a tooth, generally with a smoothly (i L 0 S S A R Y . XI rounded bottom. It differs from the developmental grooves in that it does not mark the junction of the lobes. Supplemental lobe. A lobe that does not belong to the typical form of the tooth ; an additional lobe. Supplemental ridge. A ridge on the surface of a tooth that does not belong to the typical form of the tooth ; an additional ridge. Temporary teeth. See deciduous teeth. Thick-necked. A tooth in which the mesio-distal diam- eter of the neck is nearly equal to that of the crown. (See bell crown.) Thirds. Division of the crown of a tooth ; as to length, into occluding, or incisive third, middle third, and gingival third ; as to mesio-distal breadth, into mesial third, middle third, and distal third. Transverse ridge. A ridge formed of two triangular ridges which join to form a continuous ridge across the occluding surface of a tooth. ' Triangular ridge. A ridge* running from the point of a cusp toward the central portion of the occluding surface of a tooth. Tubercle. A slight rounded elevation on the surface of a tooth. Tubercles occur frequently on the linguo-gingival ridge of the incisors, and occasionally upon various parts of other teeth. They are deviations from the typical tooth forms. DENTAL ANATOMY. NOMENCLATURE. 1. Man's food is both animal and vegetable, and his teeth are so formed as to enable him to readily masticate either kind ; therefore, his teeth differ from those of both the carnivorous and herbivorous animals, and form the type of the omnivora. They are formed for cutting, tearing, and comminuting many kinds of food. The incisors, situated anteriorly, have edges for cutting; the cuspids and bicus- pids, at the angles of the mouth, have fairly sharp, though not very long, points or cusps, suited for tearing; while the molars, situated in the posterior part of the mouth, have broad, tuberculated grinding surfaces, which serve well to grind or comminute the more solid masses. The forms of the teeth of man indicate a design that his food should be taken in rather small masses, considered from the stand- point of the habits of animals in general, and that it should be very thoroughly commingled with saliva before being passed to the stomach. 2. The adult has thirty-two teeth: four incisors, two cuspids, four bicuspids, and six molars in each jaw. Scien- tific men express the denomination and number of the teeth of the mammalia by formulae, in which the denomination is represented by the initial letter, followed by a horizontal line, with the number of that kind of teeth on one side of the upper jaw written above the line, and the number in the lower jaw written below the line. The dental formula of man is written thus: I § C { B § M | = 32. The teeth are composed of four tissues: enamel, which covers the crown; dentine, which forms the body of both crown and root; cementum, which covers the root and joins 1 NOMENCLATURE. with the enamel at the gingival line, or neck, of the tooth ; and pulp tissue, which fills the central cavity of the dentine. 3. Each tooth presents for description certain charac- ters common to all, such as crown, neck or gingival line, root, pulp-chamber, canal, and surfaces of crown and root. The crown of a tooth is that part which projects beyond the gum tissue, and is covered with enamel; while the root, covered with cementum, is that portion fixed in the bony process of the jaw, by which the whole tooth is held securely in position. The root of a tooth may be single, as in the incisors and cuspids; divided into two roots, as in the lower molars; or into three roots, as in the upper molars; or into a greater number, as in exceptional examples. The root is divided into the body or main portion; the apex, or terminal end of a root; and the neck, which marks the junction of the root with the crown. A tooth is also marked with a slight constriction at the neck, and by the junction of the enamel with the cementum. The latter, which forms a visible line encircling the tooth, is called the gingival line. It is so curved in its course as to present a convexity toward the crown on the proximate surfaces, to correspond with the line of the gum as it passes over the alveolar ridge from the labial to the lingual side of the arch. On the anterior teeth, it is also so curved in passing the labial and lingual surfaces as to present a concavity toward the crown. These are called the curvatures of the gingival line, or the gingival curvature. The neck of a tooth is common to all of the roots, whatever the number, for the point of division into two or more roots is always rootward from the neck. 4. The crowns of the incisors and cuspids present for examination four surfaces and an edge; and the crowns of the bicuspids and molars, five surfaces. These surfaces are named according to their position and use. Those of the in- cisors and cuspids presenting toward the lips, are called labial surfaces; those of the bicuspids and molars presenting toward NOMENCLATURE. 3 the cheek, buccal surfaces; all presenting toward the tongue, lingual surfaces, whether in the upper or the lower jaw.* Those surfaces which come in contact with the opposing teeth in closing the mouth are called occluding surfaces. These surfaces of the bicuspids and molars are also called grinding surfaces. 5. The surfaces of the teeth that present toward, or lie against, adjoining teeth are called proximate, or proximal, surfaces. The proximate surfaces are also more closely defined by the terms mesial and distal. These terms have special reference to the position of the surface relative to the central or the median line of the face. This line is drawn along the suture uniting the superior maxillary bones, or perpendic- ularly through the center of the face and mouth, and passes between the central incisors of both the upper and lower jaws. Those proximate surfaces which, as they are placed in the arch, and following its curve, are toward the median line, are called mesial surfaces ; and those most distant from the median line are called distal surfaces. The mesial surfaces of the central incisors, both upper and lower, proximate each other; but in all other cases a mesial surface proximates a distal. Also, a distal surface always proximates a mesial surface, except those of the third molars, upper and lower, which have no distal proximating teeth. The points at which the proximate surfaces touch each other as the teeth stand in the line of the arch, are called the points of proximate contact, or simply contact points. 6. The angles of the teeth are formed by the junction of two of the surfaces of their crowns named in paragraph 4. The incisors and cuspids have four, and the bicuspids and molars six, angles which are named by combining the names of the uniting surfaces into a compound word, always using the word mesio- or disto- as the prefix. The incisors and * Some authors use the term "palatine surfaces" for those of the upper jaw, and " lingual " for those of the lower. This seems unnecessary. B NOMENCLATURE. cuspids have mesio-labial and disto-labial angles. The bi- cuspids and molars have mesio-buccal and disto-buccal angles. All of the teeth have mesio-lingual and disto-lingual angles. The bicuspids and molars have, also, mesio-occluding and disto-occluding angles. All of these angles form lines run- ning from the occluding surface to the gingival line, except the mesio-occluding and disto-occluding angles which form lines from buccal to lingual. Besides these there are certain angles which are only points or corners on the surface of the crown. Thus, in case of the incisors and cuspids the corners formed by the junction, the cutting edge with the mesial and distal surfaces are called respectively the mesial and distal angles. Also the corners of the bicuspids and molars formed by the junction of the mesial, buccal, and occluding surfaces are called mesial angles, and the corners formed by the junction of the distal, buccal, and occluding surfaces are called distal angles.* For convenience in designating different parts of the crowns of the teeth, they may be divided into thirds as to length ; as the occluding, or incisive third, the middle third, and the gingival third. A similar division in the mesio- distal direction may also be used. 7. The incisors present a cutting edge by the junction of the labial and lingual surfaces along a line. In the cus- pids, this joining of the surfaces to form an edge is raised to a point near the center of its length, forming a cusp, hence the term cuspid, a tooth with one point. The cutting edges of the incisors, and the occluding surfaces of the bicuspids and molars, come in contact with the similar surfaces of the teeth of the opposing jaw when the mouth is closed, as in the act of biting. The incisors and cuspids of the upper jaw *The theoretic corners, or point-angles, of the bicuspids and molars are four in num- ber, and would be called the mesio-bucco-occludms, disto-bucco-occluding mesio- linguo-occluding, and disto-linguo-occluding angles. These terms are rather cumber- some and probably will not be used by the profession. The lingual point-angles are rarely spoken of, and the profession is already in the habit of using the terms given in the text to designate the buccal point-angles. , NOMENCLATURE. 5 do not occlude exactly on the cutting edges or cusps, but generally just back of them; though, for convenience, this term may be applied to them as if they did. The occluding surfaces of the bicuspids have two cusps, hence the term bicuspid, a tooth with two points; and the occluding sur- faces of the molars have four cusps, and sometimes more. 8. A cusp is a pronounced elevation, more or less pointed, on the surface of a tooth, but more especially on the occluding surface. A slight elevation is often called a tubercle, as that frequently seen near the gum on the lingual surface of the upper incisors. These are generally deviations from the typical forms of the teeth. 9. Long-shaped elevations on the surfaces of teeth are called ridges, and are named according to their location or form: as buccal ridge, lingual ridge, and marginal ridge. The marginal ridges are those elevations of the enamel which form the margins of the occluding surfaces of the bicuspids and molars, and the mesial and distal margins of the ling-ual surfaces of the incisors and cuspids. Those ridges which descend from the cusps of the molars and bicuspids toward the central part of the occluding surfaces are called triangular ridges. They are so-called because the typical form of the ridges is usually that of a triangle. They are named after the cusps to which the}- belong, as, the triangular ridge of the mesio-buccal cusp of the upper first molar, or simply, mesio-buccal triangular ridge. When a buccal and a lingual triangular ridge join they form a transverse ridge. In this way they often subdivide the central fossa of the lower molars and form supplemental fossa?. 10. A generally rounded or angular depression on the surface of a tooth is called a fossa. Foss$e occur mostly on the occluding surfaces of the molars. When a notable depression is long-shaped it is called a sulcus. Some of these pass from mesial to distal entirely through the occluding 6 NOMENCLATURE. surface of a tooth, as in the bicuspids. This term is often erroneously applied to the grooves and fissures. 11. A shallow, long-shaped depression, in the form of a line, on the surface of a tooth, is called a groove. When such a groove follows the bottom of a sulcus it is said to be sulcate. When such a groove sinks suddenly into the sub- stance of a tooth in the form of a fault it is called a fissure. The words sulcus, groove, and fissure are often used as if they were synonymous and interchangeable, which has given rise to much confusion. It is necessary that the dis- tinctions made in their use in dental anatomy be well under- stood. A groove is a very fine line in the form of a slight, sharp depression on the surface of the tooth, as represented in section in Diagram A, at a in a section cut at the point where the mesial groove passes over the marginal ridge of a bicuspid. Diagram B, from a cross section of a bicuspid, very close to the occluding surface grooves are shown at a, a. A very shallow depression with rounded bottom is also called a groove, as those seen on the labial surfaces of the incisors. A sulcate groove is one that follows a sulcus of notable^depth, the inclines of which approach each other at an angle, as represented at «, in Diagram C. A fissure is always a fault in the enamel, a point where the maroins of the plates of enamel fail to unite, as represented at a, in diagram D. These are distinctions of importance, and there should be no confusion in the use of the words by which we distinguish them. There are two varieties of grooves in the teeth, differing essentially in their nature and formation. One class mark the lines on which the parts of the enamel NOMENCLATURE. 7 first formed separately, are afterward joined, and are called the essential or developmental grooves, or developmental lines. In their relation to the development of the enamel, they are of the same nature, and resemble the sutures of the bones of the skull, which mark the junction of the separate- ly formed plates. The parts, or divisions, of a tooth out- lined by its developmental grooves are called its lobes. Most human teeth have four lobes; but some have five, and some . only three. Hence these grooves form the key to much of de- scriptive dental anatomy; and when they can be traced, enable the anatomist to unravel complex forms, and assign irregularly formed teeth to the groups to which they belong, to properly name their individual parts, and identify any additional, accidental, or unusual forms or parts that ma}' have become intermingled in the accident of a faulty forma- tion. These developmental grooves, or lines, are subject to fissures, which occur when, from any cause, there is a failure of perfect union of the parts, leaving a fault. 12. The other class of grooves have no especial relation to the developmental lines, and are called supplemental grooves. These are aptly described as wrinkles in the enamel, which, in fact, many of them are. But some are so constant in their location and form as to make up a portion of the typical tooth-form. The supplemental grooves are usually shallow, with well-rounded bottoms, and are not ordi- narily subject to fissure. 13. In the mesio-distal direction all of the teeth are a little broader at or near their occluding surfaces than at their necks ; therefore, as they stand in the well-formed arch, their proximate surfaces touch only at or near their occluding sur- faces, leaving V-shaped openings between their necks. These are called inter-proximate spaces. Normally, the inter- proximate spaces are filled with gum tissue. 14. The teeth of different individuals show considerable variety of form. Some persons have teeth with very long K MEASUREMENTS OF THE TEETH. crowns, broad in the mesio-distal direction at their occluding surfaces and narrow at their necks. These present large inter-proximate spaces. They are known as bell-crowned teeth. Again, some individuals have teeth that, in their mesio-distal diameter, are nearly as thick at their necks as at the occluding surfaces, making their inter-proximate spaces very narrow, the teeth almost, or quite, touching along the whole length of the crown. These are known as thick-necked teeth. The more common form is midway between these two extremes. The teeth of some individuals and families have very long cusps; those of others have very short cusps. Some are deeply marked by grooves and sulci, and in those of others the grooves and sulci are shallow. Thus, there is considerable variety of contour without change of type. MEASUREMENTS OF THE TEETH. 15. In the following tables the results of the measure- ment of many teeth of each denomination are given. The numbers measured of the different varieties differed, but in all denominations they were sufficient to insure reasonable accu- racy as to the average size. There are three measurements given, the average, greatest, and least, in the several posi- tions measured. With a greater number, both larger and smaller teeth might be found, so that the tables must not be taken to represent the greatest nor the least that might be obtained; but the occurrence of larger, or longer teeth must be rare. The lines of measurement, are: 1st. " Length over all: " Length of the tooth from the cutting edge, or buccal cusp, to the apex of the root. 2d. "Length of crown: " Length of the crown from the cutting edge, or buccal cusp, to the gingival line on the labial or buccal surface. MEASUREMENTS OF THE TEETH. 9 3d. " Length of root: "Length of root from the gingival line on the buccal surface to the apex of the root. 4th. " Mesio-distal diameter of crown: " This is the extent from mesial to distal in the greatest diameter, or at the points of proximate contact. 5th. u Mesio-distal diameter of neck: " This measure- ment was made at the gingival line. 6th. " Labio- or bucco-lingual diameter:'' This meas- urement was taken at the greatest diameter of the crown in the direction named. In the incisors it was on the gingival ridge. In the bicuspids and molars it was generally mid- length of the crown, but occasionally it was near the gingival line, especially in the upper second and third molars. 7th. " Curvature of the gingival line;" This is the height or extent of the curve of the gingival line toward the cutting edge, or occluding surface, as it passes from labial to lingual, measured on the mesial surface. Having these tables, the necessity for giving many measurements in the text is avoided. 10 MEASUREMENTS OF THE TEETH. Table of Measurements of the Teeth of Man, Given in Millimeters and Tenths of Millimetres.* UPPER TEETH. Central Incisor. Average. Greatest. Least. Lateral Incisor. Average. Greatest. Least. Cuspid. First Bicuspid. Second Bicuspid. First Molar. Second Molar. Third Molar. Average. Greatest. Least. Average. Greatest. Least. Average. Greatest. Least. Average. Greatest. Least. Average. Greatest. Least. Average. Greatest. Least. r^ •M n 5= ■*J — o 8^3 O *i w u > o I-, 1*4 "-5 o ^g o O o O (H O !- o - ~ ■- 4; <» .2 s 3 be 53 zp c .2 1—1 ►4 i-I ^ ^3 i—! U^ 22.5 27-0 iS.O 20.6 22.5 I7.O 21-5 27.0 16.0 17.1 22.0 14.0 10.0 ; 12.0 12.0 8.0 16.0 8.0 22.0 8.8 13.0 26.0 10.5 16.0 17.0 8.0' 8.0 8.2 9.0 7.0 20.8 7-7 24.0 9.0 17.0 7.0 20.0 7.2 24.0 8.0 16.0 6 0 63 8.0 '50 26.5 | 9.5 17-3 32.0 12.0 20.5 20.0 8.0 11.o 12.4 14.0 10.0 7-5 14.0 9.0 19.0 7-Q 10.0 7-7 13.2 9.0 16.0 13.0 17.0 9.0 11. X 15-0 8.0 8.6 11.o 7.0 6.0 10.0 6.1 10.6 8.0 14-5 5-o 8.0 " 1> o — si 0 9.0 6-3 7.o 3-o 10.0 7.0 8.0 4.0 8.0 5-5 7.0 2.0 6.4 4.4 6.0 2.8 7.0 5-o 7.0 4.0 50 4.0 5-o 2.0 7.6 5-2 8.0 2-5 9.0 6.0 9.0 3-5 7.0 4.0 7-o 1.0 7.2 4-9 9-1 1.1 8.0 b.o 10.0 2.0 7.0 4.0 8.0 8.8 0.0 6.8 5-3 0.8 8.0 6-5 10.0 i-5 b.o 4-5 7-5 0.0 10.7 7-5 11.8 j 2.2 12.0 8.0 12.0 3-Q 9.0 7.0 11.0 I 1.0 9.2 6.7 11.5 1.6 10.0 8.0 12.5 ! 4.0 7.0 6.0 10.0 0.0 0.7 2-5 0.0 There are 25.4 millimeters to the inch. MEASUREMENTS OF THE TEETH. 11 Taisle of Measurements of the Teeth of Man, Given ix Millimeters and Tenths of Millimeters. LOWER TEETH. Central Incisor. Lateral Incisor. Cuspid. First Bicuspid. Second Bicuspid. First Molar. Second Molar. Third Molar. Average. Greatest. Least. Average. Greatest. Least. Average. Greatest. Least. Average. Greatest. Least. Average. Greatest. Least. Average. Greatest, Least. Average. Greatest. Least. Average. Greatest. Least. o -a 20.7 24.0 16.0 27.0 18.0 10.5 7.0 9.6 12.0 7.0 ir.8 16.0 9.0 •n o r^ O 6 >- 00 a 16.0 o 00 6.0 T? O O u ■S P. is cd a; to ° 'V O O u 1Z1 *" 15.8 5.6 19.0 6.5 15-2 5-i ri 11 OOo U2 O O Jr< T1 Ih .s ** 13 00 i.02 .66 o 00 I U Oi .CO 5 b to 0' o o c lo bo 1-' .88 ■39 ■49 •35 .24 .27 .11 1.06 .41 •63 •39 .27 ■3t • 15 .72 •3i •3i •3i .21 ■27 .07 .86 •34 •5i •25 • 17 •23 .11 .41 .63 •31 -3i ■ 27 •19 19 15 ■37 -68 j .29 ■47 .80 .35 •31 1 -43 -27 .20 •23 •15 ■23 •19 •3i ■35 .27 ■ 15 .07 .09 • 13 •03 First Bicuspid. Second Bicuspid. First Molar. Second Molar. Third Molar. Average. Greatest. Least. Average. Greatest. Least. Average. Greatest. Least. Average. Greatest. Least. Average. Greatest. Least. .81 .89 .66 .32 .48 .28 •35 -55 -3i .27 .39 .27 19 -35 23 -39 15 :-31 .04 .07 .00 .81 • 30 • 5i .42 .29 .46 .08 •94 •35 .62 •47 •3i • 47 . n .66 .27 •39 •35 .27 •43 •03 •78 .28 •51 .36 .26 • 45 ■05 •94 •31 .66 •39 •3i •49 • L5 .62 •23 •35 .27 •23 ■39 .00 .67 .24 ■ 44 •33 •23 .41 .02 .86 •.V •59 •43 •3' •57 .09 •55 •19 •31 .27 •19 •31 .00 H m p- gs. ^5 2. o P^ r> f rp -t < P 0> (t cn p i-t fQ> cn O tri p o'P- cn 12. p p O i-1 P P P cn T3 T3 i-t pL. Cu t cc>> t<0> £<^> ro> n 2 < rD *-t *^ a i-t < -1 n n ft cn P' i-t cn p >-( cn P cn P r* sr p P ' SW • rp orq »Q ■ $(N £■ 1> m r+ fl) n p p re !-t < f1 O W W w H H n M H > 3 n « r1 '4 M n w cn >> w « 0 W 0 w t/3 d (a W w ^ W M 2! '4 -• H K M 0 to '4 Tl Length over all. Length of crown. Length of root. Mesio-distal diameter of crown. Mesio-distal diameter of neck. Labio- or bucco- lingual diameter. Curvature of the gingival line. MEASUREMENTS OF THE TEETH. 15 Table of Measurements of the Deciduous Teeth of Man, Given in'Inches and Hundredths of an Inch. Averages only. £ ~ o T3 O 6 1h — CI co £ CU cu § s UPPER TEETH. Central Incisor. Lateral Incisor. Cuspid. First Molar. Second Molar. h-f I ^r .63 .23 .39 .25 I I —1 u CO o V o O u a cr) O P u ^.M Ss cfl 1-} •3 O 15 d1 u o cu ■-* -m S O .c 11 cd a 62 j .25 •74 -25 •59 LOWER TEETH. Central Incisor. .68 ■45 •53 •39 .22 I .46 •25 .18 .20 .16 .20 .14 • 19 .14 •27 .20 •27 .2t .28 .20 •33 .27 ___ ___ --- •32 •25 •55 Lateral Incisor. Cuspid. First Mclar. Second Molar. •59 ■19 ■19 ■66 .23 ■35 •39 •15 ■39 •'5 •32 ■13 • 45 .62 I .24 .38 .15 .11 .15 13 __ .19 | .14 .17 : -15 •30 J..25 .27 .21 .62 .21 ' .44 -38 .28 .34 -25 16 UPPER CENTRAL INCISORS. UPPER CENTRAL INCISORS. Note.—Usually, in the descriptions of the teeth, those of one side only will be men- tioned, without reference to which side in the text. Accompanying the illustrations, the side to which the tooth belongs will be given. The student will readily determine to which side a given example belongs by comparing it with the text and correctly naming its surfaces. Students should remember that the cuts are representations of individual teeth while the text describes the average forms, therefore the cuts will not always exactly correspond with the text. 16. The right and left upper central incisors are situ- ated in the extreme anterior part of the dental arch, one on each side of the median line, their mesial surfaces proxi- mating each other. The crown presents four surfaces (five including the cutting edge), four angles, and a cutting edge, or occluding surface. The general contour of the crown is similar to a wedge, with rounded angles, and merging into a rounded form at the thick end, or the neck of the tooth. It is slightly bent on its shorter diameter, so as to make the flattened labial surface convex ; while the other, the lingual, is concave. The crown is also slightly bent in the mesio- distal direction, so that the labial surface is convex and the lingual concave in this direction. Therefore, there is a general convexity of the labial, and a general concavity of the lingual surface. 17. The labial surface of the crown of the upper central incisor (Fig. 1), in its general form, is an imperfect square, with its gingival side rounded. The mesial margin is a little longer than the distal, so that the cutting edge slopes away toward the distal angle (Fig. 1, A). Both the angles, formed by the proximate surfaces and cutting edge, are slightly rounded, the distal more than the mesial, after which the proximate surfaces converge toward the long axis of the tooth, making the crown a little narrower at the neck than at the cutting edge. 18. The lingual surface of the crown (Fig. 2) is con- cave in all directions, forming a fossa, bounded by the cutting edge (a), the mesial and distal marginal ridges (n, m) and the linguo-gingival ridge, or cingulum (d). The marginal Fig. 4. Fig- ■">■ Fig. 1* (Par. 17).—Right Upper Ckntral Incisor. Labial Surface, a, Cutting edge ; b, mesial surface ; c, distal surface ; d, labial surface ; e,e, labial grooves ; g, mesial angle ; h, distal angle ; i, body of root ; k, apex of root. Fig. 2* (Par. IS) —Right Upper Central Incisor, Lingual surface, a, Cutting edge ; b, mesial surface ; c, distal surface ; d, linguo-gingival ridge, or cingulum: /, gin- gival line; g, mesial angle ; h, distal angle ; /, body of root; A-, apex of root; m, distal marginal ridge ; n, mesial marginal ridge. Fig. 3* (I'm. IK)—Left Upper Central Incisor, Lingual Surface, showing lingual pit. The cutting edge, a, is considerably worn. The mesial and distal marginal ridges, 6, c, are prominent; d, linguo-gingival ridge; /, gingival line ; g, mesial angle ; h, distal angle ; i, body of root ; k, apex of root; m, lingual pit. Fig 4* (Par. 19).—Right Upper Central Incisor, Mesial Surface, a, Mesial angle ; d. linguo-gingival ridge ; /,/, gingival line, showing its labio-lingual curvature; /. body of root; k, apex of root. Fig. 5* (Par. 21).—Left Upper Central Incisor, Lingual Surface. Young, un- worn tooth. The developmental lines, or grooves, are made diagrammatically promi- nent to show the form of the lobes, a, Middle lobe ; b, mesial lobe ; c, distal lobe ; d. lingual lobe; e, e, linguo-gingival groove ; /,/, lingual developmental grooves ; g, mesial angle; h, distal angle._______________________________________________ * Illustration, \y, diameters. UPPER CENTRAL INCISORS. 19 ridges are strong elevations of the enamel, running from the mesial and distal angles of the cutting edge along the borders of this surface to near the gingival line, where they join the linguo-gingival ridge. The linguo-gingival ridge is a strong elevation of the enamel forming the linguo- gingival border of the crown. It is sometimes elevated into a tubercle. The lingual fossa is usually smooth, and the ridges by which it is bounded are not prominent. In many instances, however, there is a deep pit at the junction of the linguo-gingival ridge with the lingual surface proper; and in some a groove extends from the pit for a short distance along the border of each marginal ridge (Fig. 3). These latter may, or may not, be fissured. In a few examples the enamel of this surface has irregular wrinkles, or ridges and grooves, running from the linguo-gingival ridge toward the cutting edge. In malformed teeth this surface is often very imperfect. 