. ' • .' - •' ■' ' P :v 7 : . - ‘ ' ;; >7:. j ; .• [ • •. ; ■: , | -i ■ ’ i • i , i_ . - • THE COMPARATIVE ANATOMY OF THE TEETH OF THE VERTEBRATA. BY JACOB L. WORTMAN, A.M., M.D., i ii ANATOMIST TO THE 11. S. ARMY MEDICAL MUSEUM, WASHINGTON, D. C. REPRINTED from THE AMERICAN SYSTEM OF DENTISTRY, 1886. THE COMPARATIVE ANATOMY OF THE TEETH OF THE VERTEBRATA. JACOB L. WORTMAN, A. M., M. D. A study of the dental organs of the Vertebrata is one replete with much interest when viewed from the standpoint of the naturalist. The circumstance that their modification is so intimately associated with the food-habits of the animal, being principally concerned in the prehension and comminution of the food, and that to these same habits we must look for the most powerful influences and incentives to modification in general, causes them to assume more than ordinary importance in the estimation of the philosophic anatomist who earnestly addresses himself to the problem of vertebrate evolution. fhe fact, too, that the perfect condition in which they have been so often preserved in the fossiliferous strata of the earth’s crust has frequently furnished flie only evidence which we possess of the existence of forms long since extinct, causes them to be regarded as objects of still greater interest. When we reflect that with nothing more to guide his judg- ment than the dental series of an animal the expert palaeontologist can, generally, not only indicate with great certainty the character of the food upon which the animal subsisted, but its general characteristics and relationships as well, even though the date of its existence be removed to a remote period in geologic history, but little surprise can be felt that so much thoughtful attention has been bestowed upon this set of organs. No series of anatomical structures has proved of greater utility to the systematist who has endeavored to indicate the exact relationship or philogenetic history of mammalian forms than the teeth. Generally, the student who attempts to master the subject is discouraged almost at the very threshold of his undertaking by the apparently great diversity of tooth-forms to be met with in the mammalian class; but if looked at from a developmental point of view, and if a little careful attention Is bestowed upon the plan of organization of the teeth of certain groups, it is not difficult to discover that there are certain central or primitive types from which it is easy to derive other related forms of dentition hy simple addition, subtraction, or modification of parts already pos- sessed. Careful attention to this subject for several years past, with the assist- ance of the light which American palaeontology is now able to throw upon the question, has convinced me more and more of the truth of this assertion; and I feel well assured that we are now in a position to 352 DENT A L ANA TO MY. lay down some broad principles in regard to dental evolution, at least among certain groups of the Mammalia, where they have been subjected to the greatest amount of modification. Although there are many questions concerning the origin and details of tooth-evolution of many aberrant forms which remain to be solved, yet the discoveries which have been made in palaeontology within the last twenty-five years leave scarcely a living group of animals, the development of whose teeth lias progressed beyond the primitive stages, from which we have not gained some important information relative to the phases through which they have passed to reach their present con- dition. The possibility of reducing our knowledge of the dental struc- tures of the Mammalia to a broad and comprehensive basis was long since recognized by Prof. Cope, to whom probably more than any one else we are indebted for a genuine philosophic insight into the forms and structure of these teeth. Scarcely less important are the contribu- tions of John A. Ryder and Dr. Harrison Allen, whose learned researches into the probable causes of tooth-modification have marked notable stages in the progress of the subject and have opened new and inter- esting fields for investigation. Nor should we omit a mention of the researches of Flower, nor those of Tomes, Waldeyer, Frey, Hertwig, Magitot, and Legros, into the histology and development in later times. Commonly, teeth are defined as hard bodies attached to the parietes of the mouth or oral extremity of the alimentary canal, whose chief function is the seizure and comminution of the food. Morphologically considered, however, they are specialized dermal appendages situated in the buccal cavity, and characterized by the presence of certain calcified tissue developed from the true derm or corium of the integument, known as dentine. It will be seen from this definition that the term “tooth,” strictly speaking, is limited to those structures of the oral cavity which alone possess such tissue, although it is a recognized fact that to other epithelial or cuticular structures, found in many inverte- brate and some few vertebrate forms, the term “tooth” has likewise been applied. While they all subserve the same purpose, and are therefore analo- gous, their cliief distinction consists in this—viz. in the latter, so far as they have been investigated, these organs consist of a corneous or horny substance, which is invariably derived from the more superficial epidermal layer, and is therefore eederonie in origin. In the former a papilla arises from the corium, being sunk into a fold or pit, and eventually undergoes more or less calcification from its summit down- ward by a deposition in its substance of lime salts, forming dentine. The dentine thus formed is a hard, elastic substance, consisting of closely-set parallel tubuli, branching as they go, and whose crown may or may not be invested with an exceedingly hard and unyielding sub- stance derived from the deeper layers of the epidermis, known as enamel. These are, then, enderonic in origin. Those of eederonie source include the so-called teeth of Annulosse, Mollusca, Insectse, etc. among the invertebrates, as well as the horny teeth of Ornithorhynchus, palatal plates of the tiirenia, and the horny TEETH OF THE VERTEBRA TA. 353 teeth of the lampreys among vertebrates. If the term “ tooth ” is applicable to these structures, then we must likewise include the “ baleen ” of the Cetacea and the beaks of birds and reptiles, which by common consent are far removed from true teeth. For all such I think the term oral armature is preferable, from the fact that their produc- tion not infrequently depends upon the modification of organs widely different in origin. On the other hand, those of enderonic source are found only within the limits of the Vertebrata, and range in form from the simple cone usual among fishes to the higher complex grinding organs of certain herbivorous mammals. They all agree in being developed from the corium of the lining membrane of the mouth, which is continuous with, and really a part of, the integument, invaginated at an early period. There is a possible exception in the pharyngeal teeth of fishes, which Ryder considers to be of hypoblastic origin or developed from the base- ment-layer of the mucous membrane of the alimentary canal, and which are practically the same as those of epiblastic origin, as far as their relation to the surface is concerned. When we speak of teeth being modified dermal appendages, it will not be amiss to cite the evidence upon which such a generalization rests. This is best afforded by a study of the relationship and development of the dermal armature of certain elasmobranch fishes, of which the shark is a good example and furnishes us with one of the earliest, and there- fore one of the most primitive, conditions of the Vertebrata. In these fishes the defensive power of the integument is augmented by the production of numerous hard bodies in its substance, which have been termed “ dermal denticles ” by Gegenbaur. These structures, which are likewise known as “ placoid scales,” are distributed over the whole of the integument in shark-like fishes, and are ordinarily Fig. 187. Vertical Section through the Skin of an Embryonic Shark: c, corium; c, c, c, layers of corium; d, uppermost layer; p, papilla; E, epidermis ; e, its layer of columnar cells; o, enamel layer (from Gegenbaur, after Hertwig). rhomboidal in form, with their apices directed obliquely backward. They consist of a solid body, which is inserted by its base into the DENTAL ANATOMY. corium, with an exposed part, which is covered with a substance indis- tinguishable from the enamel of the teeth. The structure of the body is likewise coincident with true dentine, and becomes fused with a basal plate of osseous material. Their development is as follows: First, a papilla arises from the uppermost layer of the corium, being covered in by the epidermis (see Fig. 187). From the deepest layer of the epi- dermis, or that which corresponds with the Malpighian layer, a special epithelial covering is furnished, which eventually becomes, by a process of histological differentiation, the enamel of the exposed part. The papilla, before the conversion of its substance into dentine, exhibits a central cavity, from which fine branched canals radiate to the surface. Eventually, calcification takes place, beginning at the summit, and the salts of lime are deposited in the substance of the papilla, giving rise to the dentine. Gegenbaur observes :1 “ The placoid scale has therefore the structure of dentine, is covered by enamel, and is continued at its base into a plate formed ol osseous tissue; as they agree with the teeth in structure, they may be spoken of as dermal denticles.” Now, in the early embryonic stages the integument bearing these dermal denticles is pushed into the oral cavity, where they become somewhat enlarged, and appear in the adult form as teeth. Tomes says:2 “No one can doubt, whether from the comparison of the adult forms or from the study of the development of the parts, that the teeth of the shark correspond to the teeth of other fish, and these again to those of reptiles and mammals ; it may be clearly demonstrated that