19. The mesial and distal surfaces each present the outline of the letter V, with its lines curved with the con- vexity toward the lips, and the acute angle at the cutting edge (Fig. 4). The mesial surface is almost straight from the angle of the cutting edge to the gingival line. It is convex from labial to lingual, but nearly flat toward the gingival line ; while in some there is even a slight concavity, centrally at, or near, the gingival line. In the labio-lingual direction, the distal surface is rounded, as in the mesial. In the majority of examples it is also convex in the direction of the long axis of the tooth, so that it bellies out toward the lateral incisor. 20. At the gingival line, the tooth is a little constricted, forming a slight furrow ; or rather, the root at the neck is a little smaller than the crown, and the enamel slopes down to the size of the root, giving the appearance of a continuous ridge of enamel around the neck. This line does not run horizontally around the neck of the tooth. On the proxi- c 20 UPPER CENTRAL INCISORS. mate side it forms a curve with the convexity toward the crown, and on the labial and lingual surfaces it forms a curve with the concavity towards the crown. This line marks the limit of the attachment of the peridental mem- brane and gum tissue to the root of the tooth. 21. Developmental lines (Fig. 5). When any of the incisors first appear through the gums there are three little eminences, or tubercles, on the cutting edge with grooves crossing from labial to lingual between them (a,g,h). These grooves run some distance on- the labial surface, becoming broader and shallower till they disappear. In many, these lines appear on the lingual surface between the marginal ridges and the fossa (/,/). Occasionally, they are seen as far as the linguo-gingival ridge. The little tubercles are soon removed from the edge by wear, leaving it straight, or slightly •curved. These lines divide this part of the crown of the tooth into three labial lobes. Calcification begins in these tubercles as separate pieces, or plates, and the grooves are the marks of the after-confluence of these plates. This is common to the incisors and cuspids. These teeth are some- times fissured across the cutting edge, marking an imperfect confluence of the primary plates. The calcification of the linguo-gingival ridge, or cingulum, is also begun as a sepa- rate plate, forming the lingual lobe, but afterward it becomes united to the other parts by confluence, leaving a groove, often very slight, indeed, and soon obliterated by wear, marking the line of union. This is the linguo-gingival groove (e, e). In smooth regularly formed teeth it begins at the gingival line just lingual of the summit of its labio- lingual curvature, and runs across the marginal ridge at right angles with its length, then runs almost horizontally across the lingual surface to the distal marginal ridge. This ridge is now crossed at right angles, and the gingival line reached. The length of the groove usually includes from a quarter to a third of the circumference of the tooth. When £-"■ '■'■*:•'.'■£ I Fig. 10. jy Fig. 11. Fig. C>* (Par. 24).—Right Upper Lateral Incisor, Labial Surface, a, Cutting edge; c, distal surface; e, labial grooves; /, gingival line ; g, mesial angle; h, distal angle. Fig. 7* (Par. 25).—Right Upper Lateral Incisor, Mf.sial Surface, a, Mesial angle ; d, linguo-gingival ridge : /,/, gingival line ; /, body of root; k, apex of root. Fig. 8* (Par. 2ii).—Right Upper Lateral Incisor, Lingual Surface, without lingual pit. a, Cutting edge; 6, mesial marginal ridge; c, distal marginal ridge; d, linguo-gingival ridge; /, gingival line ; g, mesial angle; h, distal angle; i, body of root; it, apex of root; to, lingual fossa. Fig. 9* (Par. 2Gi.—Right Upper Lateral Incisor, Lingual Surface, with lingua- pit, a, Cutting edge ; b, mesial marginal ridge ; c, distal marginal ridge, with linguo- gingival groove crossing it; d, linguo-gingival ridge, or cwigulum ; /, gingival line ; g, mesial angle ; h, distal angle ; i, body of root; k, apex of root; to, lingual pit. Fig. 10* (Par. 26).—Right Upper Lateral Incisor, showing a linguo-gingival Assure. a, Cutting edge ; b, linguo-gingival groove fissured ; /, gingival line ; g, mesial angle ; h, distal angle. Fig. 11* (Par. 27).—Upper Lateral Incisor, Mf.sial Surface. Very short root, a mesial angle ; d, linguo-gingival ridge ; /, gingival line. ♦Illustration, 1>£ diameters. UPPER LATERAL INCISOR. 23 the linguo-gingival ridge is prominent, or rises in the form of a tubercle, this groove is subject to much variation in its course. Often, there is a deep pit in the centre of its length ; i. e., centrally in the lingual surface, at the margin of the* linguo-gingival ridge (Fig. 3, m). From this, fissures may extend laterally. Occasionally, especially in the lateral incisors, a sulcus, or a fissure divides the linguo-gingival ridge from one of the marginal ridges, and extends into the cementum (Fig. 10). This is the linguo-gingival fissure. 22. The root of the upper central incisor (Figs. 1 to 5) is about one and a fourth, to one and a half times as long as the crown. It is conical in form, tapering from the crown to the apex ; less rapidly near the neck, and more rapidly as the apex is approached. Therefore, the body of the root seems a little swollen. However, the root of this tooth presents great variety of figure, as do the roots of teeth generally. The root is nearly round at the neck. The cur- vature of the lingual surface is the arc of a smaller circle than that of the labial. The proximate surfaces are slightly flattened. The flattened portion of the mesial surface is a little broader than the distal. These two converge to the lingual, giving the form of a prism Avith its angles rounded. UPPER LATERAL INCISOR. 23. The description of the lateral incisor may be much abridged, 1 iccause of its resemblance to the central in its general form and developmental lines. The tooth is a little shorter, and from mesial to distal the crown is about a third narrower. 24. The labial surface of the lateral incisor (Fig. 6) is more rounded in the mesio-distal direction than in the cen- tral. The mesial angle is acute, and the cutting edge slopes away in a curve to a rounded and obtuse distal angle. The cutting edge, at the time of eruption, presents three tubercles, and the grooves crossing the edge between these are pro- jected on the labial surface as shallow labial grooves. 24 UPPER LATERAL INCISOR. 25. The mesial (Fig. 7) and distal surfaces present the characteristic V-shape of all the incisors. From labial to lingual the mesial surface is rounded near the cutting edge, but much flattened near the gingival line. Sometimes a slight concavity exists at this point. Occasionally the mesio- labial angle has a flattened or sunken point of enamel near the middle of its length. This is sometimes broad and of notable depth, and in this case is generally in the labial portion of the mesial surface. In others, it is a small im- perfection in the mesial border of the labial surface. The distal surface is convex in all directions. In its occlusive third, it rounds out tVeely toward the cuspid, but becomes more flattened toward the gingival line. 26. The lingual surface (Fig. 8) of lateral incisors, is very irregular in the extent of its concavity. Some are almost flat, while others are deeply concave. The mesial and distal marginal ridges are proportionately broader, and stronger, than in the centrals. In the majority of examples the lingual surface is the broadest part of the crown. The rounding of the proximate surfaces is at the expense of the labial surface, so that a moderately acute angle is formed by the junction of the proximate surfaces with the lingual. Generally, the lingual surface is almost smooth, but in many, a pit, with or without lateral fissures, will be found at the junction of the lingual surface proper, with the linguo-gin- gival ridge (Fig. 9). In some of these, the linguo-gingival ridge is unusually short, so that the marginal ridges are folded in together at their gingival ends, forming a deep sulcus be- tween them, and there is a deep pit at their junction. Again, some are found in which there is a deep groove, which is often fissured, dividing one marginal ridge from the linguo- gingival ridge, and extending into the cementum (Fig. 10). This is sometimes nearly central, giving the appearance of a failure of the lingual lobe, or of a division of the lobe cen- trally, or of the displacement of the lobe to one side. This is the linguo-gingival fissure. Fig. 12. Fig. 15. Fig. 16. Fig. 12* (Par. 29).—Left Lower Central Incisor. Labial Surface. Long root, a, Cutting edge; e, labial grooves; ft gingival line; g, mesial angle : h, distal angle. Fig. 13* (Par. 29).—Left Lower Lateral Incisor. Lap.ial Surface. Long root, a, Cutting edge ; e, labial grooves ; /, gingival line; g, mesial angle ; h, distal angle ; t, body of root; k, apex of root. Fig. 14* (Par. 29).—Right Lower Lateral Incisor, Labial Surface. Short root. a, Cutting edge ; e, labial grooves ; /, gingival line ; g, mesial angle ; h, distal angle; ;', body of root; k, apex of root. Fig. 15* (Par. 30).—Lower Central Incisor, Lingual Surface, a, Cutting edge ; b, mesial marginal ridge ; c, distal marginal ridge ; d, linguo-gingival ridge : g, mesial angle ; h, distal angle ; in, lingual ridge. Fig. 16* (Par. 3d).—Lower Central Incisor, Distal Surface, a, cutting edge ; the edge is worn away as represented by the line ; d, linguo-gingival ridge; /,/, gingival line ; i, groove along the distal side of root; k, apex of root. Fig. 17* (Par. 31).—Right Lower Lateral Incisor, Distal Surface, showing root deeply grooved, a, Cutting edge ; d, linguo-gingival ridge ; /, gingival line ; i, deep groove in root; k, apex of root. * Illustration, 1% diameters. THE LOWER INCISORS. 27 27. The root of the upper lateral incisor (Figs. 6 to 11) is conical, but considerably flattened on its mesial and distal sides, which is generally maintained to the apex. The root is generally straight, and about one and a half times as long as the crown. In many specimens the apex is curved to the distal. Occasionally the root is very crooked. 28. The upper lateral incisor presents much variety of size and form. It is not uncommon to find narrow laterals associated with broad centrals. The lateral incisors are more often imperfectly developed than the other anterior teeth. In these, the crown of the tooth is frequently conical, with a rounded, or even a moderately sharp, point. THE LOWER INCISORS. 29. The lower incisors have outlines similar to the upper lateral, but are, in every way, more slender. Their developmental lines are the same, but the grooves are much less marked, and generally cannot be seen except in unworn teeth. The cutting edge of the lower central (Fig. 12) is very nearly at right angles with the long axis of the tooth, and its angles are square and sharp. From mesial to distal, the cutting edge is the widest part of the crown, and from it the proximate surfaces converge equally to the gingival line, reducing the mesio-distal diameter about a third. The lower lateral differs from the central by the cutting edge sloping away to the distal, the mesial angle being acute, and the distal, obtuse and rounded. The distal surface is also convex from the angle to the gingival line, bellying out toward the cuspid. 30. The lingual surface (Fig. 15) of the lower incisors is concave from the cutting edge to the linguo-gingival ridge, over which there is a convexity (Fig. 16). ]STear the cutting edge this surface is generally nearly flat in the mesio-distal direction, but is sometimes concave or slightly convex; it becomes convex progressively toward the linguo-gingival 28 THE CUSPIDS--THE UPPER CUSPID. ridge. In many there is a slight ridge on the center of this surface with a shallow concavity on either side, which marks the junction of the lobes, running from near the cutting edge to the linguo-gingival ridge (Fig. 15). The mesial and distal surfaces are convex near the cutting edges, but become flattened, and sometimes slightly concave, toward the gin- gival line. 31. The roots of the lower incisors are slender and much flattened in their mesio-distal diameter, and not unfrequently slightly grooved on the mesial and distal sides. The labial surface of the crown and root, in its length, forms nearly the arc of a circle (Fig. 17), though the curve of the surface of the crown is usually a little greater than that of the root (Fig. 16). The lingual surface of the root is almost straight, but in the apical third it is convex, sloping away to form the apex. The roots of these teeth are generally straight, but occasionally the apex is curved to the distal. THE CUSPIDS. 32. "We have four cuspids, one on each side in the upper and the lower jaw. They are sometimes called canine teeth, or eye teeth. They are situated at the angles of the mouth, between the lateral incisors and the first bicuspids. They are third from the median line, and are large, and strong teeth of simple form, firmly implanted in the alveolar process by a long, strong root. THE UPPER CUSPID. 33. The labial surface of the crown of the upper cuspid (Fig. 18) is a little narrower in the mesio-distal direction than in the central incisor, with nearly equal length from the point of the cusp to the gingival line. Instead of a straight, or only slightly curved cutting edge, as in the in- cisors, the central portion of the crown is extended into a well-formed point (a), with cutting edges sloping away to the mesial and distal angles (g, h). Of these cutting edges, Fig. IS. Fig. 19. Fig. 20. Fig. 21. Fig. 22. Fig. 23. Fig. 24. Fig. IS * (Par. 33).—Right Upper Cuspid, Labial Surface, a, Point of cusp; cf, labial ridge ; e, labial grooves; /, gingival line ; g, mesial angle ; h, distal angle; /, body of root; k, apex of root. Fig. 19* (Par. 34).—Right Upper Cuspid, Lingual Surface. The more common form. a, Point of cusp ; d, linguo-gingival ridge ; e, e, lingual grooves ; /, gingival line; g, me- sial angle ; h, distal angle ; i, body of root; k, apex of root; m, lingual ridge. Fig. 20* (Par. 34).—Right Ii ii:r Cuspid, Lingual Surface of irregular form, a, Point of cusp ; d, linguo-gingival ridge; e, e, lingual grooves ; /, gingival line ; g, mesial angle ; /(, distal angle ; m, prominent lingual ridge; n, two small tubercles on linguo- gingival ridge. Fig. 21 * (Par. 351.—Right Upper Cu>piu, Mesial Surface, a, Point of cusp; b, mesial marginal ridge ; c, point of slight concavity of mesial surface ; d, linguo-gingival ridge ; e, labial groove; /,/, gingival line ; g, mesial angle; i, body of root; k, apex of root. Fig. 22 * (Par. 37).—Left Upper Cuspid, with a very long and crooked root. Fig. 23 * (Par. 37).—Right Upper Cuspid, with a peculiarly crooked root. Fig. 24 * (Par. 37).—Left Upper Cuspid, with a very small and short root. * Illustration, Vy.. diameters. 1 THE UPPER CUSPID. 31 the distal is a little the longer, and from the angle to the gingival line, the distal surface is a little shorter, than the mesial. In unworn teeth, the angle formed by the union of the cutting edges to form the cusp is usually about ninety degrees, or a square. The point is a little rounded at first, but is soon much rounded or flattened by wear. Both the mesial and distal margins of the labial surface of the crown, from the angles to the gingival line, slope toward the central axis of the tooth,—the distal slope being the greater. This narrows the crown at the neck nearly one-third from the width of its widest point. The curvature of the gingival line on this surface, marking the termination of the crown, is about a quarter circle. The surface is convex in all direc- tions, and is much more rounded in the mesio-distal direc- tion, than in the incisors. In the direction of its length, the convexity is about the same as in the incisors. The greater convexity mesio-distally is caused by a strong labial ridge (cf) running from the point of the cusp to the gingival line. This ridge occupies so much of the surface.of the tooth that its margins are imperfectly defined. It belongs to the mid- dle lobe, developed from the middle plate, which, in the in- cisors, is the smallest of the three, but in this tooth is much the largest. There are two labial furrows (e), or a flattening of the convexity between the central line of the ridge and each angle, marking the junction of the lobes. These fur- rows are usually lost by becoming shallower before reaching the center of the length of the crown. In well-formed teeth, this surface presents a uniform enamel, free from pits or sulci. 34. The lingual surface (Fig. 19) presents the same o-eneral marginal configuration as the buccal, though it is somewhat narrower toward the gingival line. This is caused by the arc of convexity of the lingual surface of the neck of the tooth being a smaller circle than that of the buccal, and by the flattening of the proximate surfaces on lines which 32 THE UPPER CUSPID. converge rapidly to the lingual. This surface is usually almost straight from the cusp to the linguo-gingival ridge, or cingulum, but is sometimes slightly concave. The linguo- gingival ridge is sharply convex and longer from the gingival line to the point of convexity than in the incisors. (Fig. 21, d.) Mesio-distally this surface is slightly convex in its central part on account of the lingual ridge which runs from the point of the cusp nearly, or quite, to the cingulum. On each side of this, and between it and the marginal ridges, there is a slight but well-defined concavity and furrow, marking the confluence of the lobes. The marginal ridges arise from the mesial and distal angles and unite with the linguo-gingival ridge or cingulum. These ridges are usually large near the angles, and much less pronounced toward the gingival ridge. The latter is prominent, and is often raised into a tubercle, or slight cusp. Occasionally this part of the enamel is thrown into irregular folds, with grooves between, which are sometimes fissured. More rarely the small cusp may be divided by a groove (Fig. 20, n). The linguo-gingival groove is often pronounced in unworn teeth. 35. The mesial surface, near the angle (Fig. 21), is convex in all directions, but becomes flattened, and occasion- ally slightly concave, near the gingival line (c). 36. The distal surface is similar to the mesial, but is more convex, usually being well rounded in the labio-lingual direction to the gingival line. But in the direction of the long axis of the tooth this surface, on account of the projec- tion of the distal angle, is first convex, and further toward the gingival line is concave, especially near, and at the neck of the tooth. The labio-lingual curvature of the gingival line is about 2.5 m.m., varying from 1.0 m.m. to 3.5 m.m. on the mesial surface, and a little less on the distal. 37. The root of the upper cuspid is the longest in the human mouth, averaging, according to my measurements, 17.5 m.m., and varying from 11.0 to 21.0 from the apex to a , distal angle; i, body of root; k, apex of root. Fig. 28* (Par 39).—Left Lower Cuspid, Lingual Surface, a, Point of cusp; d, linguo-gingival ridge; e, lingual grooves ; /, gingival line ; g, mesial angle; h, distal angle ; i, body of root; k, apex of root; I, distal marginal ridge ; in, lingual or triangular ridge ; n, mesial marginal ridge. Fig. 29* (Par. 40).—Right Lower Cuspid, Mesial Surface, a, Point of cusp; d, linguo-gingival ridge; /, gingival line ; g, mesial angle. The root is short and thick. * Illustration, V., diameters. THE LOWER CUSPIDS. 35 the gingival line afr the labial surface. It is irregularly conical in form, tapering from the neck to the apex. Its labio-lingual diameter is a little greater than its mesio-distal, which gives the root a flattened appearance, but it is seldom entirely flat on either the mesial or distal surface. In most examples the body of the root is straight, and tapers to a slender point, which is often curved to the labial and distal, though the form of this root presents great variations. Fre- quently, it is very crooked, perhaps, because, when it is taking its place in the arch, it is often crowded by the teeth mesial and distal to it, so that its growth in a right line is inter- rupted. (See Figs. 22, 23 and 24.) THE LOWER CUSPIDS. 38. In their general figure, the lower cuspids so closely resemble the upper, a description of their differences will be sufficient. They are slightly smaller than the upper cuspids, and the crowns are a little longer, which makes them appear more slender. The mesial surface of these teeth is usually nearly straight the entire length of the root and crown (Figs. 25 and 26); so that the increased width of the crown over the root is mainly on the distal. This causes a marked prominence of the distal angle. In many examples this gives the tooth the appearance of being bent, with a considerable concavity on the distal side. In young, unworn teeth, the cusp is rather more prominent and pointed than in the upper cuspid, and the distal cutting edge is proportionately longer; but, as the point of the cusp comes directly in occlusion with the upper teeth, it is soon worn to a blunt point, or a flat surface, inclining to the labial and sloping away to the distal. 39. The lingual surface (Fig. 28) is very smooth, and the ridges are less prominent than in the upper cuspid. A tubercle on the linguo-gingival ridge is rare. The develop- mental lines, or grooves, are the same as those of the upper cuspids, but less prominent. Yet, generally, they can be D 36 THE BICUSPIDS. seen in unworn teeth. Fissures are seldom seen in any part of this tooth. 40. The root of the lower cuspid (Figs. 25 to 29) is shorter than the upper, and generally more flattened in the mesio-distal diameter, often presenting deep furrows. In rare instances, there is a division of the root near the ex- tremity. The root is nearly straight, and in many examples the lingual surface of the root is nearly a straight line, while the labial surface, root, and crown, present a nearly regular convexity. The root is not so often abnormally crooked as that of the upper cuspid, though, like the upper, it is much inclined to end in a slender apex ; which is often slightly bent in the labial direction. THE BICUSPIDS. 41. There are eight bicuspids, or premolars, two on each side in the upper jaw, and two on each side in the lower jaw. Hence they are called the first and second bicuspids. They are situated between the cuspids and first molars, and are the fourth and fifth teeth from the median line. The bicus- pids, though unlike the incisors and cuspids in the contour of their crowns, have the same number, and a similar dis- tribution of primary parts, or lobes. They are, indeed, formed on the same general plan. The change of form is the result of a different relative development of the parts, by which the cingulum, or linguo-gingival ridge, is elevated into a powerful lingual cusp; which, in the upper bicuspids, is almost or quite as high as the buccal cusp, but in the lower bicuspids, especially in the first, is less prominent. The middle lobe also forms a relatively larger part of the buccal portion of the crown than in the incisors and cuspids; while the mesial and distal lobes are relatively smaller. By the development of the lingual cusp of the upper bicuspids ; the linguo-gingival groove of the incisors and cuspids (q. v. 21) is carried to the central part of the crown, which it traverses Fig. 30. Fig. 33. Fig. 30* (Par. 42).—Left Upper First Bicuspid, Occluding Surface. av Point of buccal cusp; b, lingual cusp; c, buccal ridge; cf, mesial marginal ridge ; e, distal marginal ridge; /, triangular ridge of the buccal cusp ; g, distal angle ; li, mesial angle : i, triangular ridge of the lingual cusp ; J, central groove ; o, mesial groove; p, distal groove; n, m, tri- angular grooves; r, *, buccal grooves. Fig. 31 * (Par. 46!.—Left Upper First Bicuspid, Buccal Surface, a, Buccal cusp; c, buccal ridge; e, e, buccal grooves; /, gingival line; g, mesial angle ; h, distal angle ; i, buccal root; k, lingual root. Fig. 32 * (Par. 48).—Right Upper First Bicuspid, Mesial Surface, a, Buccal cusp; b, lingual cusp; c, mesial angle ; d, mesial surface and point where there is often a con- cavity ; /,/, gingival line ; c, lingual root; g, buccal root. Fig. 33 * (Par. 50).—Upper First Bicuspid, with three roots. Mesio-buccal angle. Fig. :U*(Par. form. -Upper First Bicuspid, with three short roots and of a peculiar Fig. 35 * (Par. 50).—Upper First Bicuspid, with a single, very long, crooked root. * Illustration, l1.^ diameters. UPPER FIRST BICUSPID. 39 from mesial to distal in a deep sulcus (Fig. 30). It is natu- rally divided by the mesial and distal pits, found at its j unction with the triangular grooves, into three parts : mesial (o), central {I), and distal (p). In the lower bicuspids the lino-ual lobe is often very small and the course of the grooves i rregular. UPPER FIRST BICUSPID. 42. The outline of the occluding surface of the upper first bicuspid (Fig. 30), when seen in a line with the long axis of the tooth, is irregularly quadrilateral, or trapezoidal, in form. The bucco-lingual diameter is about two-ninths greater than the mesio-distal. The flattened proximate surfaces converge toward the lingual, so that the mesio-distal measurement of the buccal portion is a little greater than the lingual. The buccal and lingual margins are convex. The buccal convexity forms an arc of about a quarter circle, and merges into the proximate surfaces by obtuse, but well- defined, angles (A, g). The lingual margin forms an arc of nearly half a circle, and merges into the proximate margins without any angular prominence. 43. The occluding surface has two prominent cusps— the buccal (a), and the lingual (6)—and is transversed from mesial to distal by a deep sulcus. The buccal cusp is the larger, and forms the terminal point of the buccal surface. From the point of this cusp, four ridges lead away at right ano-les. Two of these form cutting edges, which slope away mesially and distally to the mesial (A) and distal angles (g), where they join the marginal ridges. The central buccal ridge (c) leads away centrally on the buccal surface toward the o-ingival line, forming the convexity of this surface. The triangular ridge (/) slopes down to the central part of the crown and joins a similar ridge from the lingual cusp (/) to form the transverse ridge, or ends in a central sulcate oroove (I). The lingual cusp (b) is in the form of a crescent; its convexity forms the occluding margin of the lingual 40 UPPER FIRST BICUSPID. surface. Instead of a well defined point it usually presents a blunt edge, which runs around its elevated central portion, and joins with the marginal ridges at both angles. The lingual triangular ridge (i) leads down from the central point of the cusp to the central groove, to join its fellow from the buccal cusp in the formation of the transverse ridge, or is divided from it by a deep central sulcate groove. This ridge is seldom prominent. Very frequently the central incline of the lingual cusp is a plain surface. 44. The mesial {d) and the distal (s).—Left Lower First Bicuspid, Mesial Surface, a, Buccal cusp; b, lingual cusp or ridge ; c, distal marginal ridge ; d, triangular ridge of buccal cusp ; e, mesial marginal ridge; /, gingival line. Fig. 46* (Par 59).—Left Lower First Bicuspid, Lingual Surface, a, Buccal cusp; b, lingual cusp or ridge ; c, distal marginal ridge ; d, triangular ridge of buccal cusp; e, mesial marginal ridge; /, gingival line. Fig. 47* (Par. 59).—Right Lower First Bicuspid, Lingual Surface, a, Buccal cusp; b, lingual cusp or ridge ; d, triangular ridge of buccal cusp ; e, e, marginal ridges; f, gingival line. * Illustration, 1% diameters. LOWER FIRST BICUSPID. 49 into a small cusp, located centrally, or to either side of the central line of the tooth. 56. The marginal ridges are occasionally quite small, but in most examples they are well developed, and cause the mesial and distal surfaces to stand out prominently, giving the tooth a strong bell-crowned appearance. The length of the marginal ridges varies with the size of the lingual lobe. 57. The buccal surface (Fig. 44) is convex in all direc- tions. The cusp is to the distal of its perpendicular line, and its figure similar to the upper first bicuspid, except that its surface is more convex. As the occlusion is directly on the point of the cusp, as with the anterior lower teeth gen- erally, it is soon so worn down that its prominence is lost. 58. The mesial and distal surfaces are convex from buccal to lingual. In the direction from the occluding mar- gin to the gingival line, they are generally concave, after passing the convexity of the immediate occluding margin. This latter stands out boldly to both mesial and distal, giv- ing a marked bell-crowned form. The concavity of the mesial and distal surfaces is not so well seen from the buccal view as from the lingual. The greater over-hang of the crown (over the root), and the greater concavity, are toward the lingual portion. 59. The lingual surface (Figs. 46 and 47) is smoothly rounded from mesial to distal, and slightly convex in the direction of the length of the tooth. It is only about half as long as the buccal surface (in unworn teeth), and when viewed at right angles with the long axis of the tooth, all of the occluding surface on the lingual side of the buccal cusp is seen except the pits and sulci. 60. The neck of the tooth is much constricted, and has a deep gingival line. This makes the proper adjustment of a band for an artificial crown particularly difficult. The cur- vature of the gingival line is much less than in the upper 50 LOWER SECOND BICUSPID. first bicuspid, the average being less than one millimeter (q. o. table of measurements). 61. The root of this tooth is somewhat flattened at the neck, on lines that converge rapidly toward the lingual, and often grooved on its mesial and distal sides, and sometimes the root is bifurcated. It tapers regularly, with a tendency to a more nearly round form at the apex, and generally ends in a slender point. The root is generally straight, or the lingual surface is straight, and the buccal surface convex. This convexity extends from the apex of the root to the point of the crown, which gives that characteristic appear- ance so peculiar to the anterior lower teeth. LOWER SECOND BICUSPID. 62. The lower second bicuspid is a little longer than the lower first, and of much the same figure, except that the lingual cusp is proportionately higher and more nearly, but never quite, on a level with the buccal, but the lingual sur- face is about equal to the buccal in mesio-distal breadth. These teeth are regular in general contour, but the grooves of the occluding surface are much diversified. These differ- ences may be classified under three forms: 1st. The central groove joins the triangular grooves in such a wray as to form a half circle with the convexity to the lingual, with or with- out a transverse ridge crossing its line. In these, when the transverse ridge is high, only a deep pit appears on either side (Fig. 48). 2d. The lingual cusp is divided by a sulcate groove, which runs over centrally, or nearly so, to the lingual surface, making a three-cusped tooth (Fig. 49). The central groove forms an angle at the junction with the lingual, or is crescentic in form. It joins with the triangular grooves in such a way that the point of junction cannot be told except by finding the mesial and distal grooves, which are often very indistinct. 3d. The central groove is straight, and generally sulcate, with a deep pit at each end. In many F»g- 51. Fig. 52. Fig. 53: Fig. is * (Par. 62).—Left Lower Second Bicuspid, Occluding Surface, a, Buccal cusp ; b, lingual cusp or ridge; c, e, marginal ridges ; d, /, pits ; g, h, triangular grooves. Fig. 49 * (Par. 62).—Right Lower Second Bicuspid, Occluding Surface, with three cusps, a, Buccal cusp; b, mesio-lingual cusp; c, disto-lingual cusp; d, lingual groove; e, e, mesial and distal grooves. Fig. 50 * (Par. 62).—Right Lower Second Bicuspid, Occluding Surface, with straight central groove, a, Buccal cusp; b, lingual cusp; c, e, marginal ridges ; d, triangular ridge of the buccal cusp ; /, central groove; g, h, triangular grooves. Fig. 51 * (Par. 63).—Left Lower Second Bicuspid, Buccal Surface, a, Buccal cusp; d, buccal ridge; e, e, buccal grooves; /, gingival line; g, mesial angle; h, distal angle ; i, body of root; k, apex of root. Fig. 52 * (Par. 63).—Left Lower Second Bicuspid, Lingual Surface, a, Buccal cusp; b, lingual cusp; c, distal marginal ridge; d, triangular ridge of the buccal cusp; e, mesial marginal ridge; /, gingival line. Fig. 53 * (Par. 64).—Left Lower Second Bicuspid, Mesial Surfa< e. a, Buccal cusp; b, lingual cusp; d, triangular ridge; e, mesial marginal ridge ; /, gingival line; i, groove in mesial side of the root. * Illustration, 1% diameters. E THE MOLAR TEETH. 53 examples these pits are crossed bv the triangular grooves almost at right angles with the central (Fig. 50). By trac- ing the fine mesial and distal grooves carefully in young, unworn teeth, it will be found that the lingual lobe is much larger than in the lower first bicuspid, and almost as large as in the upper bicuspids. In the three-cusped forms the two lingual lobes are usually a little broader from mesial to dis- tal than the buccal portion of the tooth. 63. The buccal surface (Fig. 51) of the lower second bicuspid does not differ from the other bicuspids, except in being shorter, the cusp lower, and the lingual surface (Fig. 52) broader and smoothly convex; in the three-cusped forms it is often somewhat flattened and grooved in its occluding third, giving the crown a squarish appearance, or even triangular when the lingual lobes are large. 64. The mesial and distal surfaces in these bicuspids are a little flattened, but remain convex from buccal to lingual. From the occluding margin to the gingival line, they are nearly straight, though some are convex and some in part concave. 65. The root of the lower second bicuspid is larger and longer than in the first bicuspid. It is flattened on the mesial and distal surfaces on nearly parallel lines, and in some examples concave or grooved. In rare instances the root of this tooth is deeply grooved on the buccal and lingual sides, with a tendency to a division into a mesial and distal prong. In the greater number it tapers regularly to a slender apex, but in many, the apex is large and obtuse. The root is generally straight, but occasionally very crooked. THE MOLAR TEETH. 66. The molar teeth are very different in form and plan of construction from those previously described. They are particularly designed for grinding or comminuting food; for this purpose they have broad occluding surfaces, broken 54 THE UPPER MOLARS—UPPER FIRST MOLAR. by ridges, grooves, and fossae. The ridges are raised at inter- vals into powerful cusps, which fit with more or less accuracy into the sulci and fossae of the opposing teeth. There are twelve molars, three on each side of both jaws. They are the sixth, seventh and eighth teeth from the median line, and are commonly named the first, second and third molars, upperand lower. The last is also called the wisdom tooth, or Dens Sapientia. The upper and lower molars are much alike in size and general contour, but in the detail of the arrange- ment of their lobes, cusps, fossae and grooves, they are dif- ferent. It will, therefore, be necessary to describe the upper and lower molars separately. THE UPPER MOLARS. 67. The three upper molars are similar, but present minor differences of detail, consisting mostly in a less pro- nounced, or typical development of certain parts, or lobes, of the second and third molars (Fig. 62). The upper first molar being the typical form, will be described first, and afterward the deviations from this type that occur in the second and third molars. UPPER FIRST MOLAR. 68. The occluding surface of the upper first molar (Fig. 54), when seen in a line with the long axis of the tooth, pre- sents an outline of irregular rhombic form, with the mesio- buccal and disto-lingual as acute angles. The angles are rounded, with more or less convexity of the marginal lines. This surface presents two principal fossae, and four devel- opmental grooves. These grooves divide the crown into four lobes, or primary developmental parts, each of which is surmounted by a strong cusp. These lobes, or cusps, are the mesio-buccal (Fig. 54, a), disto-buccal (6), mesio-lingual (c), and disto-lingual (d). Of the grooves which outline these parts, three rise from the central pit of the central fossa: The mesial (A), which runs to the mesial margin ; the buccal Fig. 51. Fig. 55. Fig. 56. Fig. 57. Fig. 5s. Fig. .54* (Par. 08).—Right Upper First Molar, Occluding Surface, with four cusps. a, Mesio-buccal cusp ; 6, disto-buccal cusp; c, mesio-lingual cusp: d, disto-lingual cusp; /, mesial marginal ridge; g, distal marginal ridge ; h, mesial groove; i, buccal groove; j, distal groove; k, k, disto-lingual groove ; m, mesio-buccal triangular groove ; n, disto- buccal triangular ridge ; this unites with the distal ridge from the lingual cusp to form the oblique ridge : o, disto-buccal triangular groove ; p, mesio-buccal triangular ridge; q, central pit. Fig. 55* (Par. 70).—Right Upper First Molar, Occluding Surface, with five cusps. a, Mesio-buccal cusp; 6, disto-buccal cusp ; c, mesio-lingual cusp; d, disto-lingual cusp; e, fifth cusp ; /, mesial marginal ridge; g, distal marginal ridge ; h, mesial groove ; i, buccal groove; j, distal groove : k, k, disto-lingual groove ; I, k, mesio-lingual groove. The lingual cusps are faceted by wear; m, mesio-buccal triangular groove; n, disto- buccal triangular ridge. Fig. 56* (Par. 78).—Right Upper First Molar, Buccal Surface, a, Mesio-buccal cusp; 6, disto-buccal cusp; c, mesio-lingual cusp ; d, disto-lingual cusp; e, buccal ridge ; /, gingival line; g, mesial angle ; h, distal angle; <', buccal groove ; k, mesial root; I, dis- tal root; m, lingual root. Fig. 57* (Par. 79).—Right Upper First Molar, Lingual Surface, o, Mesio-buccal cusp ; b, disto-buccal cusp ; c, mesio-lingual cusp ; disto-lingual cusp; e, fifth cusp; /.gingival line ; g, disto-lingual groove; h, mesio-lingual groove: ?', lingual groove; k, mesial root; I, distal root; m, lingual root. Fig. 58* (Par. 80).—Right Upper First Molar, Mesial Surface, a, Mesio-buccal cusp; 6, disto-buccal cusp; c, mesio-lingual cusp ; d, disto-lingual cusp: c. fifth cusp; /,/, gingival line: h, mesio-lingual groove; k. mesial root ; ?, distal root; m, lingual root. ♦Illustration, 1% diameters. * UPPER FIRST MOLAR. 57 (i), which runs in a deep sulcus to the buccal margin, and over it onto the buccal surface; and the distal (,/.), which runs distally, and lingually, over the oblique ridge and ends in the distal fossa (k). The remaining groove—the disto-lingual (k, k)—begins a little to the buccal of the cen- tral part of the distal margin, and runs diagonally in a straight line, or in a curve with the concavity toward the disto-lingual angle, to the lingual margin, and over it onto the lingual surface, to become the lingual groove. Except that part crossing the distal marginal ridge, this groove is usually deeply sulcate. 69. In their origin, each lobe begins its calcification as a separate piece, plate, or cusp, and moves apart from the others as growth proceeds, till the proper dimensions of the occluding surface are attained ; then the plates coalesce on the lines marked by the grooves named above, and thus complete the occluding surface. When the union has been completed, nothing but fine lines remain, which, on any plain parts, are often quickly obliterated by wear. But on parts meeting at an angle, forming a sulcus, as in the buccal and disto-lingual grooves, there is usually a sharp groove, and at any point where the union has been imperfect, there is a fis- sure. Fissures are most frequent at the ends of the grooves, near the central pit, or where they are deeply sulcate, and in the central portion of the disto-lingual groove; but they may occur in any part of these lines. 70. In many examples of the upper first molars, but in no others, there is a small fifth lobe or cusp (Fig. 55, e). This is situated on the lingual side of the mesio-lingual lobe, from which it is divided by a fifth groove, the mesio-lin- gual (I, A-), which runs from the lingual portion of the mesial margin diagonally to the lingual margin, and joins the lin- gual groove. This cusp, when it occurs, is always bilateral, /. e., on both the right and left upper first molars. It is hereditary, appearing regularly in the teeth of children when 58 UPPER FIRST MOLAR. present in the teeth of both parents. It occurs also, in a modified form, when present in but one parent. Therefore, the cusp will be found in all possible varieties of develop- ment, from its largest size, as represented in Fig. 55, to the merest line marking its position on the lingual side of the mesio-lingual cusp. 71. The occluding surface of the upper first molar has four marginal ridges, broken by the grooves described (68), so as to form four principal eminences or cusps. These are the buccal, lingual, mesial, and distal-marginal ridges. Of these, the mesial and distal-marginal ridges are close upon the mar- gins of the surface. But the buccal and lingual are carried toward the central line of the tooth by the inclination of the buccal and lingual surfaces. In the upper molars the central inclination of the lingual surface is much greater than that of the buccal; while in the lower molars the cen- tral inclination of the buccal surface is the greater. The buccal-marginal ridge begins at the mesio-buccal angle, in the form of a blunt cutting edge, and rises in a curved line to the summit of the mesio-buccal cusp (Fig. 54, a), from which it descends distally to the buccal groove (i). From the summit of this cusp the mesio-buccal triangular ridge (p) descends to the mesial side of the central pit (q). This ridge is divided from the mesial-marginal ridge by the mesio-buccal supplemental groove (ra), which is sometimes deep, but gen- erally shallow, and occasionally absent. From the buccal groove the marginal ridge rises rapidly to the summit of the disto-buccal cusp (6), then descends in a curve to the disto- buccal angle, to join the distal-marginal ridge (g). From the point of this cusp the disto-buccal triangular ridge (or tri- angular ridge of the disto-buccal cusp, n), runs down to the distal side of the central pit, where it joins a ridge from the mesio-lingual cusp to form the oblique ridge, or is divided from this ridge by a sulcate distal groove. 72. The lingual-marginal ridge begins at the mesio- UPPER FIRST MOLAR. 59 lingual angle as a rounded edge, and rises in a curve to the summit of the mesio-lingual cusp (c), and descends, continu- ing its curve, into a ridge, meeting the triangular ridge of the disto-buccal cusp to form the oblique ridge. However, in a less pronounced form, this marginal ridge descends from the summit of the mesio-lingual cusp to the distal, where it is deeply broken by the disto-lingual groove. On the distal side of this groove it rises abruptly to the summit of the disto-lingual cusp (d), from which it slopes away in a curve to join the distal-marginal ridge. 73. The mesial-marginal ridge (/) is a strong band of enamel running from the mesio-buccal to the mesio-lingual angle of the tooth. It forms the mesial boundary of the occluding surface, and the angle of junction of the mesial and occluding surfaces, or the mesio-occluding angle. It is low in the center of its length, and rises toward both angles. It is crossed near its center by the mesial groove, usually as a fine line, which is often obliterated by wear, early in life. Occasionally one or more small tubercles appear in the cen- tral portion of this ridge, inclosed in what seems to be a division of the mesial groove. 74. The distal-marginal ridge (g) is a band of enamel forming the distal boundary of the occluding surface, from angle to angle, and forms the angle of junction between the occluding and the distal surfaces, or the disto-occluding angle. It is low in the center of its course, and is crossed by the distal end of the disto-lingual groove as a fine line, usually a little to the buccal of the central point. 75. The central fossa is irregularly circular, and is formed by the central inclines of the mesial marginal ridge, mesio-buccal cusp, disto-buccal cusp, mesio-lingual cusp, and oblique ridge. It is made irregular, and sometimes angular, by the depth of the several sulcate grooves, or by the prom- inence of the triangular ridges, especially that from the mesio-buccal cusp. The central incline of the mesio-lingual 60 UPPER FIRST MOLAR. cusp is generally a plane surface, but is sometimes slightly concave, and in about a fourth of its examples there is a low, triangular ridge running from its apex directly to the cen- tral pit. The oblique ridge is occasionally cut through by a deep and wide distal sulcus, thus connecting the central with the distal fossa. In a few examples two supplemental grooves or wrinkles arise from the mesial groove at about the center of its length, and run, one toward the mesio-buccal, and the other toward the mesio-lingual angle of the tooth. These are often of such depth and width as to form a small sup- plemental mesial fossa. Of these grooves, the mesio-lingual is generally absent, and the mesio-buccal (ra) is generally present, though often very shallow. 76. The distal fossa is formed by the distal incline of the disto-buccal and mesio-lingual cusps and oblique ridge, which meet the central incline of the disto-lingual cusp and distal-marginal ridge. It is traversed by the disto-lingual groove, which is generally sulcate, and deepens into a pit at the point where it receives the distal groove. From the buccal portion a supplemental groove, the disto-buccal, runs well up toward the summit of the disto-buccal cusp, divid- ing its triangular ridge from the distal-marginal ridge. 77. In some molars many supplemental grooves or wrinkles radiate from the centers of these fossae on the in- clines of the ridges and cusps, and in some poorly developed teeth these are deeply fissured. The growth of enamel seems to have reached out toward the line of junction in the form of spiculae, and to have failed to fill out the space. 78. The buccal surface of the upper first molar (Fig. 56) is irregularly convex. Its length is about equal to the mesio-distal breadth at the gingival line, while the width at the widest point, near the occluding margin, is about three- tenths greater. Therefore, the mesial and distal margins converge toward the neck. The mesial margin is almost straight, after the rounding of its angle, but the distal is Fig. 59. Fig. 60. Fig. 61. Fig. 62. Fig. 59 * (Par. 81.)-Right Upper First Molar, Distal Surface, a, Mesio-buccal cusp; b, disto-buccal cusp; d, disto-lingual cusp; /, /, gingival line; *, mesial root- I, distal root; in, lingual root; o, concavity of the distal surface. Fig. 60 * (Par. 82).—Left Upper First Molar, Buccal Surface. A bell-crowned tooth. Fig. 61 * (Par. 85).—Upper Front Molar showing the mesial and distal roots united at their apexes. Fig. 62 * (Par. 86).—The Right Upper Molars showing the progressive diminution of the disto-lingual cusp from the first to the third molar. * Illustration, \% diameters. UPPER FIRST MOLAR. 63 convex. The occluding margin is surmounted by the buccal cusps (a, b), between which there is a deep notch, through which the buccal groove passes from the occluding to the buccal surface. This groove passes centrally toward the gingival line about half the length of this surface, dividing the occluding portion into a mesial and distal buccal ridge. In some examples the groove continues across the gingival line to the bifurcation of the roots. There is a bucco-gingi- val ridge of enamel (e) near the gingival line which the buccal groove generally does not cross. The mesio-distal convexity is greatest on this ridge, and diminishes toward the occluding margin. The summit of this convexity is on the mesial half, in a line with the mesial root, and the sur- face falls away rapidly toward the distal. 79. The lingual surface (Fig. 57) is divided in a line with the long axis of the tooth into a mesial and distal lobe by the lingual groove (i). Both lobes are smoothhT convex from mesial to distal, and in a less degree from the gingival line (/) to the occluding margin. The gingival line is nearly horizontal, and so sunken as to give the appearance of a gin- gival enamel riclge. The occluding margin is surmounted by the mesio- and disto-lingual cusps (c, d), of which the mesial is usually the larger. In the five-cusped molar the fifth cusp (e) is seen on the mesio-lingual lobe. The mesial and distal margins are convex, converging rapidly toward the lingual root. In the few examples in which the lingual groove is not apparent, the lingual surface is smoothly con- vex from mesial to distal. SO. The mesial surface (Fig. 58) is nearly flat in all directions, and its margins are rounded to the buccal and lingual surfaces. Toward the mesio-buccal angle, the flat surface is continued well up to the occluding surface, which it meets in a fairly sharp angle; but as it approaches the lingual portion it is progressively rounded toward the oc- cluding surface. In many molars, near the gingival line, half way from buccal to lingual, this surface is a little concave. 64 UPPER FIRST MOLAR. 81. The distal surface (Fig. 59), in its lingual half, is convex in all directions, but in its buccal half there is often a concavity formed by a considerable distal protrusion of the disto-lingual lobe. This is a shallow, but marked, depression (o), running from the bifurcation of the distal and the lingual roots toward the disto-buccal cusp. It crosses the gingival line at a point about one-third distant from the buccal to- ward the lingual margin. In many examples this depression is more central, and receives the distal termination of the disto-lingual groove, which may often be traced as a fine line nearly or quite to the gingival border of the enamel. This complication of the surface makes the finishing of fillings, and the fitting of bands for crowns, specially difficult. This depression falls short of forming a concavity in about one- fourth of the first molars. 82. The root of the upper first.molar (Figs. 56 to 61) is divided into three prongs or roots*—the mesial, or mesio- buccal (k), the distal or disto-buccal (£), and the lingual (m). These are usually quite widely separated, giving the tooth a firm seat in its alveolus. The lingual root is the largest; it diverges boldly to the lingual, and is straight, or slightly curved with the convexity to the lingual. It is conical, and tapers regularly to an obtuse rounded apex. In most molars it has a groove, a continuation of the lingual, on its lingual side, running nearly, or quite, its whole length. In rare examples this root bifurcates. 83. The mesial root (k) is larger than the distal; broad from buccal to lingual, and thin from mesial to distal, with the lingual portion the thinner, and grooved on its flattened sides. It diverges first to the mesial and buccal, and then *The divisions of the root of a tooth are usually called "the roots," and the upper molars are said to have three roots. The neck of the tooth is, however, common to all, and, more strictly, there is one root divided into several prongs, or fangs. Some authors follow this form of expression. For instance, Prof. Judd says: " The root of a molar is divided into three fangs." I see no good reason, however, for not calling them roots, the term generally in use by the dental profession. FiS- 63- Fig. 64. Fig. 65. Fig. 66. Fig. 67. Fig. 68. Fig. 63 * (Par. 86).—Right Upper Second Molar, Occluding Surface, with three of the cusps faceted by wear. Tooth large and especially well developed, a, Mesio-buccal cusp; 6, disto-buccal cusp; c, mesio-lingual cusp; d, disto-lingual cusp;/, mesial marginal ridge ; g, distal marginal ridge; h, mesial groove; i, buccal groove ; j, distal groove ; it, disto-lingual groove; I, distal fossa ; to, mesio-buccal triangular groove ; n, central pit; o, mesio-buccal triangular ridge ; p, disto-buccal triangular ridge ; r, mesio-lingual tri- angular ridge. Fig. 64 * (Par. 86).