the teeth of the shark are nothing more than highly-developed spines of the skin, and therefore we infer that all teeth bear a similar relation to the skin.” Thus the generalization is reached that teeth are but spe- cialized dermal appendages. With this statement of the nature of teeth in general, we are now pre- pared to begin a more special inquiry into the organization of a single tooth. For this purpose I have selected the third lower premolar of the dog as an average and easily-procurable example of a generalized type among the higher forms, which will serve to illustrate the compo- sition and nomenclature of the several parts of which all teeth, with tew exceptions, are made up. For convenience of description, the several parts of most teeth can be divided into crown, fang, and neck, although there are many in which no true fangs are formed, owing to the persistent and continuous growth of the tooth; in all such no distinctions of this kind can be recognized. In the particular tooth under consideration, however, we can distinguish without difficulty an enamel-covered crown, which corresponds with the exposed part of the tooth in the recent state ; two more or less cylindrical fangs or roots, by which the tooth is implanted in the aveoli and attached to the jaw bone; and a slight constriction at the point where the fangs join the crown, known as the neck (see Fig. 188). The crown in form resembles a laterally compressed cone, with an anterior and posterior cutting edge. It is covered by a dense shiny white substance of great hardness, the enamel, which ceases at the point where the fangs com- 1 Elements of Comparative Anatomy. 2 A Manual of Dental Anatomy. TEETH OF THE VERTEBBATA. 355 mence. At the base of the crown the enamel is thrown into a conspic- uous fold or ridge, which completely encircles the tooth at this point, and is called the cingulum. Of the two cutting edges, the posterior is the more extensive, and is interrupted in its descent from the summit of the crown by a deep transverse notch, which constricts oh a prominent cusp known as the posterior basal tuber- cle. A slight indication of a second cusp of this kind is seen immediately behind it as an elevation of cingu- lum. The anterior is the shorter, and descends from the apex of the crown to the cingulum without inter- ruption. It is placed nearer the inner than the outer border of the tooth, and curves somewhat inward at its lower extremity. Fig. 188. The fangs are two in number, occupying an antero- posterior position, and give firm support to the crown. They are covered by a softer substance, resembling bone- tissue, known as cementum or erusta petrosa of human odontography. This material is continued over the entire surface of the crown as an excessively thin stratum in the unworn teeth of the Carnivora and several other orders, but can be demonstrated only by the most delicate manipulation and the use of the microscope. It assumes a more important relationship with the crown, as we shall pres- ently see, in the herbivorous species of mammals. Third Lower Premolar of a Dog (Canis fa- miliaris), enlarged. Of the two fangs, the posterior is the larger, but the shorter, and takes the greater share in the support of the crown, although the cleft which separates them at their summits is placed directly beneath the summit of the crown. It is broad at its base, and tapers somewhat abruptly to an obtuse point. It is traversed by a vertical groove upon its anterior moiety, which fits into a corresponding ridge on the side of its socket. The anterior root is the more slender and the longer of the two. It tapers more gradually, and is likewise traversed by a broad, shallow groove upon its posterior aspect. At the point of each fang will be seen a small aperture, the apical foramen, through which the nerves and nutrient vessels pass to the pulp. So far, we have spoken only of the external appearance of the tooth and of those substances which make up its outer coverings; but if both the cementum and enamel were removed, it would still preserve its original form, so great is the preponderance of the dentine as a constit- uent element. This can best be seen in a longitudinal vertical section, since at no part in an unworn tooth is the dentine exposed in these ani- mals. Although the dentine is quite thick, and constitutes by far the greatest part of the tooth, it nevertheless does not form a solid body; on the contrary, a considerable cavity is hollowed out in its centre, this being largest in the part which makes up the body of the crown, and extending down each fang. This cavity lodges the dentinal pulp, the ion native and nutrient organ of the tooth, and is in communication with the exterior by means of the apical foramina of the fangs. While this structure, in common examples of enamel-covered teeth, is observable with the unassisted eye, a more minute study of the organiza- tion of the various tissues must be conducted with the aid of the micro- 356 DENTAL ANATOMY. scope. This necessarily requires a considerable amount of experience and skill in the manipulation and preparation of material, so that to the unpractised observer a proper determination of the things which one may see is not always an easy matter. On this account I have chosen to follow the conclusions of the recognized authorities, especially the excellent treatise on dental anatomy by Charles S. Tomes, in this brief statement of the histology, rather than trust the accuracy of my own observations on the same. Since the histology of human teeth has been more fully made out than perhaps the histology of those of any other animal, it is here taken for illustration, although Tam fully aware that important deviations from the structure here described are to be met with among the Vertebrata. Dentine.