—Right Upper Second Molar, Occluding Surface. Tooth of medium size and imperfect form, a, Mesio-buccal cusp; b, disto-buccal cusp; c, mesio- lingual cusp ; d, disto-lingual cusp ; /, mesial marginal ridge; g, distal marginal ridge ; h, mesial groove; i, buccal groove : I, distal fossa. The disto-lingual groove is imperfect; m, mesio-buccal triangular groove; o, mesio-buccal triangular ridge; p, disto-buccal tri- angular ridge, which unites with the ridge from the mesio-lingual cusp to form the oblique ridge. Fig. 65 * (Par. 87).—Right Upper Second Molar, showing a form of deformity peculiar to this tooth that is not very common. Fig. 66 * (Par. 89).—Right Upper Second Molar with the roots inclined to the distal. Fig. 67 * (Par. 90).—Right Upper Second Molar, Distal Surface, with the three roots compressed into one. Fig. 68 * (Par. 90).—Right Upper Second Molar, showing the distal and lingual roots united. * Illustration, 1% diameters. UPPER SECOND MOLAR. 67 curves to the distal. It tapers mostly at the expense of the lingual thin edge, and ends in a flattened or rounded apex. 84. The distal root (I) is the smallest of the three. It diverges to the distal and buccal, and is straight, or slightly curved, sometimes to the distal, sometimes to the mesial, so that its apex approaches the mesial root. It is flattened on its mesial and distal sides, but less than the mesial root, and tapers to a more rounded form, ending in a slender point. 85. The roots of the first molar are, perhaps, more regular in form than those of any other of the molar teeth ; but even in these, considerable deviation from the forms de- scribed will occasionally be observed (Figs. 60 and 61). UPPER SECOND MOLAR. 86. The most constant difference between the occlud- ing surfaces of the upper first and second molars is that the latter tooth is smaller, and has an almost constant tendency to a relatively smaller size of the disto-lingual lobe. This tendency is well seen in Fig. 62, in which the upper molars of a very well formed denture are drawn from the cast. This shows the disto-lingual lobe progressively diminished, so that in the third molar it is very small. When a large number of second molars are examined, it is found that the central fossre are not so regularly formed as in the first, being often small, even in large, well-developed teeth (Fig. 63), and the cusps comparatively low, with a relatively greater inclination of the lingual surface toward the summit of the mesio-lingual cusp. In many examples the distal fossa is reduced to a mere pit (Fig. 64), and the lingual marginal ridge is continued from the summit of the mesio-lingual cusp distally to the disto-lingual angle, where it joins the distal marginal ridge, as in Fig. 64, making practically a three-cusped tooth. In many of these, after the tooth is a little worn, the disto-lingual groove cannot be traced across the lingual marginal ridge, nor on the lingual surface; but a fine line is generally seen crossing the distal marginal ridge. F 68 UPPER SECOND MOLAR. 87. The upper second molar is liable to a peculiar deformity, which I have not seen in any other tooth. The crown is greatly flattened from mesial to distal, occasionally to such an extent as to present none of its usual lines. In the extreme cases (Fig. 65) there is one central, long-shaped sulcus running from buccal to lingual, formed by the central inclines of a ridge encircling the occluding surface. This ridge may be broken at intervals by shallow grooves crossing it. Usually, much of the central inclination of this ridge is wrinkled, and often there are many small fissures. The mesial surface is commonly deeply concave, and the distal convex. 88. In well-developed second molars, the form of the buccal, lingual, mesial, and distal surfaces is much the same as in the first molar, though rather more convex, and concavities in the mesial and distal surfaces are not so frequent. However, in teeth with large disto-lingual cusps, the concavity of the distal surface is about the same as in the first molar (81). The lingual groove is generally not sulcate, though it is usually seen as a fine line in unworn teeth. It is often near the disto-lingual angle, and, when sulcate, it generally disappears by becoming shallower about half-way toward the gingival line; rarely, it runs across the gingival line, and in that case the lingual root is grooved, as in the first molars. 89. The neck of the tooth is less regular in its outline than in the first molar. In the average, it is more flattened from mesial to distal, and on lines that converge more to the lingual. On the buccal surface, the mesio-buccal ridge is relatively more prominent, and at the neck the surface slopes away more toward the distal, so that, in many examples, the distal root seems compressed between the mesial and lingual roots. 90. The roots of this tooth are the same in number and general form as in the first molar; but they spread less, and F'g- 69- Fig. 70. Fig. 71, Fig. 72. Fig. 73. Fig. 69* (Par. 91).—Right Upper Third Molar, Occluding Surface, of typical form. a, Mesio-buccal cusp ; b, disto-buccal cusp; c, mesio-lingual cusp; d, disto-lingual cusp; e, mesial marginal ridge : /, distal marginal ridge; g, mesio-buccal triangular ridge ; h, disto-buccal triangular ridge; i, mesial groove; k, buccal groove; I, distal groove; m, disto-lingual groove ; n, central pit; o, mesio-buccal triangular groove. Fig. 70 * (Par. 91).—Right Upper Third Molar, Occluding Surface, three cusped. a, Mesio-buccal cusp : b, disto-buccal cusp ; c, lingual cusp ; e, mesial marginal ridge ; h, distal marginal ridge; t, mesial groove; k, buccal groove ; I, distal groove; o, mesio- buccal triangular groove. Fig. 71 * (Par. 91).—Right Upper Third Molar, < k gliding Surface. Young, un wom tooth of imperfect fonn. a, Mesio-buccal cusp; b, disto-buccal cusp; c, lingual cusp, e, mesial marginal ridge; /, distal marginal ridge; g, mesial groove ; h, buccal groove; i, distal groove. Several supplemental grooves are seen radiating from the central pit. Fig. 72 * (Par. 94).—Right Upper Third Molar. Buccal Surface. The roots are not quite complete, and show the funnel-shaped openings at the apexes. Fig. 73 * (Par. 94).—Left Upper Third Molar with five roots. * Illustration, 1}% diameters. UPPER THIRD MOLAR. 71 are curved more to the distal (Fig. 66). There is also much more variety of the comparative size of the root and crown than in the first molar. In many the root is single, with the outlines of the division marked by grooves of variable depth (Fig. 67); or two of the roots may be connected, while the third is free. Sometimes this union is the lingual with the distal (Fig. 68); but oftener it is the lingual with the mesial root. UPPER THIRD MOLAR. 91. The upper third molar deviates from the typical form of the first more than does the second. Yet, in dent- ures of the best form, it presents the same developmental lines, fossae, and cusps (Fig. 69). The disto-lingual lobe is very much smaller, and in many examples is entirely want- ing. Of the examples in my possession, about twenty-one per cent, show a diminutive disto-lingual cusp; thirty-two per cent, show a pit in the position of the distal fossa, and some portion of the disto-lingual groove, but no depressed groove over the lingual-marginal ridge; it is similar to that shown in Fig. 64. The remainder, or about forty-seven per cent., have no disto-lingual lobe. Of the latter, three-fourths are properly three-cusped teeth in which the oblique ridge becomes the distal marginal ridge, and the distal groove runs over to the distal surface (Fig. 70). In the remainder, there is only an irregular ridge (Fig. 71), forming a central fossa, so marked with wrinkles or supplemental grooves that the developmental lines are not satisfactorily made out. Many examples of the three-cusped teeth show much relative dimi- nution of the disto-buccal lobe. 92. The upper third molar is the smallest of the mo- lars,* and is more irregular in its size and conformation than the first or the second molar. On account of the small size of the disto-lingual lobe, the distal portion of the tooth is much smaller than the mesial, and in the three-cusped * Forty upper first molars balanced sixty upper third molars. 72 THE LOWER MOLARS—LOWER FIRST MOLAR. teeth the crown becomes triangular, with its angles well rounded. (Fig. 70.) 93. The mesial surface resembles that in the upper first and second molars, but is more rounded, so that a concave portion is less frequent, and the distal surface is well rounded. The lingual and buccal surfaces are more rounded than the same surfaces in the other upper molars. The gingival line is usually horizontal in its course around the neck of the tooth, except that, in a few examples, there is a slight curva- ture on the mesial surface. Many upper third molars are much flattened from mesial to distal. These generally have a considerable concavity on the mesial surface. Occasionally teeth are found that are also abnormally small and have the appearance of supernumeraries. More rarely this tooth fails to develop, and is wanting entirely. 94. The root of the upper third molar has, in the more regular forms, the three divisions common to the upper mo- lars, though relatively smaller and not so widely separated as those of the first and second molars ; and often end in slender conical points ; many have but a single root; but in most of these the three roots are outlined by grooves of more or less depth. A considerable number have more than three roots, which are irregular in size or form (Fig. 73). Four, five, six, and even seven or eight divisions are sometimes met. The neck is then generally broad from buccal to lingual, and the occluding surface of the crown irregularly formed. Many of the three cusped teeth have the root with- out divisions. THE LOWER MOLARS. 95. The lower molars differ so much from each other, especially the first and second, that each must be separately described. THE LOWER FIRST MOLAR. 96. The lower first molar is the sixth tooth from the median line in the lower jaw. It proximates the lower second Fig. 75. Fig. 76. Fig. 77. Fig. 74 * (Par. S6).—Left Lower First Molar, occluding Surface, of typical form. a. Mesio-buccal cusp; 6, disto-buccal cusp; c, mesio-lingual cusp; d, disto-lingual cusp; e, distal cusp; /, mesial marginal ridge ; g, distal marginal ridge; h, mesio-buccal tri- angular ridge; i, disto-buccal triangular ridge; k, disto-lingual triangular ridge; I, mesio-lingual triangular ridge ; m, distal triangular ridge; n, mesial groove ; o, buccal groove ; p, disto-buccal groove; r, distal groove ; s, lingual groove. Fig. 75* (Par. 96).—Right Lower First Molar, Occluding Surface, a, Mesio-buccal cusp; b, disto-buccal cusp; c, mesial lingual cusp ; d, disto-lingual cusp; e, distal cusp ; /, mesial marginal ridge; g, distal marginal ridge; h, mesial triangular ridge; i, disto- buccal triangular ridge; k, mesio-lingual triangular ridge ; I, disto-lingual triangular ridge; n, mesial groove; o, buccal groove; p, disto-buccal groove ; r, distal groove ; .«, lingual groove ; t, mesio-buccal triangular groove. Fig. 76* (Par. 104).—Left Lower First Molar, Buccal Surface, a, Mesio-buccal cusp; 6, disto-buccal cusp; c, distal cusp; d, bucco-gingival ridge; e, buccal pit; /, gingival line ; g, mesial root; h, distal root; i, buccal groove; k, disto-buccal groove. Fig. 77* (Par. 104).—Left Lower First Molar, Buccal Surface, with prominent cusps. References the same as for Fig. 76. * Illustration, 1% diameters. LOWER FIRST MOLAR. iO bicuspid with its mesial surface, and the lower second molar with its distal. Next to the upper first molar, it is the larg- est tooth in the denture.* The outline of the occluding sur- face (Figs. 74 and 75), when seen in a line with the long axis of the tooth, is trapezoidal, with the buccal line the longest. The buccal angles are about equally acute, while the lingual angles are equally obtuse, and all are more or less rounded. The buccal margin is convex, but made irregular by two buccal grooves. The lingual margin is nearly straight, but sometimes slightly concave, or notched in the center of its length, by the lingual groove ; but more generally it is slightly convex. The mesial and the distal margins are nearly straight in the best formed teeth ; though the distal is some- times considerably convex, as the fifth, or disto-lingual cusp is more or less prominent. All of these margins vary as to their convexity; the rule being that, in teeth of large size and symmetrical development, they approach nearer to straight lines. 97. Each of these marginal portions is surmounted by the mesial, buccal, lingual, and distal marginal ridges, which form a continuous elevation of irregular height around the margins of the occluding surface, and on which there are five cusps. The central inclinations of these ridges form the central fossa. On the mesial, lingual, and distal, the summits of these ridges are close on the margins of the surface; but on the buccal, there is an inward inclina- tion of the buccal surface that carries the summit of the ridge considerably toward the central axis of the tooth. 98. The occluding surface has five developmental grooves (Figs. 74 and 75)—the mesial, buccal, disto-buccal, lingual, and distal—which divide it into five developmental parts, or lobes. These are the mesio-buccal (a), disto-buccal (6), mesio-lingual (c), disto-lingual (d), and distal (e) lobes 5 each bearing a cusp of the same name. The mesial groove * Forty-two upper first molars balanced forty-six lower first molars. 76 LOWER FIRST MOLAR. (n) runs from the central fossa over the mesial marginal ridge to the mesial surface. On the mesial marginal ridge it is usually a fine line which is soon obliterated by wear. Occasionally, this is divided into two branches, with a small tubercle on the mesial marginal ridge between them (Fig. 74,/). In many examples there is a supplemental groove which rises from the mesial groove at about the center of its length, and runs toward the mesio-buccal angle. This is the mesio-buccal triangular groove (Fig. 75, t). It divides the mesial marginal ridge from the triangular ridge of the mesio buccal cusp. More rarely there is also a similar groove running toward the mesio-lingual cusp. When these are deep, they form a mesial supplemental fossa (Fig. 75, t). The buccal groove (o) runs in a deep sulcus from the central pit to, and over, the buccal marginal ridge to the buccal surface, and divides the mesio-buccal from the disto-buccal cusp. The disto-buccal groove (p) also runs from the central pit over the buccal ridge, more or less near the distal angle, as the distal cusp is large or small. It divides the disto-buccal lobe from the distal. The lingual groove (s) runs from the central pit in a deep sulcus to, and over, the lingual marginal ridge onto the lingual surface, and divides the two lingual lobes. The distal groove (r) runs distally over the distal marginal ridge, and divides the disto-lingual lobe from the distal. Frequently this groove can be traced some distance toward the gingival line on the distal surface. The mesial and distal grooves form a line traversing the whole extent of the occluding surface, from mesial to distal, in the center of which a V-shaped deflection is formed with its point to the lingual, the base receiving the point of the triangular ridge (i) of the disto-buccal cusp. 99. In most examples, the central fossa occupies all the occluding surface within the circle of the summit of the marginal ridges, though, occasionally, one or more supple- mental fossae are divided from it by high triangular, ridges LOWER FIRST MOLAR. 77 running down from the cusps (Fig. 75, h, k). The surface of the fossa is made irregular in most of these teeth by deep sulci on the lines of the grooves, separating the cusps and triangular ridges. 100. The occluding surface of the lower first molar has five cusps, one on each of the five lobes, or three on the buccal marginal, and two on the lingual marginal ridge.* These cusps are formed by the grooves previously described (98), which pass over the ridges in depressions of variable depth, thus subdividing the crests of the ridges into obtuse elevations. Usually, these are not so high and prominent as the cusps of the upper molars. The mesio-buccal (Figs. 74 and 75, a) is the largest and strongest of the buccal cusps, and occupies rather more than one-third of the buccal mar- ginal ridge. From its crest a triangular ridge (A) runs down centrally to the junction of the mesial and buccal grooves, and is divided from a similar triangular ridge from the mesio-lingual cusp, by the mesial groove. When these two ridges are high, they form, in conjunction with triangular grooves between them and the mesial marginal ridge, a mesial supplemental fossa. The disto-buccal cusp (6) is of less extent from mesial to distal, but has a longer triangular ridge, though not so high, which ends in the point of the V-shaped deflection of the mesial and distal grooves, or at their junction. 101. The lingual cusps (c, d) are about equal in size and height (perhaps the mesial is a little the higher on the average). They have strong triangular ridges (&, I) which terminate in the angles formed by the junction of the lingual groove with the mesial and distal grooves in the central pit. 102. The distal cusp (e) occupies the distal portion of the buccal ridge, and forms the disto-buccal angle of the occluding surface. It is the distinguishing mark of the * In some rare cases lower first molars have but four cusps, and then the tooth is like the lower second molar. (Pars. 109,110.) 78 LOWER FIRST MOLAR. lower first molar, being but very rarely absent in that tooth, and never present in the lower second molar. It is the smallest of the five cusps, and varies most in its relative size. In some examples it is almost or quite as large as the disto- buccal cusp. In others, especially in small and poorly de- veloped teeth, it may be reduced to a mere tubercle, occupy- ing the buccal portion of the distal marginal ridge and the immediate disto-buccal angle. The central incline of this cusp is commonly nearly flat, but occasionally has a rounded crest. It ends in a point at the junction of the distal and disto-buccal grooves. 103. In this tooth there is often a deep pit at the junc- tion of the mesial, distal, and lingual grooves. Less fre- quently there is also a pit at the junction of the mesial and buccal grooves, and at the junction of the distal and disto- buccal grooves. The grooves are often fissured for a short distance from the pits, especially in the deeply sulcate lingual groove. In some very poorly developed teeth fissures may be found in any part of the grooves. 104. The buccal surface of the lower first molar (Figs. 76 and 77), when seen at right angles with the long axis of the tooth, is irregularly trapezoidal in form, with the occluding margin about two-sevenths longer than the gingival. The mesial and distal margins converge toward the gingival, and the angles with the occluding surface are about equally acute. The occluding margin is broken into three elevations, or cusps, by the buccal and disto-buccal grooves. The gingival line is straight, or slightly curved, with the concavity toward the occluding surface. The mesial and the distal lines are slightly convex. The buccal surface is con- vex in all directions; but the line of convexity from mesial to distal is broken toward the occluding margin by the buccal and disto-buccal grooves, which pass over from the occluding surface. The buccal groove, (i) is usually a little to the mesial of the central line of the surface, and often ends Fig. 78. Fig. 79. Fig. 81. Fig. 82. Fig. 78 * (Par. 105).—Left Lower First Molar, Lingual Surface, o, Mesio-buccal cusp; 6, disto-buccal cusp ; c, distal cusp; d, mesio-lingual cusp; r, disto-lingual cusp; /,/, gingival line; g, mesial root; h, distal root. The roots arc spread wide apart; /, lingual groove. Fig. 79* (Par. 105).—Left Lower First Molar, Lingual Surface. This tooth has prominent cusps, and the roots are straight and close together. References the same as for Fig. 7S. Fig. 80* (Par. 106).—Left Lower First Molar, Mesial Surface, a, Mesio-buccal cusp; d, mesio-lingual cusp ; e, mesial surface, point of concavity ; /,/, gingival line ; g, mesial root with broad groove. Fig. M* (Par. 107).—Left Lower First Molar, Distal Surface. The cusps are very prominent, o, Mesio-buccal cusp : b, disto-buccal cusp; c, distal cusp ; d, mesio- lingual cusp; e, disto-lingual cusp; /, gingival'line; g, mesial root: h, distal root. Fig. s_> * (Par. 108).—Lower First Molar, with three roots ♦Illustration, 1% diameters. LOWER FIRST MOLAR. 81 in a deep pit (e), about half way from the occluding to the gingival margin. Exceptionally, this groove is continued to the bifurcation of the root. The disto-buccal groove (k) is near the disto-buccal angle, and its course is toward the gingival line, with a distal inclination. It is usually lost to sight by becoming shallower, but in some examples it may be traced to the gingival line. The enamel terminates in a marked inclination toward the gingival line, forming the bucco-gingival ridge. 105. The lingual surface of this tooth (Figs. 78 and 79) is slightly convex in all directions. It forms a fairly sharp angle with the occluding surface, but is rounded away toward the mesial and distal surfaces. On account of the lingual convergence of the mesial and distal surfaces, the lingual surface is much shorter from mesial to distal than the buccal. The occluding margin is deeply notched by the passage of the lingual groove (z), which usually termi- nates near the center of the surface by becoming shallower. 106. The mesial surface (Fig. 80) is very irregular in outline, and often the occluding margin is deeply concave. The gingival curvature is generally marked, and the buccal and lingual lines convex. The buccal margin of this surface is more convex than the lingual, and its curvature is at such an incline as to render the gingival line much longer than the occluding margin. The surface is slightly convex, though almost flat; but, in the central portion, near the gingival line, it is sometimes slightly concave from buccal to lingual. It is rounded away toward the buccal and lingual angles. With the occluding surface it forms a sharp angle in the central portion, but this is rounded toward the buccal and lingual angles. In the direction of the long axis of the tooth, there is usually a concavity at the junction of the enamel and cementum. 107. The distal surface (Fig. 81) is smoothly convex from buccal to lingual. From the occluding surface to the 82 LOWER FIRST MOLAR. gingival line it is straight Or slightly couvex, but forms a considerable concavity at the junction of the crown with the root, which occasionally forms a sharp angle, but generally is well rounded. The occluding margin is often deeply notched by the distal groove. In a few examples the disto- buccal groove is deep after passing over the marginal ridge, and in its distal inclination forms a slight concavity at the disto-buccal angle. 108. The root of the lower first molar is divided into two prongs (Figs. 76 to 82), and this division is usually close to the crown, closer than in any other tooth in the mouth. The mesial root (g) inclines first to the mesial, and then curves regularly toward the distal. It is broad from buccal to lingual, and is much thinned from mesial to distal, so that in cross sections it measures double as much one way as the other. It is usually slightly concave on both mesial and dis- tal surfaces, and tapers regularly, but not rapidly, from the bifurcation to the apex, and ends in a flattened, but well rounded, point. The distal root (h) inclines to the distal at first, and afterward is nearly or quite straight. In some ex- amples the apical half, curves to the distal, but more often toward the mesial, so that the apexes of the two roots are inclined toward each other. It is narrower from buccal to lingual than the mesial root, and more nearly round, being but rarely concave or grooved on either mesial or distal sur- face. It tapers quite regularly, and more nearly to a point than the mesial root, though the apex is generally well rounded. The form of the root is regular, rarely deviating much from its type. Occasionally, however, the division of the root is incomplete. The mesial root is occasionally di- vided, giving the tooth three roots (Fig. 82); and I have seen a few in which the distal root was divided also, giving four roots. Fig. 84. Fig. 85. Fig. 86. Fig. 83* (Par. 110).—Right Lower Second Molar, Occluding Surface, of typical form, a, Mesio-buccal cusp ; b, disto-buccal cusp ; c, mesio-lingual cusp; d, disto-lingual cusp; e, mesial marginal ridge; /, distal marginal ridge ; g, mesio-buccal triangular ridge; h, disto-buccal triangular ridge ; i, mesio-lingual triangular ridge; k, disto-lingual trian- gular ridge; I, mesial groove; m, buccal groove ; n, lingual groove ; o, distal groove. Fig. 84* (Par. 113).—Right Lower Second Molar, Occluding Surface, of imperfect form, o, Mesio-buccal cusp; b, disto-buccal cusp ; c, mesio-lingual cusp; d, disto-lingual cusn; e, mesial marginal ridge ; /, distal marginal ridge ; g, mesio-buccal triangular ridge; h, disto-buccal triangular ridge; i, mesio-lingual triangular ridge; k, disto-lingual trian- gular ridge; I, mesial groove; m, buccal groove; n, lingual groove; o, distal groove. Fig. 85* (Par. 115).—Left Lower Second Molar, Buccal Surface, a, Mesio-buccal cusp; b, disto-buccal cusp; c, buccal groove; d, buccal pit; /, gingival line; g, mesial root; h, distal root. Fig. 86 * (Par. 116).—Left Lower Second Molar, Lingual Surface, a, Mesio-buccal cusp; b, disto-buccal cusp; c, mesio-lingual cusp ; d, disto-lingual cusp; e, lingual groove; /, gingival line ; g, mesial root; h, distal root. Fig. 87* (Par. 117).—Left Lower Second Molar, Mesial Surface, a, Mesio-buccal cusp; c, mesio-lingual cusp; d, point of proximate contact of mesial surface; /, gingival line; g, mesial root. Fig. 88* (Par. 118).—Left Loaver Second Molar, Distal Surface, a, Mesio-buccal cusp; b, disto-buccal cusp; c, mesio-lingual cusp; d, disto-lingual cusp; e, point of proxi- mate contact of the distal surface; /, gingival line; h, distal root. Fig. 89* (Par. 119).—Right Lower Second Molar, Lingual Surface. The roots are curved very much to the distal. * Illustration, 1% diameters. o LOWER SECOND MOLAR. 85 THE LOWER SECOND MOLAR. 109. The most characteristic difference between the lower first and second molars is the absence of the fifth lobe in the second, and the general change of form which this absence implies; the other parts of the tooth being similar. 110. When the occluding surface is seen in a line with the long axis of the tooth (Fig. 83), the outline of the crown is nearly a parallelogram, with angles rounded and the lines slightly convex, the buccal most. The summits of the mar- ginal ridges are close on the mesial, distal, and lingual margins, while the buccal is carried over toward the lingual by the inclination of the buccal surface. The central inclines of the marginal ridges form a deep central fossa, in the cen- ter of which there is usually a deep pit. 111. There are four developmental grooves, all arising from the central pit. The mesial groove (I) runs to the me- sial margin and crosses the mesial marginal ridge as a fine line, which is often obliterated by wear. The distal groove (o) runs in a similar way to the distal surface. These two grooves divide the occluding surface from mesial to distal, and centrally, between the summits of the buccal and lingual marginal ridges. The buccal groove (m) runs from the cen- tral pit to the buccal margin, and over it to the buccal surface, dividing the buccal ridge into two buccal cusps, while the lingual groove ()i) runs to and over the lingual marginal ridge, dividing it also into two lingual cusps. The two divide the tooth from buccal to lingual, into nearly equal parts. The mesial portion is usually slightly the larger. The four grooves form a cross through the occluding sur- face, dividing it into four lobes, or developmental parts, on each of which there is a cusp and a triangular ridge. In some examples the grooves do not exactly meet at the cen- tral pit. The lingual may rise to the mesial of the buccal, or vice versa; or, a similar variation may occur in the central ends of the mesial and distal, causing irregularity of the 86 LOWER SECOND MOLAR. contour of the central fossa. Occasionally, the lobes are of unequal size, or the cusps are unequally developed, giving rise to imperfect forms (Fig. 84). 112. On the average, the cusps of the lower second molar are higher and more pointed, and the triangular ridges are more prominent than in the first molar. The mesio-buc- cal and mesio-lingual cusps are generally a little larger than the disto-buccal and disto-lingual; also, the mesio-buccal and mesio-lingual triangular ridges (Fig. 83, g, i) are usu- ally more prominent. The crests of these ridges do not run directly toward the central pit, but the two mesial ones meet mesially of the central pit, while the two distal meet distally of it. When they are high, they form transverse ridges by their junction, which separate a mesial and a distal supple- mental fossa from the central fossa. When this occurs, there are usually triangular supplemental grooves deflected from the mesial and distal grooves, to the mesial and distal of the triangular ridges, which run towards the angles of the tooth and separate the triangular from the marginal ridges, widen- ing and deepening the supplemental fossae. A deep pit is often found at the point where these triangular grooves arise from the principal grooves. Supplemental fossae occur much more frequently, or are more pronounced, in the mesial than in the distal portion of the tooth, but in many exam- ples the triangular ridges are so widely divided by sulcate, mesial and distal grooves that no supplemental fossse are seen. 113. In some examples the lower second molar presents differences in the comparative size of its lobes, and the grooves may be deflected from their normal course. Occa- sionally, the distal groove is divided, and passes over the dis- tal marginal ridge in two divisions, with a small tubercle between them. In poorly developed teeth there may be many supplemental grooves, or wrinkles, running from the developmental grooves up onto the central inclines of the ridges and cusps. LOWER SECOND MOLAR. 87 114. Fissures occur oftenest near the central ends of the grooves; though they may appear in any part of their length ; and in poorly developed teeth the supplemental grooves may be deeply fissured. 115. The buccal surface of the lower second molar (Fig. 85) is convex in all directions, except that it is partially divided into two sections, or ridges, by the buccal groove (4. The upper first deciduous molar has but three cusps, a, Mesial groove; b, distal groove; c, buccal groove; d, mesio-buccal cusp; e, disto-buccal cusp;/, lingual cusp. * Illustration, V/2 diameters. UPPER FIRST DECIDUOUS MOLAR. 95 is such that it leaves the general proportions unchanged, ex- cept that the roots are proportionally longer. 131. The deciduous teeth are, however, marked with a much greater constriction at their necks. The enamel, in- stead of thinning away to the gingival border as in the per- manent teeth, retains its thickness almost to the gingival line and terminates abruptly, leaving a sudden constriction of the neck of the tooth. This varies in degree, but is com- mon to all of the deciduous teeth, and distinguishes them from the permanent teeth. 132. The buccal and lingual surfaces of the deciduous molars are inclined toward the occluding surface much more than those of the permanent, so that the immediate occlud- ing surface, is narrow in proportion to the greatest bucco- lingual thickness of the crown. This gives the crown as seen in the mouth the appearance of being very long from mesial to distal; though this characteristic is less marked in the upper than in the lower deciduous molars (see Figs. 106, and 107). 133. The enamel of the deciduous teeth is usually whiter than that of the permanent teeth, and they are proba- bly of coarser texture. The difference in color is often strongly contrasted when some of the permanent teeth, as the central incisors, have taken their places by the side of the remaining temporary teeth. UPPER FIRST DECIDUOUS MOLAR. 134. The occluding surface of the upper first decidu- ous molar (Fig. 107) when seen in the line of the long axis of the tooth, presents an irregular quadrangular form in which the buccal line is the longest. The mesio-buccal angle is acute, the mesio-lingual is obtuse, and both distal angles are nearly right angles. The buccal margin is irregularly convex, and the lingual margin regularly rounded. Both the buccal and the lingual surfaces are much inclined cen- trally ; or toward the occluding surface. 96 UPPER FIRST DECIDUOUS MOLAR. 135. This tooth has three lobes, divided by three grooves. The mesial (a) and distal (b) grooves, run from the mesial to the distal margin in a deep sulcus and divide the lingual from the buccal lobes. Their junction is in a pit in the central fossa. The buccal groove (c) rises from the same pit, at the junction of the mesial and distal grooves, and runs over the buccal marginal ridge to the buccal surface. This groove is generally without a sulcus; or, at most, there is but a slight furrow. 136. The buccal marginal ridge is a high cutting edge which rounds up from the mesio-buccal angle and runs to the distal and buccal till it reaches the point of the mesio- buccal cusp {d). Its direction is then to the distal, descend- ing slightly to the buccal groove, then horizontally to the disto-buccal angle of the tooth to join the distal marginal ridge, the latter portion forming a small disto-buccal cusp (e). In unworn teeth, the buccal groove causes a marked but slight depression where it crosses the ridge, breaking it into two cusps; the mesial being the larger and more pointed. This division is generally defaced very early by wear, so that the ridge presents an almost straight rounded edge. 137. The lingual cusp (/) is in the form of an elevated crescentic edge with its convexity to the lingual, which runs from the mesial termination of the mesial groove (a) to the distal termination of the distal groove (b). The central and the lingual inclines of this cusp are nearly equal slopes, while the buccal incline toward the buccal marginal ridge is less abrupt than the central. The mesial and distal marginal ridges are not marked by more than a very slight thickening of the enamel, and are cut through by the mesial and distal grooves. 138. The buccal surface (Fig. 102) is remarkable for its bucco-gingival ridge (a), which stands boldly out from the gingival line from one to three millimeters and extends from the mesio-buccal to the disto-buccal angle. At the mesio- LOWER FIRST DECIDUOUS MOLAR. 97 buccal angle it terminates abruptly in a marked prominence, and diminishes gradually as it passes from the mesial to the disto-buccal angle. 139. From the summit of the bucco-gingival ridge to the summit of the buccal marginal ridge, or the mesio-buccal cusp, is nearly a flat surface, except a slight depression along the buccal groove. In many examples there is a slight con- cavity extending from mesial to distal along the occlusive margin of the bucco-gingival ridge, and from the point of the mesio-buccal cusp a strong ridge runs to the mesio-buc- cal prominence of the gingival ridge. 140. The mesial and distal surfaces are quite smoothly flattened. The lingual surface is convex. The neck pre- sents the characteristic constriction common to deciduous teeth. LOWER FIRST DECIDUOUS MOLAR. 141. The occluding surface of the lower first deciduous molar (Fig. 106), when viewed in a line with the long axis of the tooth, presents the outline of a parallelogram, modified by the rounding of its angles and more or less convexity of its lines. In many, the distal portion of the tooth is broader than the mesial, giving the tooth an ovoid outline. There are two fossae. The principal fossa occupies nearly three- fourths of the distal portion of the surface, while the small mesial fossa occupies the immediate mesial portion. 142. The tooth has four lobes of irregular form, divided by four grooves. These grooves all run from the principal fossa. The mesial groove (a, a) rises from the central pit and runs to the mesial, passing over the transverse ridge into the mesial fossa, where it is deflected sharply to the lingual, passing over the marginal ridge near the mesio-lingual angle. This groove varies considerably in its course in different ex- amples. In the principal fossa it usually inclines to the buccal and then toward the lingual, but there is generally an ano-le at the origin of the buccal groove. The buccal groove 98 LOWER FIRST DECIDUOUS MOLAR. (b) rises from the mesial groove, some distance to the mesial of the pit, and runs over the buccal ridge onto the buccal surface, in a slight sulcus, dividing the buccal marginal ridge into two cusps, the mesio- and disto-buccal. Its position de- termines the relative size of the buccal lobes. The lingual groove (c) runs from the central pit over the lingual mar- ginal ridge onto the lingual surface, and is deeply sulcate on the central incline of the ridge. The distal groove (d) arises from the central pit and runs over the distal marginal ridge nearly centrally, to the distal surface. It is often deflected to the buccal in the first part of its course by the point of the disto-lingual triangular ridge. 143. This tooth has four cusps corresponding with the four lobes. The mesio-buccal lobe (e) is very irregular in its outline. It forms the entire mesial marginal ridge, and from one-third to three-fourths of the slopes of the mesial fossa. The mesial marginal ridge is usually high in young, unworn, teeth. It is curved, and from the mesio-buccal angle it be- comes the buccal marginal ridge, and rises to the distal to form the point of the mesio-buccal cusp (e). From the point of the cusp it fails away to the distal and buccal to the buccal groove. A prominent triangular ridge descends from this cusp to the lingual and distal, and joins a similar tri- angular ridge from the mesio-lingual cusp (g) to form the transverse ridge, thus dividing the mesial from the principal fossa. Exceptionally, a deep sulcus divides these triangular ridges and connects the fossse. From the buccal groove, the buccal marginal ridge passes almost directly to the disto- buccal angle where it joins the distal marginal ridge. In the centra] portion it rises slightly to form the low disto-buccal cusp (/). The triangular ridge from this cusp is usually low, or wanting. 144. The lingual marginal ridge (g, h) rises abruptly from the mesial groove to the summit of the mesio-lingual cusp (g), and then falls away toward the distal and lingual LOWER FIRST DECIDUOUS MOLAR. 99 to the lingual groove. In most specimens, in unworn teeth, the mesio-lingual cusp is sharp, and its point is carried by the lingual incline far toward the central line of the tooth, so much so as to be in marked contrast with the general form of the lingual cusps of the lower molars. From its apex a triangular ridge descends to join that from the mesio-buccal cusp in forming the transverse ridge. From the lingual groove, the lingual marginal ridge rises to the point of the disto- lingual cusp (A) and then falls away in a curve to form the distal marginal ridge. This cusp is generally rather low, but varies much in these teeth. In some cases, there is a sharp triangular ridge descending into the central fossa, but more generally this ridge is slight. 145. The distal marginal ridge is usually made to ap- pear prominent by the depth of the principal fossa. It is crossed near its center by the distal groove. 146. The principal fossa is generally deep and well rounded. The distal triangular ridges, the only ones descend- ing into this fossa, are generally not prominent, but occasion- ally they are sufficiently so to render the fossa very angular. I have observed many in which the enamel in this fossa was very imperfect and the bottom of the fossa broad and rough. 147. The mesial fossa (i) is usually sharp and deep with smooth sides and has a central pit that is frequently the seat of caries. 148. The buccal surface of the lower first deciduous molar (Fig. 104) is remarkable for its prominent bucco-gin- gival ridge (a), which runs from the mesio-buccal to the disto-buccal angle, and stands out prominently over the junc- tion of the crown with the root. From mesial to distal, this ridge slopes toward the occluding surface, making the crown longer at the mesio-buccal angle than at the disto-buccal. From this ridge the surface slopes rapidly toward the occlud- ing margin, and more rapidly at the mesial than the distal portion. From the mesio-buccal cusp a strong ridge of H 100 PULP CHAMBER. enamel runs to the more prominent portion of the bucco-gin- gival ridge near the mesio-buccal angle. Otherwise this surface is nearly flat. The lingual surface is usually well rounded, but is broken toward the occluding surface by the prominence of the mesio-lingual cusp, and the sulcus of the lingual groove. 149. The mesial and distal surfaces are slightly round- ed. The disto-buccal and disto-lingual angles are about equal in prominence, but the mesio-buccal and mesio-lingual angles are very unequal. The mesial surface slopes rapidly to the lingual, making the lingual surface much shorter than the buccal. The mesio-buccal angle is acute and prominent, while the mesio-lingual is very obtuse and rounded. 150. The root of this molar is divided into two prongs which are spread widely apart. They are thin from mesial to distal, and slightly grooved ; and, from buccal to lingual, broad. They taper regularly to broad flat apexes, which are occasionally bifurcated a short distance. 151. The roots of the deciduous teeth are the same in number and general characteristics as in the teeth of the same denomination in the permanent set, except that those of the molars are more divergent. This spreading of the roots accommodates the crowns of the permanent bicuspids, which are developed between the roots of the deciduous molars. Those of the lower jaw are thin from mesial to distal, broad from buccal to lingual, and grooved along their flattened sides. The mesial and distal roots of the upper deciduous molars are, also, thin, grooved, and widely divergent. The lingual root stands boldly to the lingual, forming a wide space be- tween the three, for the crowns of the upper bicuspids. In many examples the lingual and distal roots are joined by broad thin connections for the greater part of their length. THE PULP CHAMBER. 152. Every tooth has a cavity in the center of the crown, and one or more canals extending through the long PULP CHAMBER. 101 axis of the root, or roots, to the apex. This cavity contains a tissue composed of cellular elements imbedded in a semi- gelatinous matrix, filling every part of the space, and is richly supplied with blood-vessels and nerves. This is known as the pulp of the tooth.* 153. The central cavity in the tooth is usually divided into a crown, or coronal portion, and a root portion. Its parts arc familiary known as the pulp chamber (crown cavity), and root canal, or root canals. The pulp chamber is comparatively large, and the root canals are small, tapering from the pulp chamber to a minute opening at the apex of the root, known as the apical foramen. In those teeth that have prominent cusps, as the bicuspids and molars, there is a prolongation of the pulp toward the point of each cusp. These are known as the horns of the pulp; and the prolonga- tions of the chamber are designated the horns of the pulp chamber. 154. The size of the pulp chamber and of the root canals varies greatly in teeth of different denominations; and also in different teeth of the same denomination. In the early formative stages of the teeth, it is very large, and diminishes in size as growth proceeds, until the tooth is fully formed. Afterward this diminution goes on slowly, until, in old age, it is often nearly obliterated. In the forma- tive stage, i. e., during the growth of the root of the tooth, the root canal is large and funnel-shaped, with the open end of the funnel toward the apex of the root (Fig. 72). As growth proceeds, and the root approaches completion, this diminishes rapidly till the root is fully formed, when it is contracted to a small foramen. This, however, continues to diminish slowly. Therefore, the size of the pulp chamber, the root canals,'and the apical foramen, are greater in youth * As the form of the pulp chamber gives the exact form of the pulp, no separate description of the pulp will bo given. It is not my purpose to give any histo- logical descriptions in this work. 102 PULP CHAMBER. than in old age. However, after adult age is reached, the diminution in size is usually not great. During'this time, the horns of the pulp chamber are shortened by the same process of formation of dentine on their surface that is going on in all parts of the pulp chamber and root canal. There- fore, the horns of the pulp become shorter, or recede, as age advances. Indeed, the whole pulp, very slowly, becomes smaller. 155. Certain processes, when present, also, serve to di- minish the size of the pulp chamber more rapidly ; especially abrasion of the teeth, a matter that seems to depend largely upon the character of the occlusion. When the occlusion is such that there is much rubbing, or sliding motion, of the teeth against each other, wear goes on rapidly. This seems to induce depositions of dentine on the walls of the pulp chamber, which reduces its size ; and, especially, causes the recession of the horns of the pulp. In this way, exposure of the pulp from the wearing away of the dentine is delayed or prevented. In many instances the pulp chamber is almost obliterated in the molars and bicuspids, and recedes root- wise of the gingival line in the incisors and cuspids. Slowly progressive caries or erosion of the teeth often induce similar deposits. 156. In the incisors and cuspids, the pulp chamber and the root canal are not sharply differentiated. The latter tapers, gradually, from the full size of the largest crown por- tion to a small foramen at the apex of the root. In teeth with more than one root, the transition from pulp chamber to root canal is usually sharply defined, the former being very large as compared with the pulpal end of the latter. Indeed, the general form of the pulp is a diminished counter- part of the form of the surface of the tooth, except that it is in every way more slender. 157. Our studies thus far have been of the outer surfaces of the teeth. The pulp chambers are within, and, therefore, an Fig. 10s. Fig. 109. in Fig 110. Fig. 108* (Par. 160).— The Pulp Chamber of the Upper Central Incisor, a, b, Mesio-distal sections of young teeth, showing the three short horns of the pulp; c, mesio- distal section of a tooth from an adult; d, e, labio-lingual sections. Fig. 109* (Par. 160).—Pulp Chamber of the Upper Lateral Incisor, a, b, Mesio- dis'al sections; c, labio-lingual section; d, labio-lingual section of a very' long lateral incisor. Fig. 110* (Par. 163).—Pulp Chamber of the Upper Cuspids, a, b, Mesio-distal sec- lions ; c, d, labio-lingual sections. * Illustrations, actual size. PULP CHAMBERS OF THE UPPER INCISORS. 105 in the study of them, dissections must be made to expose them to view. It is often necessary for the dentist to enter the pulp chambers of the teeth of his patients, and there per- form delicate operations with a precision which demands the most accurate knowledge of these cavities. Therefore this work of exposure, and examination, of the pulp chambers of teeth should be thorough, as a preparation for operations in the mouth. The form of dissection necessary will be given in connection with the teeth as they are individually de- scribed. THE PULP CHAMBERS OF THE UPPER INCISORS. 158. The pulp chambers and root canals of the upper central, and lateral incisors are so similar, the description of one will do for all. Dissection.—1st. Saw the tooth through on the gingi- val line, at the labial surface, at right angles with the long axis. 2d. Saw the tooth from the labial to lingual, along the central line of the long axis from end to end. 3d. Saw the tooth from mesial to distal, along the central line of the long axis from end to end. 159. A very thin saw in a strong frame should be used, otherwise the lengthwise dissections should be made to one side of the central line in the first instance, and after- ward the whole length of the pulp chamber exposed by grinding on a stone; or the lengthwise exposure may be made by catching the tooth in the vise and removing one- half with the file, or by grinding on an emery wheel. After the pulp chamber is exposed so that half of its concavity remains in the half of the tooth, and has been ground smooth and flat, it should be inked on an inked pad (such as is used for the rubber stamps lor printing), and a print made from it. This will give the form of the tooth and pulp chamber in silhouettes similar to the accompanying illus- trations. The printing is facilitated by sticking the tooth to 106 PULP CHAMBERS OF THE UPPER INCISORS. a piece of hard wax for convenience in handling. Ordinary modeling compound, or sealing wax, is convenient and effec- tive.* The printing is usually better done by laying ordinary writing paper on a sheet of semi-soft rubber, about one-eighth inch in thickness. This is specially useful when the ground surface cannot be perfectly flat, as in curved roots. 160. In the upper central and lateral incisors (Figs. 108 and 109), there is no distinct division of the pulp cavity into pulp chamber and root canal; but there is one straight canal, from the interior of the body of the crown to the apex of the root, of which the crown portion is the larger. In young teeth, this has ver}r distinctly the form of the surface of the tooth and root, except that it is much more slender. The largest diameter of the cavity is about level with the gingi- val line on the labial surface. From this point, the pulp chamber, or canal, extends toward the cutting edge of the tooth, about two-thirds the length of the crown, sometimes a little more, often less, and ends in a thin edge, broad from mesial to distal. In young teeth this edge has three short horns (Fig. 108, a, b), or prolongations, extending toward the three small cusps seen on the edge of unworn incisors (21). 161. From the level of the gingival line toward the apex of the root it tapers very gradually and regularly to a narrow canal. Just within the apex of the root, almost at the end, there is usually a sudden contraction of the diameter of the canal, lessening it from one-third to one-half. This is the apical foramen; but this contraction of the canal is not usually present for one or two years, or more, after the tooth has taken its place in the arch (q. v., 154). 162. The canal becomes smaller from youth to old age. In incisors just taking their places in the arch, I have found the diameter of the canal at the gingival line to be from one- fourth to one-third the diameter of the neck of the tooth. In early adult age, the canal may be said to average about a * Dr. Cattell has found that in teaching it is very convenient to have a sufficient number of small pine blocks (%x%xl inch) to which he has pupils cement the teeth with sealing wax. a b c d e f g Fin. 111. HI a b c Fig. 112. Fig. Ill* (Par. 161).—Pulp Chamber of the Lower Central and Lateral Inci.-ors. a, b, c, Labio-lingual sections, showing differences of form of the pulp chamber; d, mesio- distal section; e, /, labio-lingual sections, showing the more usual forms of the pulp chamber ; g, mesio-distal section, showing large pulp chamber. Fig. 112* (Par. 165).—Pulp Chamber of the Lower Cuspids, a, Labio-lingual section, showing a small pulp chamber; b, labio-lingual section showing a very large pulp chamber; c, mesio-distal section. * Illustrations, actual size. UPPER CUSPID—rULP CHAMBER—LOWER INCISORS. 109 fourth the diameter of the neck of the tooth, ranging down as age advances to one-fifth, or sixth, and even to one-tenth. In the lateral incisor, the chamber and canal are a little smaller than in the central, but larger in proportion to the size of the tooth. THE UPPER CUSPID. 163. The pulp chamber and root canal of the upper cuspid (Fig. 110) is about the same in form as that of the central and lateral incisors, except that the coronal extremity has the central horn much extended toward the apex of the cusp of the tooth, and the lateral horns are practically absent. The canal is proportionately somewhat smaller. However, this tooth is often somewhat flattened at the neck, the long diameter being from labial to lingual. In this case the pulp canal at the neck, and from thence toward the apex of the root, is also much flattened in the same direction, but is pro- gressively rounded as the apex is approached. In some ex- amples, the labio-lingual diameter of the canal is double the mesio-distal. As age advances, and the canal becomes smaller, the opening is occasionally reduced to a mere slit. PULP CHAMBER OF THE LOWER INCISORS. 