—As we have already seen, the tooth consists of a dentine body with a central cavity lodging the pulp, an enamel-capped crown, and cementum-covered roots. The dentine is a hard, highly elastic, translucent substance of a yellowish-white tinge, having a silky lustre upon fracture. It is composed of an organic matrix highly impreg- nated with calcareous salts; through this matrix closely-set parallel tubuli radiate from the pulp-cavity toward the periphery in a direction at right angles to the surface of the tooth. Of perfectly dry dentine the following chemical analysis is given by Von Bibra: Organic matter (tooth-cartilage) 27.61 Fat 0.40 Calcium phosphate and fluoride 66.72 Calcium carbonate 3.36 Magnesium phosphate 1.18 Other salts 83 The organic basis of the matrix, although closely related to that of bone, is said not to be identical with it, and is hence called “ dentine” or “ tooth-cartilageit is perfectly structureless and transparent. After the tooth has been decalcified by submitting it to the action of dilute acid for a few days, the matrix will still preserve the characteristic shape of the tooth, and can readily be studied. As already stated, the tubuli, which are likewise known as dental tubes, permeate the matrix in all directions, opening freely upon the walls of the pulp-cavity, by which arrangement all parts of the dentine are brought into direct communication with the central nutrient organ, the pulp. They are most nearly approximated and their diameters greatest at their commencement on the walls of the pulp-cavity, but, pursuing a somewhat wavy course, gradually diminish in size, owing to the numerous branches which they give off. These branches, although not uniform in size, anastomose freely with those of the neighboring tubuli, and frequently show varicosities in their course. They termi- nate either by gradually fading out, by anastomosing with other branches, by ending in loops, or by entering the enamel and cement- urn layers. While the dental tubes may be said to be channelled out in the sub- stance of the dentine cartilage, the walls of the tubuli are not formed by this cartilage, but each tubuli is furnished with a structure known as TEETH OF THE VERTEBRATA. 357 the dentinal sheath, which accompanies it throughout all its plexiform radiations. The structure of these dentinal sheaths is not certainly known, owing to the impossibility of isolating them without decalcifica- tion of the dentine. Some histologists believe that they are calcified, while others express doubt as to the correctness of this conclusion. One very marked peculiarity which they possess is their great indestructibil- ity. Dentine when submitted to the action of strong acid for a suf- ficient length of time to completely destroy the intervening cartilage, or when boiled in caustic alkali, will still exhibit these dentinal sheaths, for it is indeed only in this way that their presence can be demonstrated satisfactorily. One writer (Magitot) denies their existence altogether. Enclosed within each dentinal sheath is a soft fibril, the dentinal fibrils, which take their origin from the cells of the odontoblastic layer of the pulp, presently to be noticed, and of which there are sufficient reasons for believing them to be nothing more than processes or pro- longations. There is, however, considerable discussion upon the exact nature and relationship of these fibrils. Magitot maintains that they are continuous with a layer of reticulate cells which lie beneath the odontoblasts; these freely communicate with processes of the odonto- blasts, so that there is a very direct communication between the den- tinal fibrils and the nerves of the pulp. He would therefore ascribe to them a sensory function. Klein, on the other hand, holds that the odontoblasts are concerned only in the formation of the dentine matrix, and that the dentinal fibrils are long processes of deeper cells extended between the odontoblasts. Whichever of the various views now held may ultimately prevail, this much appears to be settled—viz. that the dentine is extensively invaded, so to speak, by soft plasmic material derived from the pulp, by which it is not only nourished, but also ren- dered highly sensitive. o In the outermost layer of the dentine, which underlies the cementnm, numerous globular spaces are found, in which many of the dentinal tubes end; these are filled with soft living plasma. These spaces, if such indeed they may be properly termed, give to this layer a dis- tinctly granular appearance, whence it was called by Tomes the a gran- ular layer.” Other structures, known as the interglobular spaces, pos- sessing a ragged outline and short pointed processes, may frequently be seen in dried sections of dentine. They are said by Tomes to be most abundant at a little distance below the surface, and he believes them to pertain rather to a pathological than to a normal condition. The Tooth-pulp.