164. (Fig, 111). The coronal portion of the pulp chamber of the lower incisors is much flattened. At the level of the gingival line, the long diameter is from labial to lingual. The chamber extends toward the cutting edge of the tooth, about two-thirds the length of the crown, and in this extension its diameter is progressively diminished from labial to lingual, and extended from mesial to distal, follow- ing the contour of the surface of the tooth, and ends in a thin edge. In young teeth this has three short projections toward the slight cusps seen on the cutting edges of the young, unworn teeth. The root has usually a narrow slit- like opening for the greater portion of its length (d, g), cor- responding with the form of the flattened roots. In many instances, however, the root canal is divided into two por- 110 PULP CHAMBER OF THE LOWER CUSPID. tions, or canals, for a part of its length (e,f). In the adult, these canals are usually very small. The point of separation into two canals is irregular, but is usually slightly root-wise from the level of the gingival line. It may occur at about the level of the gingival line, or the canal may remain single for half the length of the root, and then be divided for a space, the two uniting again before reaching the apex. Gen- erally, there is but one apical foramen. Instances occur in which there are two, each canal remaining distinct to the *end. As age advances, the canals of the lower incisors often become very minute. PULP CHAMBER OF THE LOWER CUSPID. 165. The pulp chamber, and the root canal of the lower cuspid (Fig. 112) are variable in size and form. At the neck of the tooth the chamber is usually irregularly flattened, with the longer diameter from labial to lingual, and the labial portion wider than the lingual. The coronal portion extends about two-thirds of the length of the crown toward the point of the cusp, ending in a point, or horn, which is often very slender. The form of the root portion of the canal depends on the form of the root. It is sometimes nearly round, but more frequently it is sharply flattened for the greater portion of its length, becoming more rounded toward the apex. Occasionally, this canal is divided for a part of the length of the root. Also, the root is sometimes divided, a very small prong appearing on its lingual side. In this there is usually a very small canal that is difficult to enter with a broach. In some lower cuspids the canal is very small (a), in others, very large (b). In a few instances I have seen it more than one-third the diameter of the root in the adult. In this case the diameter of the canal is greater than the thickness of the walls of dentine and cementum by which it is inclosed. This renders the pulp very liable to exposure in excavating carious cavities. Aaoflftft be d „ Fig. 114. 0 a b c a b Fig. 115. Fig. 116. Fig. 113* (Par. 1671.—Pulp Chamber and Root Canals of the Upper First Bicusri is. 6, Bucco-lingual section of single-rooted tooth, with single canal divided near the apex of the root; a. cross-section of the same, mid-length of the root; d, bucco-lingual section of a single-rooted tooth with two canals, which connect at one point; c, cross-section of the same, a little root-wise from the pulp chamber; e, bucco-lingual section of a double- rooted tooth, showing the more usual form of the chambers and canals. Fig. 114* (Par. 168).—Pulp Chamber and Root Canals of the Upper Second Bi- cuspid, a, Bucco-lingual section, showing chamber with a long, slender horn, also two canals which unite in the apical third of the root; b, cross-section of a root with two canals: c. cross-section of a root with a single large canal; d, bucco-lingual section of a single root with two canals ; ej, bucco-lingual sections, showing the more usual form of the pulp chamber and root canal of this tooth. Fig. 115* (Par. 169).—Pulp Chamber and Root Canals of the Lower First Bi- cuspid, a, Bucco-lingual section, showing the more usual fonn ; b, bucco-lingual section, showing a' peculiar and very unusual division of the root canal; c, cross-section in the body of the root. Fig. 116* (Par. 169).—Pulp Chamber and Root Canal of the Lower Second Bi- cuspid." a, b, Bucco-lingual sections, showing the more usual forms of the chamber and canal in this tooth. ______ * Illustrations, actual size. PULP CHAMBER—BICUSPIDS—UPPER FIRST BICUSPID. 113 PULP CHAMBERS OF THE BICUSPIDS. 166. Dissections.—1st. Divide the crown from the root on the gingival line with a fine saw. 2d. Divide the tooth from buccal to lingual through its length with a fine saw; or remove the distal half of the tooth with the file, or stone. These two dissections will usually exhibit the pulp chamber and root canals sufficiently, though in the single-rooted teeth with two canals it is better to divide the root crosswise at the middle of its length, or at several points. PULP CHAMBER OF THE UPPER FIRST BICUSPID. 167. The pulp chamber and root canals of this tooth differ from those of the incisors and cuspids, by having the coronal chamber distinguished sharply from the root canals (Fig. 113, d, e). The chamber is centrally located in the long axis of the crown of the tooth, the lateral walls being about equal in thickness. The center of the pulp chamber is about level with the gingival line, or a little toward the occluding surface. The occluding walls are thicker than the lateral, and vary in thickness from about one-third to two-thirds of the length of the crown of the tooth. The form of the pulp corresponds closely with the form of the tooth. A horn extends from the coronal portion toward the apex of each cusp. The buccal horn rises from the extreme buccal part of the pulp, while the lingual horn rises from the extreme lin- gual portion. Sometimes, especially in long cuspid teeth, they are very long and slender, extending far toward the points of the cusps, and in rare cases almost, or even quite to the enamel. As age advances, they become shorter, and in old age have almost disappeared. In thick-necked teeth with short cusps, the horns of the pulp chamber are short, and the occluding wall is usually very thick. 167. The root canals in upper first bicuspids that have two roots pass from the pulp chamber through the center of 114 PULP CHAMBER—UPPER SECOND BICUSPID—LOWER BICUSPIDS. each root to its apex, and are known as the buccal and lin- gual root canals (e). The buccal canal arises from the ex- treme buccal side of the pulp chamber, and the lingual canal from the extreme lingual side, and their course is almost parallel with the walls of these two portions of the pulp chamber. The floor of the chamber is rounded over in an arch from one canal to the other. Each canal begins in a funnel-shaped opening leading into a narrow round canal, which tapers gradually to the apical foramen. Many of the upper first bicuspids have only one root; but they generally have two root canals almost exactly similar to those with two roots. Occasionally, however, these come together and end in one apical foramen, or there may be a communication between the two canals in some part of their course {d). More rarely, the upper first bicuspid has one broad (from buccal to lingual) flat canal passing through the whole length of its single root. Sometimes this is divided near the apex (b). PULP CHAMBER OF THE UPPER SECOND BICUSPID. 168. The pulp chamber of the upper second bicuspid (Fig. 114) is similar to that of the first (q. v. 167); but the horns of the pulp are usually shorter. In this tooth there is generally but a single root canal (e, f). This is approached by an opening that is broad from buccal to lingual, and tapers gradually toward the apex of the root, ending in a narrow apical foramen. The canal is often quite large, and the demarkation of the pulp chamber, as distinguished from the root canal, very indistinct, or entirely absent. Examples are not infrequent, however, in which there are two root canals in the single root (d). They are then similar to those of the first bicuspid ; but, sometimes, the two canals end in a common apical foramen (a). PULP CHAMBERS OF THE LOWER BICUSPIDS. 169. The pulp chambers of the lower bicuspids (Figs. 115 and 116) seldom show a marked distinction from the root PULP CHAMBERS OF THE UPPER MOLARS. 115 -anals. There is, however, usually a coronal bulbous portion which connects with the pulp canal proper by an extended funnel-shaped constriction (a, b). In the lower first bicuspid, the coronal extremity ends in a horn, which extends toward the point of the buccal cusp. This horn may be short and obtuse, or long and pointed. There is, generally, a well marked protrusion toward the lingual cusp, but no extended horn. It is rather an enlargement of the bulb in that direc- tion. In the lower second bicuspid this protrusion is more considerable, and in some examples it is elongated into a slender point, especially in young teeth (Fig. 116, b). In the three cusped lower second bicuspids (Par. 62, Fig. 49) there are two of these on the lingual side, spreading toward the mesial and distal. They are generally short, but by their protrusion are brought rather nearer the surface of the tooth than other horns of the pulp; and are, therefore, more liable to be opened into when excavating proximate cavities. 170. The root canals of the lower bicuspids are usually large in the first half, tapering to a fine canal in the apical third, of their length. The canal of the lower first bicuspid is usually nearly round, and that of the second is considerably flattened; and in both they are usually straight. Bifurca- tions of these canals are rare, but occur occasionally. In the illustrations (Fig. 115, b) one is shown dividing in such a way that the division would not be likely to be detected by a broach. PULP CHAMBERS OF THE UPPER MOLARS. 171. Dissections.—1st. With a fine saw, separate the crown from the root level with the gingival line. 2d. Cut away the mesial surface of the crown and the mesial surface of the mesial and lingual roots, with the file or a corundum stone, till the canals in each are fully exposed. As the mesial root is generally curved, some care is required to fully expose the full length of the canal without cutting too far in the central portion of its length. If the curved j 116 PULP CHAMBERS OF THE UPPER MOLABS. surface is made smooth, good prints can generally be made by using a piece of semi-soft rubber under the paper, and, while pressing it to the paper, rolling the tooth so as to bring all of the length of the curved surface in contact. 3d. Cut away the buccal surface so as to expose the pulp chamber and canals of the two buccal roots, observing the same precautions as in the second dissection. In the first dissection, both crown and root should be examined. First, clean the portion of the chamber in the crown to study carefully its horns and its general shape or contour with relation to the outer surface of the tooth. The root canals should be cleaned with the broach, and their size and direction carefully studied ; also, the position of the openings leading from the pulp chamber should be studied with regard to their relation to the several points of the sur- face of the crown. This latter is especially important. Several dissections should be made of each of the upper molars. 4th. Grind away the root portion of the first dissec- tion, printing occasionally till the bifurcation of the roots is reached. 172. The pulp chamber of the upper molars is very distinct from the pulp canals; the latter often leaving the former by very small openings (Fig. 117). The average diameter of the pulp chamber is about equal to the thickness of the lateral walls by which it is surrounded, sometimes more, sometimes less. The occluding wall is usually con- siderably thicker. The form of the pulp chamber is gener- ally similar to that of the crown of the tooth ; but the horns in the young tooth are often quite slender as compared with the cusps, and penetrate far toward the enamel. The length of these diminishes as age advances. In teeth much flattened from mesial to distal, as often occurs in the upper first molars, and especially with the second, the equal thickness of the lateral walls is usually maintained pretty closely, so bu&v Fig. 117. 0&eoo& Fig. 118. Fig. 119. a & Fig. 120. Fig. 121. Fie. 117* (Par. 17-J).—Pulp Chamber and Root Canals of the Upper First Molar. a, c, mesio-distal sections, showing the pulp chamber and the canals in the mesial and distal roots; 6, bucco-lingual section, showing the pulp chamber and the canals in the mesial and lingual roots; d, Bucco-lingual section, showing the chamber and the canals in the distal and lingual roots. Figs. 118,* 119* (Par. 17;!).—Pui.i- Chamber and Root Canals of the Upper First Molars, cross-sections, a, Centrally through the pulp chamber; 6, section just at the floor of the pulp chamber ; c, section a little rootwise from the pulp chamber, showing the canals, and the form of the molar triangle. Fig. 120* (Par. 177).—Pulp Chamber and Root Canals of the Upper Second Molar. o, Mesio-distal section, showing pulp chamber and the canals in the mesial and distal roots; 6, bucco-lingual section, showing the chamber and the canals in the distal and lingual roots Fig. 121* (Par. 178).—Pulp Chamber and Root Canals in the Upper Third Molar. a, Bucco-lingual section, showing pulp chamber and the canals in tbe mesial and lingual roots; b, mesio-distal section of the single rooted tooth, showing the fonn of the pulp chamber and the mesial and distal root canals. * Illustrations, actual size. V PULP CHAMBERS OF THE UPPER MOLARS. 119 that the flattening of the pulp chamber seems out of propor- tion to the form of the tooth. 173. The floor of the pulp chamber is rounded or arched in the center, and falls away toward the mouths of the canals. The latter are situated in the position of the angles of a triangle {the molar triangle) (Figs. 118 and 119), the mesial line of which is the longest, the buccal the short- est, and the distal the intermediate length. For the upper first molar, this triangle is well shown in the illustrations repre- senting sections a little rootwise from the floor of the pulp chamber (c). This is best seen in the specimen itself; and the position and the direction of the canals, with relation to the walls of the pulp chamber and the main points of the surface of the crown, should be carefully studied. 174. The opening into the lingual root (Fig. 117, b) is the simplest and most direct. Generally, it begins in a fun- nel-shaped opening inclining to the lingual, which quickly narrows to the dimensions of a moderately small canal, and continues to taper to the apical foramen. It is usually straight, or but slight!v curved. 175'. The opening into the mesial canal is under the mesio-buccal cusp, close against the mesio-buccal angle of the pulp chamber. It often happens that this canal begins in a groove in the angle of the chamber (Fig. 119, 6), mak- ing this the thinnest point in the dentinal walls surrounding it. In voung teeth, the mouth of the canal is of a flattened funnel-shape, which is quickly contracted into a very fine canal; but in the adult, it often begins as a fine canal. Its course at first is to the buccal and mesial, and then curves to the distal. It is usually distinctly flattened, and often has a thin edge to the lingual. It is often a very difficult canal to clean with a broach. To find this canal the point of the broach should be directed into the mesio-buccal angle of the pulp chamber ; and, while held against the wall within this angle, it is slid toward the root. It will rarely fail to glide into the canal. 120 PULP CHAMBERS OF THE UPPER MOLARS. 176. The distal canal usually begins abruptly as a fine opening (Fig. 117, a, c), situated at the disto-buccal angle of the floor of the pulp chamber (Figs. 118 and 119); so that a broach pressed into that angle will easily glide into it. But in some instances, especially in the upper second molars, the opening is in the floor of the pulp chamber at a little distance from the immediate angle toward the center of the floor, and then, in positions which limit the use of the eye, it is often difficult to find. In teeth much flattened at the neck, the opening of this canal may begin very close to the mouth of the mesial canal (Fig. 120, a), or close against the distal wall of the chamber, half way from the buccal to the lingual wall, or, anywhere between this point and the disto-buccal angle. The first direction of the canal will vary according to its position. If it is found in a fairly well-defined disto- buccal angle of the chamber, its direction will be a little inclined to the distal, and the broach will penetrate it easily ; if in the floor of the chamber, it will sometimes be straight, as in the former case ; but more generally the first direction will be to the distal and buccal, wTith considerable curve afterward. If found close to the mesial canal, its course is usually first sharply to the distal, when it curves rather abruptly toward the apex of root. If found along a smooth or curved distal wall, the course will generally be to the distal and buccal, with but little curve. This canal is usually very fine from its beginning, and almost, or quite round. 177. While the canals are similar in all of the upper molars, there are differences in the form of the floor of the pulp chamber that may be briefly generalized. The pulp chamber of the upper second molar (Fig. 120) is usually much more flattened from mesial to distal than in the first molar. This changes the relation of the mouths of the canals some- what, rendering the distal angle of the triangle formed by them more obtuse, and brings the mouth of the distal canal nearer the mesial line of the triangle, so that it seems to be PULP CHAMBERS OF THE LOWER MOLARS. 121 found along the distal wall of the narrowed chamber. In others, it is found in the extreme buccal portion crowded close against the mouth of the mesial canal. 178. The position of the openings of the canals in the upper third molar (Fig. 121) is usually much the same as in the first and second, varying so as to resemble either. Occa- sionally there is more than the usual number; and others with only one or two. When there is but one, it is com- monly quite large. Four, five, or even seven, or eight, arc sometimes found. PULP CHAMBERS OF THE LOWER MOLARS. 179. Dissections. 1st. Saw the tooth through the gin- gival line dividing the crown from the root. This cut will pass through the body of the pulp chamber and give a good view of the roof, and floor, and a good idea of the general form. The root canals should be cleaned and examined with the broach. 2d. Saw the tooth through from end to end centrally from mesial to distal, or grind or file away the buccal side till the pulp chamber and the root canals are exposed. As there are usually two canals in the mesial root, an exact central cut from mesial to distal will generally fail to expose the canals, and the cut will be better made at a slight angle, so as to expose either the buccal or lingual canal of the mesial root. 3d. Grind, or file, away the mesial surface of the crown and root till the pulp chamber and the whole length of the canals in the mesial root are exposed. As this root is usually curved, the cutting must be done with care, and the curve followed. 4th. Cross sections of the roots should be made at intervals. An excellent study is to begin grinding at the apex of the roots, printing occasionally, and continuing the grinding until the pulp chamber is reached. This will display 122 PULP CHAMBERS OF THE LOWER MOLARS. cross sections, at intervals, of the entire root canals. Enough of these dissections should be made, of each of the lower molars, to make the student familiar with each class. 180. The pulp chamber of the lower molars (Figs. 122, 123,124) has the same general form as the surface of the crown, but is generally rather more angular. The wall of the chamber toward the occluding surface is convex toward the pulp; the horns extend from the extreme angles toward the apex of each cusp. The floor, through the central por- tion, is arched or convex from mesial to distal, and concave from buccal to lingual. The mesial wall of the cavity is flat, and longer than the distal. The mesio-buccal and mesio- lingual angles are sharp and projecting, while the distal angles are rounded (Fig. 123, d, g). The size of the chamber varies much. In youth, its diameter is often as much as two-fifths of the crown, and seldom less than one-third. This diminishes as age advances, and in old age, it is often very small, or especially where there has been considerable abra- sion of the teeth, the pulp chamber may be almost obliterated. 181. The root canals of the lower molars proceed from the mesial and distal portions of the pulp chamber (Fig. 122, a, b). The mesial canal, at its mouth, is usually about as broad from buccal to lingual as the whole breadth of the chamber, including its angular projections. Either at, or a little rootwise from the floor of the pulp chamber, it is usu- ally divided into two very small canals which diverge at first, and approach each other afterward, but usually remain distinct, each ending in its own apical foramen (Fig. 122, c). Occasionally, however, they are united in the apical third of the root, and end in a common apical foramen. Again, there may be a communication between them in the apical portion of the root, each canal remaining otherwise complete in itself. A few have one broad flattened canal (Fig. 123, d, e, f). These canals are usually minute, and very difficult to thor- oughly clean with the broach, though the mesio-buccal canal flfl QSS.. Fig. 122. Fig. 12:;. a b r d Fig. 124. a b r Fig. 12\ Fig. 122* (Par. 180).—Pulp Chamber and Root Canals of the Lower First Molar. a, b, Mesio-distal sections, showing the fonn of the pulp chamber and root canals; c, bucco-lingual section, showing the canals in the mesial root. Fig. 123* (Par. 180).—Cross-sections Through the Crown and Root of the Lower First Molar, showing the pulp chamber and the root canals, d, g. Sections through the pulp chamber; e, h, sections a little rootwise from the pulp chamber; /, ?', sections near apex of root. Fig. 124 * (Par. 180).—Pulp Chamber and Root Canals of the Lower Second Molar. a, c, Mesio-distal sections, showing the form of the pulp chamber and root canals: 6, bucco-lingual section of the distal root and crown ; d, bucco-lingual section through the mesial root and crown, showing two canals with communication in the apical third of the root. This communication is not very common. Fig. 125* (Par. 182).—Pulp Chamber and Root Canals of the Lower Third Molar. a, o, In double-rooted teeth ; 6, single-rooted teeth. * Illustrations, actual size. PULP CHAMBERS OF THE LOWER MOLARS. 125 is usually easily found, if the pulp chamber is thoroughly opened. By placing the point of the broach in the mesio- buccal angle of the chamber and pushing it gently on, it will generally glide into the canal. The first direction inclines to the mesial and buccal, after which it curves to the distal and lingual (Fig. 122, c). Generally these curves are easy, without short bends. The broach easily glides into the mesio-lingual canal by placing the point in the mesio-lingual angle of the pulp chamber and sliding it toward the root. The first inclination is to the mesial, but occasionally to the lingual, after which it curves to the distal and buccal. 182. The distal canal is approached by a funnel-shaped opening, of which the central part of the distal wall of the pulp chamber becomes a portion. Its direction is a little to the distal, and is generally very nearly straight to the apex. At first, it is flattened, with the long diameter from buccal to lingual, and progressively becomes rounded, and tapers regularly to the apical foramen. It is generally much larger than the canals of the mesial root and is easily cleaned with the broach. If the mouth is wide open, and the handle of the broach brought against the upper central incisors with the point directed against the posterior wall of the pulp chamber, it will easily glide into the canal, and pass to the apical foramen. This particular position for easily entering the distal canal is important, for all the lower molars. Occa- sionally, the lower third molar has but one root canal (Fig. 125, b) which is then generally very large. More rarely, only a single canal will be found in the lower second molar, but generally, the canals of the second and third lower molars are similar to those of the first. The pulp chambers are usually smaller, and oftener irregular in outline. The lower third molar has, occasionally, a very large pulp chamber. 126 VARIATIONS OF PULP CHAMBERS—DECIDUOUS TEETH. VARIATIONS OF THE FORM OF PULP CHAMBERS. 183. Many variations of form occur in the pulp cham- bers and root canals. The roots of the teeth may be abnor- mally crooked; and then the canals will be abnormally crooked. In many instances, the pulp chamber will have in it secondary formations, called nodules, which ma}* be ad- herent to the walls, or block the mouths of the canals and prevent a broach gliding into them. These also occur occasionally, within the canals, partially blocking the way of the broach. Sometimes the pulp chamber will be filled with nodular deposits so completely that there seems to be no room for the tissues of the pulp. These deposits will have to be removed before the root canals can be reached and entered, after which the canals will generally be found open. Thes2 deposits occur within the pulp chambers of any of the teeth ; but they cause annoyance more frequently in the molars. 184. Occasionally lateral openings occur from the root canals to the surface of the root. I have seen more of these from the canals of the lower molars than from those of any other teeth. Generally, they follow the course of the dental tubules, and open on the side of the root. They may diverge to one side and curve toward the apex of the root. These cannot often be detected, except in dissections of the root, and occur so rarely they may be ignored in practice. 185. Sometimes the horns of the pulp approach abnor- mally near the points of the cusps of some of the teeth, as in the upper first bicuspids, and in the mesio-buccal cusp of the upper first molar. Then the pulp is more liable to exposure in excavating carious cavities. PULP CHAMBERS OF THE DECIDUOUS TEETH. 186. The pulp chambers of the deciduous teeth are proportionally larger, and the thickness of their walls less, than those of the corresponding permanent teeth. The pulps Fig. 12C Fk;. 126 (Par. 187).—The Elliptical Form of the Arch. The crowns of the teeth of both upper and lower jaws, drawn from the casts of a very regularly formed arch, and the line of a true ellipse (in dots) placed over them, showing the comparative size of the upper and lower arch, and the deviations from perfect regularity. ARRANGEMENT OF THE TEETH. 129 are, in consequence, exposed with much less penetration of tooth substance, and, therefore, more liable to exposure from caries, or in the use of cutting instruments. The root canals are generally larger than in the permanent teeth of the same denomination, but are of the same general form. Also, the same rules for finding the root canals in the permanent molars apply to the deciduous. ARRANGEMENT OF THE TEETH. 187. The upper teeth are arranged in the form of a semi-ellipse, the long axis passing between the central inci- sors (Figs. 126 and 127). In this curve, the cuspids stand a little prominent, giving a fullness to the corners of the mouth. In different persons there is much variation of the form of the arch within the limits of the normal. Occasionally the bicuspids and molars form a straight line, instead of a curve. and frequently the third molars are a little outside the line of the ellipse. In the examination of casts of the most per- fect dentures it is found that the two sides do not perfectly correspond, and that certain teeth deviate slightly from the perfect line. The incisors are arranged with their cutting edo-es forming a continuous curved line from cuspid to cus- pid, and this line is continued over the cusps of the cuspids, and the buccal cusps of the bicuspids and molars to the dis- tal surface of the third molars. From the first bicuspid to the third molar the lingual cusps of these teeth form a sec- ond line of elevations. Between these two, the lingual and buccalcusps, there is a continuous but irregular valley, or sulcus. 188. The lower teeth are arranged similarly (Fig. 135) but on a slightly smaller curve, so that the line of the ellipse, which falls on the buccal cusps of the upper bicuspids and molars, will fall upon the buccal surfaces near the gum on the lower teeth (Fig. 126). Therefore in occlusion the upper teeth project a little to the labial and buccal of the lower at all points of the arch (Fig. 131). The incisors and 130 ARRANGEMENT OF THE TEETH. cuspids occlude so that the cutting edges of the lower in- cisors and cusps of the cuspids make contact with the lingual surfaces of the similar teeth of the upper jaw near their cut- ting edges (Fig. 128). In this, however, there is much variety within the limits of a normal occlusion. Sometimes the lower incisors strike tbe lingual surfaces of the upper near the linguo-gingival ridge, and may strike at any point between that and the cutting edges. In abnormal occlusions the lower incisors may miss the upper, striking the gums posterior to them, or they may occlude anterior to the upper incisors. The broad cusped occluding surfaces of the bicus- pids and molars of the opposing dentures rest on each other in such a way that the lingual cusps of the upper teeth fit with more or less accuracy into the general sulcus formed between the buccal and lingual cusps of the lower teeth. The buccal row of cusps of the lower teeth, in a similar way, are fitted into the sulcus formed between the buccal and lin- gual cusps of the upper teeth (Figs. 129 and 130). This arrangement is such that when the teeth are in occlusion, it leaves the buccal inclines of the buccal cusps of the upper teeth outside the buccal surface of the lower teeth (a). And, also, leaves a ledge formed by the abrupt lingual inclines of the lingual cusps of the lower teeth along the lingual line of the occlusion (b). This brings the occluding surfaces of the teeth in the best form of apposition for the purposes of mas- tication. The forms presented to the cheek and to the tongue hold these soft tissues a little apart from the actual contact points of the occlusion, and thus prevent them from being caught and pinched, or crushed, between the teeth in the act of mastication. In youth, while the permanent teeth are taking their places, and before the cusps are properly fitted to the sulci, we often find the cheeks or tongue wounded by being caught between false occluding points. With the after movements of the teeth by which they are more perfectly arranged, this difficulty disappears. Fig. 11: Fig. 128. Fig. 129. Fig. 127* (Par. 187).—Arrangement of the Teeth in the Arch. The arch of the upper jaw. (Par. 210).—The Teeth and the Gums and the Rugje of the Roof of the Mouth. Fig. 128 (Par. 188).—Labio-Lingual Position of the Incisors in Occlusion. Fig. 129 (Par. 188).—Bucco-Lingual Position of the Bicuspids in Occlusion. Fig. 130 (Par. 188).—Bucco-Lingual Position of the Molars in Occlusion. * Illustration, actual size. K ARRANGEMENT OF THE TEETH. 133 189. The line from before backward on which the occlusion occurs is not quite a plane; in the lower jaw it presents a slight curve, or concavity, and in the upper jaw a convexity (Fig. 131 c to d). This concavity of the line of the occluding surfaces of the lower teeth is a little greater than the convexity of the upper, so that the cutting edges of the lower incisors pass a little beyond, and to the lingual of the cutting edges of the upper incisors. 190. In the occlusion, the relative mesio-distal position of the particular teeth of the upper jaw to the lower is im- portant (Fig. 131). At their cutting edges, the upper central incisors are about one-third wider from mesial to distal than the lower centrals. The upper central, therefore, occludes with the lower central, and also with from one-third to one- half of the lower lateral incisor. The upper lateral occludes with the remaining portion of the lower lateral, and the me- sial portion of the lower cuspid. The upper cuspid is usually rather broader from mesial to distal than the lower, and in occlusion covers its distal two-thirds and about half of the lower first bicuspid so that its lingual, or triangular ridge, is between the cusp of the lower cuspid and the buccal cusp of the lower first bicuspid, the point of its cusp overlapping the lower teeth. The buccal cusp of the lower first bicuspid occludes in the space between the upper cuspid and upper first bicuspid. This order is now maintained between the bicuspids. The buccal cusp of the upper first bicuspid over- laps (to the buccal) the space between the two lower bicus- pids, and its lingual cusp occludes in the sulcus between them, while the buccal cusp of the lower second bicuspid occludes in the sulcus between the two upper bicuspids. The cusps of the upper second bicuspid occlude between the lower second bicuspid and lower first molar. The broad sur- faces of the molars come together, so that the mesial two- thirds of the upper first molar covers the distal two-thirds of the lower first molar : and the distal third of the upper 134 ARRANGEMENT OF THE TEETH. first molar covers the mesial third of the lower second molar. This brings the transverse ridge of the upper molar between these two lower teeth. This order is continued between the remaining molars, but less perfectly as the teeth are more irregularly formed. The upper third molar is usually smaller than the lower third molar, yet it generally extends over its distal surface. 191. The inclination of the teeth is the deviation of their long axes from the perpendicular line. The direction of the inclination is expressed by some accompanying word. The upper incisors and cuspids are so arranged that their crowns are inclined more or less forward, or toward the lip, and slightly toward the median line. The mesial inclina- tion is continued in the bicuspids and molars, diminishing from before backward, and is usually lost at the second or third molar. As a rule, the bicuspids and molars of the upper jaw are also slightly inclined toward the cheek, but in many dentures this inclination is slight, or wanting in the bicuspids and first molars to re-appear in the second and third molars, though it may be absent even in these without necessary malformation. 192. The lower incisors and cuspids are also inclined with their crowns toward the lip, but in less degree than the upper. And even the perpendicular position of these is not inconsistent with a normal arrangement. They have, however, a mesial inclination, but usually much less than the corresponding upper teeth. The lower bicuspids, with- in the limits of the normal arrangement, vary considerably in their inclinations. Sometimes they have a strong mesial inclination, and at other times they are nearly or quite per- pendicular. In many dentures, they also have a lingual inclination, but may be perpendicular or even have a slight buccal inclination. The lower molars usually have a slight mesial and lingual inclination (Fig. 135). In many exam- ples, however, the mesial inclination is wanting, especially in the second and third molars. \ ) UM -\tSi I *ig. 131.—Actual size. Fig. 131 (Par. 188).—Arrangement of the Teeth. Labial and buccal aspect of the upper and lower teeth as arranged in the arch. (Par. 197).—The Alveolar Process of the Upper and Lower Jaws, with the Teeth in Position. ARRANGEMENT OF THE TEETH. 137 193. All the teeth are a little broader from mesial to distal at or near the occluding surfaces than at their necks. Therefore, when arranged in the arch with their proximate surfaces in contact, there is a considerable space between their necks (Fig. 131). These are known as the inter-proxi- mate, or V-shaped, spaces. The sharp angle or apex of the V-form is toward the occluding surface, or at the con- tact point of the proximation, and the open end or base is at the crest of the alveolar process. In normal conditions, this space is filled by the soft tissues, or gums (136). The average arch measures about 127 millimeters (5 inches) from the distal surface of the right third molar to the distal sur- face of the left third molar, following the curve of the arch. This represents the average mesio-distal measurement of the crowns of the teeth of the upper jaw taken collectively. The average measurement of the teeth at their necks is about 89 millimeters (3.5 inches). The remaining 38 milli- meters (1.5 inches) represent the average sum of the inter- proximate spaces taken collectively. 194. On account of differences in the conformation of the crowns and the inclination of the teeth, the inter- proximate spaces vary much, in width in different dentures. They are much wider between bell-crowned teeth than be- tween thick-necked teeth; but some inter-proximate space exists in every normal denture. When the crowns of the incisors and cuspids are much inclined toward the lip, the necks of the teeth form a smaller circle than the line of the contact points of the proximation, and in this way the inter-proximate spaces may be considerably narrowed. Generally, the inter-proximate space is wide between the necks of the central incisors. The suture joining the max- illary bones passes between the roots of these teeth, and they are somewhat farther apart than the roots of the cen- tral and lateral incisors, or those of the lateral incisor and the cuspid. Therefore, in these latter, the inter-proximate 138 ARRANGEMENT OF THE TEETH. spaces are of less' width. Between the bicuspids the inter- proximate spaces are wider at the necks of the teeth than between the anterior teeth, on account of the greater breadth of the crowns as compared with the roots. The widest in- ter-proximate spaces are usually between the necks of the molars. 195. The points of proximate contact in the best formed arches are near the' occluding surfaces of the teeth. In imperfectly developed teeth, in which the crowns are much rounded toward the occluding surfaces, the contact points are more toward the gingival. In the incisors and cuspids they are in direct line with the cutting edges. In the bicuspids the contact is near the buccal angles and nearly in line with buccal cusps. The mesial and distal flattened surfaces of these teeth converge to the lingual to such an extent that, though they are arranged in arch form, the contact points remain close to the buccal angles. In many excellent dentures there is a decided inter-proximate space opening to the lingual, but in thick-necked teeth and those of a more rounded contour, the contact points are often more toward the lingual, and there is no appreciable lingual inter-proximate space. In the molars the contact points, as a rule, are removed rather more to the lingual, but still in the best formed dentures they will be found nearly in line with the buccal cusps. Between the upper first and second molars, the contact point is often extended toward the lingual by the prominent disto-lingual cusp of the first molar; and, even when otherwise, the general rounding of the distal surfaces of the upper molars often brings the contact point near the middle line of the teeth. In lower first molars the large distal cusp brings the contact point with the second molar close to the buccal side, with a considerable lingual inter-proximate space. If the distal cusp is small the contact point is usually extended toward the lingual, often as far as half the bucco-lingual breadth of Fig 132. Fi- 133. Fig. 134. Fig. 132* (Par. 196).—Horizontal Section of the Alveolar Process and Roots of the Teeth of the Upper Jaav, showing the roots of the teeth in position. Fig. 133* (Par. 196).—Horizontal Section of the Alveolar Process and Roots of the Teeth of the Lower Jaw, showing the roots of the teeth in position. Fig. 134* (Par. 197).—The Teeth and Lingual Portion of the Alveolar Proces- of the Upper Jaw, and the Bones of the Roof of the Mouth. * Illustration, actual size. ALVEOLAR PROCESS AND ALVEOLI. 141 the teeth. Between the second and third molars the contact point is most frequently near the central line of the teeth. In the best formed dentures the form of the proximate contact is such as to prevent food from being crowded be- tween the teeth in mastication; and, therefore, such as to keep these spaces clean and the inter-proximate gingivus in health. But many faulty forms are met with which allow food to leak through into the inter-proximate space and crowd the gum away, forming a pocket for the lodgment of debris, giving opportunity for decomposition, and resulting in caries of the proximate surfaces, or disease of the gum and peridental membrane. Exceptionally, cases are met with in which the teeth stand so widely apart that the spaces are self-cleaning. The form of the inter-proximate spaces is very variable. It is best studied in skulls in which the teeth are all present, and by careful consideration of the forms of the proximate surfaces of the teeth, together with their relative positions. THE ALVEOLAR PROCESS AND ALVEOLI. 196. The alveolar process is the projecting portion of the maxillary bones within which the roots of the teeth are lodged in alveoli, or sockets, accurately fitted to their surfaces (Figs. 132 and 133). The form of the alveolar pro- cess seems to depend on the teeth, the conformation of their roots, and their arrangement in the arch. If any teeth are misplaced, or from any cause stand out of the regular and normal line, the alveolar process is formed about their roots in this irregular position. Also, when teeth are lost, the alveolar process mostly disappears by absorption, and the remaining portions of the alveoli are filled with bone. 197. Normally, the alveolar process envelops the roots of the teeth to within a short distance of the gingival line (Figs. 131 and 134), varying from one to three millimeters in the vouno; adult. This distance increases somewhat with 142 ALVEOLAR PROCESS AND ALVEOLI. increasing age. The borders of the alveolar process are re- duced to a thin edge about the necks of the teeth on both the labial and lingual sides of the incisors and cuspids of the upper jaw. About the lingual sides of the necks of the bicuspids and molars the borders are also reduced to a thin edge, becoming slightly thickened about the second and third molars, especially of the latter. On the buccal sides of these, a thickening of the immediate borders of the alve- olar process, in the form of a marked ridge, begins about the first or second bicuspid, more commonly between these two, and extends to the distal of the third molar (Fig. 131, a). This ridge varies in different examples, from a very slight thickening of the immediate border, to a thick- ness of two or three millimeters. It forms a border stand- ing squarely out from the necks of the teeth. The alveolar process then thins away so that, in many instances, the buc- cal roots of the teeth, especially the mesial root of the first molar, have but a thin covering of bone. 198. Anteriorly, the bony covering of the roots of the upper incisors presents much variety. In some examples, the middle portion of the roots has but a slight covering of bone, but more generally it is progressively thickened from the neck to the apex. The roots of the cuspids are prominent toward the lip, and, for most of their length, have only a thin bony covering, and this forms a ridge along the line of the root, which may easily be traced with the finger through the soft tissues of both the gum and lip. In many instances, the bony covering is entirely wanting for a little space near the middle of the length of the root of the cuspid, the buccal root of the first bicuspid, the mesial root of the first molar, and, occasionally, of other teeth. 199. On the lingual side of the upper teeth (Fig. 134). the progressive thickening of the alveolar process, from the border toward the apex of the root, is much greater; so that the roots of the teeth seem to lie toward the labial and Fig. 135.—Actual size v.-: : Fig. 136.—Actual size. Fig. 135 (Par. 202).—Lower Teeth and Maxilla, as seen from above. Fig. 136 (Par. 210).—The Labial and Buccal Aspect of the Teeth and Gums of the Upper Jaw. ALVEOLAR PROCESS AND ALVEOLI. 145 buccal side of the alveolar process (Fig. 132). Even the large lingual root of the upper first molar, diverging strongly to the lingual, seldom forms a ridge or prominence of the pro- cess covering its lingual surface. 200. The anterior palatine foramen is in the median line, just behind the central incisors (Fig. 134). It is funnel- shaped with a broad opening to the palatine surface of the bone. The bone is often quite thin between this and the roots of the central incisors. The posterior palatine artery runs in a deep groove in the surface of the bone, very close to the apex of the lingual roots of the upper third, and the upper second molar. This artery is occasionally wounded, or cut, in efforts to extract the roots of these teeth. 201. In the lower jaw, the immediate borders of the alveolar process, on the labial sides of the incisors, are rather thicker than in the upper jaw, often amounting to a decided ridge. This, however, thins away rapidly, so that the middle portion of the roots has but a thin covering of bone. At the cuspid, the border of the alveolar process is very thin, but a gingival ridge, corresponding with that of the upper jaw, though not so prominent, begins at about the first or second bicuspid and runs to the second molar. This thins away over the middle of the length of the roots of the bicuspids and first molar. At the second lower molar, the rising of the external oblique ridge for the formation of the anterior bor- der of the coronoid process (Fig. 131, b), causes a thickening of the buccal bony covering of the root, while at the third molar this ridge rises to a level with the border of the alve- olar process, making the bony covering on the buccal side of the root about one-fourth of an inch thick (Fig. 133). This is of importance with reference to the extraction of the roots of this tooth. Indeed, the second and third molars of the lower jaw are fixed in alveoli, hollowed out in the lingual side of the body of the bone rather than in a process or ridge on the bone, as with the teeth anterior. 146 ALVEOLAR PROCESS AND ALVEOLI. 202. On the lingual side of the lower anterior teeth, the border of the alveolar process is a smooth thin edge, and the covering of bone over the roots progressively thickens toward their apexes. In this portion, the process is high, and the labio-lingual thickness is only sufficient to envelop the roots of the teeth and give them support (Figs. 135 and 137). From the first bicuspid backward the height of the alveolar process diminishes rapidly, and the line of the dental arch, and of the alveoli of the teeth, passes diagonally across the line of the curve of the body of the bone from the buccal side at the first bicuspid to the lingual side at the third molar, and sinks into the body of the bone (Figs. 135 and 137). The border of the alveolar process remains thin as far as the first or second molar, but the bony covering of the roots is rapidly thickened toward their apexes. 203. This is caused largely by the thickening of the body of the bone, produced by the rise of the mylohyoid ridge (Fig. 137, a), which begins in front below the apexes of the roots and rises progressively backward nearly to the border of the alveolar process at the third molar. The greatest thickness is over the lingual side of the apical half of the roots of the second molar, and of the crossing of the lingual side of the coronal half of the roots of the third molar. At the third molar this ridge is thinner, and at the distal angle often very slight, while the lingual covering of the roots of the tooth may be very thin below this ridge. Therefore, it is usually easy, when necessary in extracting, to force the roots of the lower third molar to the lingual and distal with a lever suitably arranged against the crown of the second molar, as with the Physic's forceps, or an elevator. 204. The borders of the septa of the alveoli of the roots of the anterior teeth are rounded from labial to lingual, but this diminishes rapidly from the cuspid backward. Between the molars it is a straight line, or presents but a Fig. 137.—Actual size. Fig. 137 (Par. 202).—Lingual Surface of the Left Half of the Lower Maxiila and Teeth, a, Mylo-hyoid ridge; 6, inferior dental foramen. PERIDENTAL MEMBRANE. 149 slight concavity, so that the highest points of the border of the alveolar process are on the buccal and lingual surfaces, or at the angles, of the necks of these teeth, which is important in fitting bands of crowns or any similar processes. 205. The alveolar process is composed of an outer and an inner plate of moderately compact bone, and between these, very open cancelous or spongy bone; so that in young persons it may be forced more or less to one side, or bent out of position, without definite fracture. The outer compact plate forms the outer surface of the bone, and the inner plates line the alveoli of the teeth. These latter are very thin, and supported on all sides by the cancelous structure. In the lower jaw the substance of the bone is more compact and stronger than in the upper, especially about the molar teeth, where the alveoli are in the substance of the body of the bone. THE PERIDENTAL MEMBRANE. 206. The peridental membrane invests the roots of the teeth from the gingival line to the apexes of the roots like a sack. It lines every part of the alveoli, and, passing over the gingival margins, is continuous with the periosteum and gums covering the outer plate of the alveolar process. It is one membrane, attached on one side to the root of the tooth, and on the other to the inner wall of the alveolus. It is composed of connective tissue which supports an abundant supply of blood-vessels, nerves and lymphatics. With these are intermingled strong fibres of wThite fibrous tissue which pass from the cementum of the root of the tooth to the bony walls of the alveolus. The ends of these are built firmly into each, forming a strong attachment of the root to its alve- olus. In childhood and youth, this membrane is compara- tively thick, and allows considerable motion of the tooth in its socket. As age advances, it becomes thinner and the motions of the teeth are more restricted. A bundle of nerves and one or more arteries enter the alveolus near the 150 GUMS. apex of the root (the apical space), and, subdividing, several arterial twigs and nerve bundles pass toward the gingivae, while others enter the apical foramen and pass to the pulp of the tooth. Also, the cancelous portion of the bone form- ing the alveolar process is richly supplied with blood-vessels and nerves, and these enter the alveolus and peridental mem- brane from all sides. This gives the peridental membrane a very rich collateral circulation. At the gingivae, the blood- vessels become continuous with those of the periosteum and gums.* THE GUMS. 207. The gums are the soft tissues covering the alveolar processes and investing the necks of the teeth. They are composed of connective tissue containing many white fibres intermingled, forming a firm membranous mass, continuous with the periosteum beneath,-and with the peridental mem- brane at the necks of the teeth. They are covered with a strong outer coat of squamous epithelium. They are richly supplied with blood-vessels and nerves ; but, under normal conditions, they are not very sensitive to pressure or moderate violence, but, in pathological conditions they may become very sen- sitive. 