—lt appears best to describe in connection with the dentine the pulp or formative organ, in consequence of the intimate relation which exists between them. As has already been stated, it is odged in the pulp-cavity, and is the principal, if not the only, source 0 Wood- and nerve-supply to the dentine. In the young and growing ootb, especially about the time calcification begins, it is largest and assumes its greatest functional activity and importance, from the fact that it is through its mediation that the dentine is formed; in fact, in the early stages of dental development, as we shall hereafter see, it is coincident with the dentine organ itself, of which in the adult tooth it is the inconsiderable remnant. As senile changes supervene it gradually 358 DENTAL ANATOMY. loses its formative energy, and may become entirely obliterated. Taken at the adult stage of the tooth, it is seen to consist of indistinct finely fibrous connective tissue containing numerous cells. The outermost layer of the pulp is known as the membrana eboris, and is made up of a single layer of highly specialized cells of a dark granular appearance, somewhat elongated, termed odontoblasts. These odontoblasts possess large oval nuclei, and are provided with three sets of processes, as fol- lows : the dentinal 'processes, which are identical with the dentinal fibrils, and, as we have already seen, enter the dental tubes; the lateral pro- cesses, by which they are connected with each other; and, lastly, the pulp processes, extending down to a deeper layer of cells. This latter layer of cells is somewhat intermediate in size between those more deeply seated and the odontoblasts. Three or more arteries enter at the apical foramen, and form a rich capillary plexus a short distance beneath the membrana eboris. The nerves enter by several trunks along with the arteries, and soon break up into a fine network in the substance of the pulp. According to 8011, nerve-fibres penetrate the dentinal tubuli in company with the dentinal fibrils, but this view is not fully accepted. Cementum.—The cementum in human and many other teeth of similar structure may be said to be confined to the roots, investing them exter- nally, unless the enamel cuticle or membrane of Nasmyth, mentioned above, pertains to it, which C. S. Tomes and others believe to be the case. It, like ordinary bone, consists of a gelatinous base combined with calcareous salts, and is permeated by vascular canals. Its histo- logical structure presents so many characters common to bone that it is difficult to consider it anything more than a slight modification of that tissue. Just as in bone, large irregular spaces ([lacunae), filled with pro- toplasmic substance and presenting numerous minute radiating canals (canaliculi), which anastomose with those of neighboring lacunae, are found in ordinarily thick cementum; certain differences are, however, seen to exist. The lacunae of cementum, for example, are more variable in size and are noted for the great length of their canaliculi. The direction, too, of the canaliculi is generally parallel with that of the dentinal tubuli, radiating from two sides only, whereas in bone-tissue they radiate in all directions. It has been already stated that the dentinal tubuli sometimes enter the cementum layer. When this is the case they become continuous with the canaliculi of the most deeply dis- tributed lacunse. The outermost or granular layer of the dentine goes so far toward establishing a complete transition in structure between the cementum and the dentine that it is generally impossible to draw a dividing-line and say where the one ends and the other begins. As to limit of distribution of the cementum on the surface of the teeth in man, monkeys, carnivores, and insectivores, different views have been expressed, owing to the various constructions that have been placed upon the nature and relationship of the enamel cuticle or Nasmyth’s membrane, already mentioned. Waldeyer, Huxley, and Kolliker hold that it is no way connected with the cementum, but that it is a product derived from the enamel, and is therefore epithelial in origin. C. S. Tomes, Magitot, and Wedl, on the other hand, maintain that it is a part of the cementum TEETH OF THE VERTEBRA TA. 359 extended over the entire crown of the tooth, and becomes continuous with its outermost layer in the vicinity of the neck. It is one of those excessively thin membranes (not over ftto o inch in thickness, accord- ing to Kolliker) which are peculiarly indestructible and resist the ac- tion of the strongest acids and alkalies. When stained with the nitrate of silver, it shows a peculiarly reticulated structure resembling epithe- lium, which is believed by Tomes to be due to the pitted surface on its interior, by which it is applied to the enamel-prisms. Encapsuled lacunae are likewise found in its substance, which would be difficult to explain if it were not a part of the cementum layer. Tomes has like- wise traced its connection with the outer layer of the cementum on sev- eral occasions, and is therefore firmly of the opinion that it is a continua- tion of this tissue. Enamel.