208. On the labial and buccal side of the alveolar pro- cess, the gums (Fig. 136) are thin and firm near the necks of the teeth and strongly attached to the periosteum. As they pass from the necks of the teeth toward the base of the alveolar ridge they become softer, lose their attachment to the periosteum, and become merged into the mucous mem- brane of the lips and cheeks. In the passage from the gums to these mucus surfaces several loose folds are formed, the most notable being the frenum labium of the upper jaw at the median line, passing from near the inter-proximate space of the central incisors to the median line of the upper lip. * For the details of this subject the student is referred to the author's treatise on " The Histological Characters of the Periosteum and Peridental Membrane." GUMS. 151 A similar, though less prominent fold connects the median line of the lower lip with the gums. Occasionally, folds of less prominence are seen in the neighborhood of the biscupids and first molars. 209. On the lingual side of the arch in the lower jaw, the conditions are much the same. The gums are firmly adherent to the periosteum near the teeth, but lower down are merged into the mucous membrane of the floor of the mouth. The median line of the tongue is connected with the median line of the gums by a strong fold, known as the frenum linguae. As an abnormal congenital condition this is sometimes connected so near the tip of the tongue as to prevent its protrusion over the teeth, constituting the condi- tion known as tongue-tied. This is generally corrected by the muscular efforts of the tongue, which stretch the mem- brane sufficiently to accommodate its motions. 210. On the lingual side, in the upper jaw, the gums (Fig. 127) are usually of greater extent and thicker. They cover the entire roof of the mouth, to the conjunction of the hard and soft palate, as a hard dense layer. In the anterior portion, a series of irregular ridges, known as the rugae, radiate from the median line toward both sides, stopping short of the gingivae. In a large collection of examples almost innumerable forms of these rugse may be noted. 211. The gingiva or gingiva?, is that portion of the gum tissue investing the neck of the tooth crownwise from the attachment of the peridental membrane at the gingival line. It is also termed the free margin of the gum. The length of the gingivae, from the line of attachment to the neck of the tooth, varies in different teeth in the same mouth, and in different adults, from about one to about four millimeters. It is often much greater in young persons, but the length usually diminishes as age advances. The free margin fits around the neck of the tooth closely ; but a thin, flat instrument is easily passed between it and the tooth to 152 GUMS. the attachment at the gingival line. As the gingival line of the tooth is at the border of the enamel, or at the junc- tion of the enamel and cementum, it follows that the gingivae inclose the immediate gingival border of the enamel, and cover this part of the crown of the tooth. In young persons, we often see one-half of the length of the crowns of the teeth buried in the gingiva*, even after the teeth are regarded as fully in place. As age advances, the gingivae become shorter, showing more of the crown, and finally recede to very near the gingival line. 212. The gingivae also fill the inter-proximate spaces in the form of septa passing between the teeth from labial, or buccal, to lingual. This portion of the gingiva is much longer than that on the labial and lingual surfaces of the teeth (Fig. 136). In normal conditions it reaches from the gingival line to the contact point between the teeth, com- pletely filling the space and preventing accumulations of debris. The form presented by the gingivae on the labial and buccal surfaces, is a series of imperfect semi-circles with the concavity toward the occluding surfaces of the teeth, and with the points of junction of these extending into and fill- ing the inter-proximate spaces (Fig. 136). On the lingual side of the arch, the conditions are much the same, but the points of the gingivae between the teeth are less prominent (Fig. 127). As age advances and the gingivae recede toward, or even to, the gingival line, the septa of soft tissue some- times fail to fill the inter-proximate spaces. This may in- duce pathological conditions by affording space for lodgment of debris in pockets, which favors fermentation. A like condition is also induced frequent!}- by a faulty form of proximate contact, which allows food to be forced into the inter-proximate space, and break down the gum septum by its pressure. INDEX. THE NUMBERS REFER TO THE PARAGRAPHS. Alveolar process............................................... 196 " " —at the buccal side of the lower third molar..... 201 " " —bone forming the....... .................... 2°5 " " —borders of.................................... x97 " " —buccal ridge.................................. *97 " " —lower labial portion.......................... 201. " '* —lower lingual portion......................... 2°2 " " —relation of the mylohyoid ridge .............. 203 " << —relation of the oblique ridge.................. 201 " —septa of the.................................. 2°4 " " —upper labial portion.......................... l9& " " —upper lingual portion......................... J99 Alveoli ....................................................... J96 " —relation of the roots of the teeth....................... *97 Angles—how named........................................... Apical foramen................................................ T53 Apex of the root defined........................................ 3 Arrangement of the teeth....................................... l87 Bell crowned teeth............................................. I4 • < << " —inter-proximate spaces...................... J94 Bicuspids ..................................................... " —buccal surface of the lower first...................... 57 << << >< " " second................... 63 .< « " " upper first...................... 46 ■ < " " " " second................... 52 —distal surface of the lower first........................ 58 n << << " " second.................... 64 .1 " " " upper first....................... 49 a «' '< " " second.................... 52 " —grooves of lower first................................ 54-55 " « " " second............................. 6a " " " upper first................................ 45 i« " " " second............................. 51 " —lingual surface of the lower first...................... 59 ,« >i << " " " second.................. "3 11 << " " " upper first...................... 47 ,1 " " " " " second................... 52 153 154 INDEX. Bicuspids—mesial surface of the lower first...................... 58 " " " " " second.................... 64 " " " upper first....................... 48 " " " " " second................... 52 '' —occluding surface of lower first....................... 54 " " " " " second.................... 62 " " " "upper first....................... 42 " " " " " second.................... 51 " —plan of the formation................................ 41 " —root of lower first.................................... 61 " " " " second................................ 65 " " upper first.................................... 5° " " " " second................................ 53 " —ridges of lower first................................. 54_56 " " " " second............................... 62 " " " upper first.................................. 43-44 " " " " second............................... 51 '' —lower first........................................... 54 " " second........... ............................ 62 '' —upper first.......................................... 42 " " second........................................ 51 Characteristics common to all of the teeth....................... 3 Cingulum...................................................... J8 Color of the deciduous teeth.................................... 133 Contact, proximate........................................... J 95 Crown, surfaces................... ............................ 3 Crowns of the teeth........................................... 3 " " " lower incisors.................................... 7 " " " deciduous teeth.................................. 130 Cutting edge of the incisors..................................... 7 Curvature of the gingival line................................... 3 Cusp defined................................................... 8 Cuspid, lower................................................. 38 " " —ridges........................................... 39 " " —similarity to the upper........................... 38 " —surfaces......................................... 38-39 " —root............................................ 40 '' upper.................................................. 33 " " —distal surface.................................... 36 " " —labial surface.................................... 33 " " —labial ridge..................................... 33 " " —lingual surface.................................. 34 " " —mesial surface................................... 35 " " —ridges.......................................... 34 INDEX. 155 Cuspid, upper—root............................................ 37 Deciduous first molars a distinct class........................... 129 second molars....................................... 129 teeth................................................ 128 " " —by what teeth succeeded....................... 129 " —buccal surfaces............................... 132 " —color......................................... 133 " " —lingual surfaces............................... 132 " —crowns....................................... 13° " " —necks........................................ 131 " —number....................................... 128 " " —roots.......................................... 151 " molars—lower first buccal surface..................... 148 " " " " bucco-gingival ridge............... 148 " " " " cusps............................. 14 " " " " distal surface..................... 149 " " " " fossae............................. 141 " " " grooves .......................... 142 " " " " lingual surface.................... 149 " * " lobes............................. 142 " " " " mesial fossa....................... 147 " " " " mesial surface..................... 149 " " " " occluding surface.................. 141 " " " " principal fossa.................... 146 " " " " ridges............................ 143 " " —upper first buccal surface.................... 138 " " " " bucco-gingival ridge............... 138 " " " " cusps. ........................... l3& << " " " distal surface..................... 14° " " " " grooves........................... *35 " lobes............................. *35 " «« " " lingual surface.................... 14° " " " " mesial surface.................... 14° " << <' " occluding surface.................. !34 " " ridges............................ J36 Dental arch.................................................... l87 " formulae................................................ Developmental grooves defined........^......................... << "of the incisors and cuspids............... 21 " " of the molars............................ 98 " lines defined.................................... IX << " of the incisors and cuspids.................. 2I '< " of the first upper molar.................... 98 .......... 158 Dissections.............................................. 156 INDEX. Dissections—instruments necessary for.......................... 158 " of the bicuspids..................................... 166 of the incisors...................................... 159 of the lower molars................................. 179 of the upper molars................................. 171 Fissure defined................................................. 11 Fissures, how formed .......................................... 21 Foramen, apical................................................ T53 Formulae, dental............................................... 1 ' Fossa defined.................................................. 10 Frenum labium................................................ 208 Frenum linguae............................................... 209 Gingivae....................................................... 211 " —inter-proximate spaces............................... 212 Gingival groove and fissure of the upper lateral incisor.......... 26 '' line, curvature of the.................................. 3 " defined........................................... 3 " of the upper central incisor........................ 20 Groove, defined................................................ 11 " " developmental................".................. 11 " " sulcate.......................................... 11 " " supplemental................................... 12 Gums......................................................... 207 '' labial and buccal......................................... 208 " lingual of lower jaw...................................... 209 " " " upper jaw...................................... 210 " of what composed........................................ 207 Incisors, lower................................................. 29 " —developmental lines............................ 30 " —difference between central and lateral........... 29 " " —distal surface.................................. 30 " " —form of crown.................................. 29 '' " —labial surface.................................. 29 " —lingual surface................................. 30 " —linguo-gingival ridge..... ..................... 30 " " —mesial surface................................. 30 " " —roots.......................................... 31 '' upper central......................................... 16 " " " —distal surface.......................... 19 " " " —developmental lines or grooves......... 21 " " " —fissures................................ 21 " " " —form of crown......................... 16 " " " —gingival liue........................... 20 " " " —labial surface.......................... 17 INDEX. 157 Incisors, upper central—lingual fossa........................... 18 " " —lingual pit............................. 21 " —lingual surface......................... 18 " —linguo-gingival fissure.................. 21 " —linguo-gingival ridge................... 18 " —lobes of crown......................... 21 " —marginal ridges........................ 18 '' —mesial surface.......................... 19 " " " —root................................... 22 " " lateral......................................... 23 " —distal surface.......................... 25 " " —division of lingual lobe................. 26 " " " —failure of lingual lobe.................. 26 " " —general form of crown.................. 23 " " —how different from the central.......... 23 " " " —labial surface.......................... 24 " " —lingual pit............................. 26 " " " —lingual surface......................... 26 " " " —linguo-gingival fissure................... 26 " " —mesial surface.......................... 25 " " " —marginal ridges................."...... 26 " " —root................................... 27 Inclination, defined........................................• • • • 191 " mesial—of the lower teeth.......................... 192 " " —of the upper teeth.......................... 191 Inter-proximate spaces......................................... 193 " " defined.................................. 13 " " —relation to the forms of the teeth...... 194 Introduction................................................... ! Lines—developmental.......................................... JI " " —of the incisors........................... 21 Lobes—defined................................................. ** " of the incisors............................................ 21 Lingual pit of the lateral incisor................................ 26 Linguo-gingival ridge of the incisors.............................18, 21 " " " " cuspids.............................. 34 " " groove.......................................... 21 Long cusped teeth.............................................. T4 Lobes of the incisors........................................... 21 Man's diet..................................................... 1 Marginal ridges................................................ 9 Mesial surface defined.......................................... 5 Measurements of the teeth...................................... 15 " " " —tables of............................ 15 158 INDEX. Mesial inclination of the lower teeth...........................191, 192 " " " upper " ...........................191, i92 Molar, lower first.............................................. 96 " " " —buccal pit.................................. 104 " " " —buccal groove............................... 104 " " " —buccal surface.............................. 104 " " " —central fossa................................ 98 " " " —cusps.................................100,101,102 " " " —developmental grooves...................... 98 " " " —distal surface................................ 107 " " " —fissures..................................... 103 " " " —grooves..................................... 98 " " " —lingual surface.............................. 105 " " " —mesial surface.............................. 106 " " " —mesial supplemental fossa................... 101 " " " —occluding surface...........................96, 98 " " " —pits........................................ 108 " " " —triangular ridges............................ 102 " " —root........................................ 108 " " second........................................... 109 " " " —buccal surface............................ 115 " " " —cusps.................................... 112 " " " —developmental grooves........ ........... in " " " —distal surface............................ 118 " " " —fissures................................... 114 " " " —lingual surface........................... 116 " " " —lobes..................................... in " " " —mesial surface............................ 117 " " " —occluding surface..............'.........*.. no " " " —pits...................................... 112 " " " —root...................................... 119 " " " —supplemental fossae....................... 112 " " " —supplemental grooves..................... 112 " " " —transverse ridges......................... 112 " " " —triangular ridges.......................... 112 " " " —variations of form......................... 113 " " third............................................. 120 " " " —buccal surface.............................. 125 " " " —distal surface............................... 126 " " " —five lobed.................................. 124 " " " —four lobed................................. 121 " " " —grooves.................................... 122 " " " —lingual surface............................. 126 " " " —marginal ridges............................. 122 INDEX. 159 Molar, lower third—mesial surface.............................. 126 " " " —root....................................... 127 " " " —supernumerary cusps....................... 123 " " —supplemental fossae......................... 121 Moiar triangle, typical forms.................................... 173 Molars........................................................ 66 " —number and names.................................... 66 " —upper................................................ 67 Molar, upper first.............................................. 68 " '' " —buccal surface.............................. 78 " " " —calcification of lobes........... ............ 69 " "—cusps................................7i,72,73»74 " " " —distal surface................................ 81 " —fifth lobe.................................... 70 " " " —fossae....................................... 68 " " " —grooves......................... ........... 68 " " " —lingual surface.............................. 79 '« «< "—lobes___................................... 68 " " "—mesial surface............................... 80 " " " —occluding surface............................ 68 " " " —oblique ridge................................ 75 "—ridges...................................71, 72, 73 1' " second............................................. 86 " " " —abnormal forms............................ 87 " " " —buccal surface.............................. 88 " " " —central fossa............................... 86 " " " —distal fossa................................. 86 " " " —distal surface............................. 88 " " " —form of neck.............................. 89 " " " —lingual surface............................. 88 " " " —mesio-buccal ridge.......................... 89 " " " —mesial surface.............................. 88 " " —occluding surface........................... 86 « «« —roots....................................... 9° " " " —three cusped................................ 86 " " " —variations of the disto-lingual lobe.......... 86 " third.............................................. 9i " " " —buccal surface.............................. 93 " " " —distal surface............................... 93 " " " —disto-lingual cusp.......................... 91 " " " —irregular forms............................. 91 " " " —lingual surface............................. 93 " " " —mesial surface.............................. 93 " " " —multiple roots.............................. 94 160 INDEX. Molar, upper third—occluding surface........................... 91 " " " —root....................................... 94 " " —three cusped .............................. 91 " " —variations of form.......................... 91 Neck of a tooth defined........................................ 3 Nomenclature.................................................. 1 Number of teeth of man........................................ 2 Occluding surface defined....................................... 4 Occlusion line.................................................. 189 " —relation of the lower and upper teeth................ 188 " —relative mesio-distal position of the teeth............ 190 Peridental membrane........................................... 206 Proximate contact.............................................. 195 " defined....................................... 5 surfaces defined..................................... 5 Pulp chamber........................ ........................ 152 " " —abnormally long horns.......................... 185 " —conditions which cause diminished size.......... 155 " —diminished size of, in the adult.................. 154 " —dissections necessary to show.................... 157 " —divisions.................................. ..... 153 '' —early formative stages........................... 154 '' —forms.......................................... 153 " —in single rooted teeth............................ 156 " —lateral openings................................. 184 '' of the upper incisors.............................. 160 " lower " ............................. 164 " upper cuspids.............................. 163 " lower " .............................. 165 " upper first bicuspid......................... 167 " second " ........................ 168 " lower bicuspids............................. 169 " upper molars............................... 172 " lower " ............................... 180 " deciduous teeth............................. 186 " —secondary formations............................ 183 " —shortening of the horns......................... 154 '' —variations in the form........................... 183 Ridges defined................................................. g '' —how named........................................... 9 " —marginal............................................. g " —transverse............................................ 9 " —triangular............................................ g Roots of the teeth defined...................................... 3 INDEX. 161 Roots of the teeth—divisions................................... 3 Root canal..................................................... 153 " " —distal, of the lower molars......................... 182 " " —distal, of the upper molars........................ 176 " " —lateral openings................................... 184 " " —lingual, of the upper molars....................... 174 " " —mesial, of the upper molars......................... 175 " "of the bicuspids, lower................................ 170 " " " " upper first........................... 167 " " " " " second........................ 168 " " " cuspids, lower................................. 165 " " " " upper................................. 163 " " " incisors, lower................................. 164 " " " " upper................................. 161 " " " molars, lower................................. 181 " " " " upper................................. 173 " " —size of, in the incisors.............................. 162 " " —position of, in the upper molars..................... 173 " " —upper first molar.................................... 173 " " —upper second molar................................ 177 " " —upper third molar.................................. 178 " " —variations of the, in the upper molars............... 177 Short cusped teeth.............................................. 14 Spaces, inter-proximate......................................... 193 " defined.................................. 13 Sulcate groove defined.......................................... u Sulcus defined...........'....................................... 10 Supplemental grooves defined................................... 12 Surfaces, mesial and distal, defined.............................. 5 " occluding, defined.............................••....... 7 " of the crown, named.................................. 4 " proximate, defined.................................... 5 Tables of measurements of the teeth............................ 15 Teeth—arrangement,........................................... J87 " —number................................................ 2 Thick-necked teeth............................................. T4 Thirds—Division of crown into.................................. 6 Tissues of the teeth............................................ 2 Tongue-tied..................................................... 2°9 Transverse ridges defined....................................... 9 Triangular ridges defined....................................... 9 Tubercle defined................................................ 9 Wrinkles defined............................................... 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