—The excessively hard, shiny substance investing the crown of the tooth is the enamel. It is by far the hardest tissue to be met with in the animal body, being at the same time the poorest in organic constituents. Where it exists at all, it generally forms a cap of varying thickness over the exposed part of the tooth, except in those instances where there is an excessive development of cementnra in this situation, which causes it to occupy a position between the cementum and dentine, as seen in the most exclusively herbivorous feeders, of which the horse, cow, and elephant are good examples. Even here palaeontological evi- dence is quite conclusive in support of the proposition that their earlier representatives possessed teeth with naked enamel-covered crowns. This condition of nudity of the enamel is coincident with shorter cusps and less elevated ridges of the crown, and, as we have good reasons to infer from analogy, with more omnivorous habits of feeding. It can thus be shown that this anomalous arrangement of the tissues is one acquired comparatively late in the development of these forms for the exclusive purpose of giving greater strength to the lengthened cusps, thereby affording immunity from fracture during the act of mastication. Yon Bibra gives the following chemical analysis of the enamel of an adult human tooth: Calcium phosphate and fluoride 89.82 Calcium carbonate Magnesium phosphate 1.34 Other salts Cartilage Fat w proportion of the organic to the inorganic material is therefore 3.59 (* •Hi;dl, while in dentine it is 28.01 to 71.99. Its structure consists 0 1 minute hexagonal prisms, known as enamel-fibres or enamel-prisms, W ((*se long axes, broadly speaking, have a direction at right angles to the o'!' _lU(‘ the tooth. It is a comparatively rare occurrence to find the fi M ('S) PUrsuillg a perfectly straight course from the dentine to the sur- ace, but such is found to be the case in the enamel of the manatee or sea-cow and several other forms. Usually, they are tortuous, and fre- quently decussate, as in the human subject, which renders it difficult to tiace the course of an individual fibre. A variety of patterns is pre- 360 DENTAL ANATOMY. sented by the arrangement of these prisms in the enamel of different animals, especially of the “ gnawing quadrupeds,” or rodents. The prisms, when decalcified and isolated, exhibit slight varicosities or enlargements, giving them a distinct transversely striated appearance, not unlike that of voluntary muscular fibres. They are otherwise structureless. It is maintained by Bodecker that the prisms are not absolutely in contact, but that minute spaces exist between them which are filled with active protoplasmic material, which becomes continuous with that of the dentinal tubuli, thereby furnishing a means of nutrition. Some investigators admit this interstitial substance, but attribute to it no greater function than that of simple cementing material, while others, again, claim that the prisms are in absolute contact, and that no inter- vening substance is demonstrable. Owing to the disparity in extent between the outer and inner surface of the enamel, as well as the fact that the individual prisms do not decrease in size nor branch in their course outward to the surface, considerable spaces would be left if it were not that they are occupied by numerous prisms which do not penetrate to the dentine. The prisms end in sharp-pointed extremities which are received into corresponding pits in the enamel cuticle or membrane of Nasmyth. Development.—Next in order will be briefly noticed the develop- ment, so as to complete in this connection an entire statement of the anatomy of a single tooth. It may be said that although teeth of dif- ferent types differ to a wonderful degree in their forms, which would seem to indicate differences quite as great in other respects, yet, in fact, the plan of their development is substantially the same wherever found. So far is this true that the description of the embryology of one tooth will, with little modification, answer fairly well for all teeth. The more important of these modifications in the details of development will be discussed in connection with the teeth of the various subdivis- ions of the Vertebrata. We have already stated that the teeth are derived from the lining membrane of the oral cavity, which blends with the integument at the lips. The principal differences between the integument which covers the surface of the body and the mucous membrane which lines the ali- mentary canal are those of function and origin, the structure being essentially the same. In the one the individual cells of the epidermal layer become devitalized and scale off, while in the other they are actively engaged in the secretion of mucous, gastric, intestinal, and other juices during alimentation. The devitalization and consequent “ shedding of the skin ”is greater in some forms than in others. In the frogs and salamanders, for example, the skin is kept constantly moist by an abundant mucoid secretion, and the epithelium of the integ- ument may be said to be more “ alive ” in these animals than in birds, reptiles, or mammals. The difference in origin consists in the import- ant fact that the integument is formed from the epiblastic or outermost layer of primitive embryonic growth, while the mucous membrane of the alimentary canal is derived from the hypoblastic or innermost layer of the same. In the early stages of the development of the embryo the skin is more or less invaginated into the mouth-cavity, and partakes TEETH OF THE VERTEBBATA. 361 somewhat of the nature of mucous membrane proper. The real point of blending is, in the embryo at least, not at the lips, but lies inside the borders of the jaws. If, therefore, we limit the term “ mucous mem- brane ” in this situation to that tissue which is of hypoblastic origin, then the teeth of the jaws cannot be said to be developed from the mucous membrane ot the mouth, as is commonly stated, but from the invaginated integument. In many fishes teeth are found far back in the pharynx, and aie attached to the gill-arches and pharyngeal bones. lam informed by Mr. J. A. Ryder, whose extensive knowledge of the embryology of fishes renders his statements highly authoritative, that these teeth lie beyond the limits of the invaginated integument, and are truly of hypo- blastic derivation. If this be true, the generalization that all teeth are modified dermal spines is certainly incorrect. It affords us, however, an example in which identical structures have been produced from tissue of vastly different origin in a similar manner, and in all probability attributable to the same causes—viz. repeated stimulation of a particu- lar point, which eventually gave rise to a calcified papilla. The point at which a tooth is about to be developed is marked by a proliferation of the cellular elements of the tissue in which it will ulti- mately appear. These eventually arrange themselves into three organs, which have been denominated the dentine organ, the enamel organ, and the dental sacculns. This latter organ becomes so modified in some ani- mals, in which coronal cement is extensively developed, as to merit the distinction of cementum organ. Taken collectively, they represent the tooth-germ. C. S. Tomes very justly remarks that “the tooth is not secreted or excreted by the tooth-germ, but an actual metamorphosis of the latter takes place.” The three principal tissues, dentine, enamel, and cementum, thus produced, are formed from their respective organs, and consequently separate parts of the tooth-germ. Although many adult teeth do not possess enamel upon their crowns (e.g. edentates or sloths, armadillos, etc.), yet the presence of an enamel organ in the early stages of growth is believed to be a universal feature of the development of all teeth, and is one of the strongest arguments for their community of origin, however much they may have been subsequently modified. The Enamel and Dentine Organs.—ln the earliest stages of the development of a mammalian tooth, which is here taken for descrip- tion, a slight longitudinal depression in the epithelium covering the bor- ders of the jaws is noticeable; this is somewhat augmented in depth by die addition of a ridge upon either side of it. At the bottom of this groove the deepest or Malpighian layer of the epithelium grows down lnto the corium as a continuous fold or lamina, being directed down- ward and a little inward. In cross-section this fold resembles a tubu- ai gland and extends throughout the entire length of the jaw. In the positions where teeth are to be formed the lower extremity of this iamma is considerably enlarged by the rapid multiplication of its con- stituent cells. The continuity of the fold is now broken up, and the structure which is destined to become the enamel organ appears as a pro- cess of epithelium comparable in shape to a Florence flask (Fig. 189). The outermost layer of the organ at this stage is made up of cells of 362 DENTAL ANATOMY. the columnar variety which still retain their connection with the Mal- pighian layer above, from which they were orignally derived, while the interior of the enlarged ex- tremity is composed of polyg- onal cells. Fig. 189. As development proceeds, the edges of the enlarged extremity grow more rapidly downward than the centre, which causes it to assume a bell-shaped form, with the concavity directed downward. Synchronous with this growth, a papilla arises from the corium beneath and is closely invested by the enamel organ. The appearance of this papilla marks the earliest stage in the development of the den- tine organ, but it will be well to examine more closely at this stage the structure of the enamel organ. While it retained the shape of the Florence flask its periphery consisted of colum- nar epithelium, the interior be- ing made up of polygonal cells. Coincidentally with its assump- tion of the bell shape those cells of the peripheral layer which are brought into juxtaposition with the dentine bulb or organ un- dergo great elongation and en- largement, forming very regular six-sided prismatic bodies, and are known as the enamel-cells. The polygonal cells of the interior are transformed into a stellate retic- ulum composed of cells with remarkably elongated processes ; these pass through a series of unaltered cells known as the stratum intermedium into the enamel-cells. Lastly, we have the outer layer, which is little changed, and still remains connected with the Malpighian layer by a slender cord of epithelium. This layer is called the external epithelium of the enamel organ. Three Stages in the Development of a Mammalian Tooth-germ: a, oral epithelium heaped up over germ; b, younger epithelial cells; c, deep layer of cells or rete Malpighii; d, inflection of epithelium for enam- el germ: e, stellate reticulum; f, dentine germ; g, inner portion of future tooth-sac; h, outer portion of future tooth-sac; i, vessels cut across; k, bone of jaw (from Tomes, after Frey). Before the dentine papilla makes its appearance “ a dark halo,” more vascular than the surrounding parts and corresponding to the epithelial lamina or fold which gives rise to the enamel organ, is to be seen in the submucous tissue or corium. Immediately beneath the enlarged ex- tremity of the enamel organ the dentine papilla is developed at about the time this stage is reached by the enamel organ. In its peripheral layer highly specialized cells with several sets of processes, odontoblasts— already described in connection with the tooth-pul})—make their appear- TEETH OF THE VEBTEBRATA. 363 ance, while in the remainder of the bulb numerous other cells, identical with those of the tooth-pulp, are developed. It also becomes highly vascular. Very soon the odontoblasts nearest the surface undergo metamorphosis into a gelatinous matrix, and their nuclei disappear; they are next calcified from the summit downward, and we soon recognize a thin dentine cap over the entire bulb, which gradually increases as development proceeds. The central portions of the odontoblasts remain uncalcified and form the dentinal fibrils, while the lateral processes occa- sion the numerous anastomoses of the dentinal tubuli and fibrils seen in the adult tooth. The dentine mass is gradually thickened by successive increments from within by a repetition of the process above described, so that it will thus be readily seen that the configuration of the dentine body, and consequently the entire tooth, is established as soon as calcifi- cation has fairly set in. Returning to the enamel organ, we can now briefly follow its devel- opment to completion. We have already seen that it consists ot an outer layer of columnar epithelium covering the convex portion, and is connected by a slender cord with the Malpighian layer above. It con- sists also in part of an internal stellate reticulum which passes by means of a layer of rounded cells (stratum intermedium) into the enlarged, greatly-elongated prismatic cells lining the concave lower surface, which invests the dentine organ like a cap. Before the enamel is completed the external epithelium, the stellate reticulum, and stratum interme- dium disappear altogether, but before this atrophy takes place the neck or epithelial cord of the enamel organ gives rise to the tooth-germ of the permanent tooth as a diverticulum which is developed in the same way as the germ of the first or deciduous tooth just described. The essential part of the enamel organ, or rather that.which ulti- mately results in the formation of enamel, consists of enamel-cells. These, as we have said, become greatly elongated and assume the form of regular hexagonal prisms, which agree in shape with the calcified enamel-prisms of the complete tooth. Just as in the odontoblasts of the dentine, they are transformed into a gelatinous matrix, the nucleus dis- appears, and calcification begins from above, the only difference being that the enamel-prisms calcify completely, and are therefore not tubular, while in the corresponding structures of the dentine dentinal tubnli are left. Different views have been advanced in regard to the exact desti- nation as well as the function of the several parts of the enamel organ spoken of above as disappearing by atrophy. As to the fate of the external epithelium, Waldeyer holds that after the disappearance of the stellate pulp it becomes applied to the outer surface of the enamel as the membrane of Nasmyth, which would certainly seem to be its most natural fate ; but Kolliker, Magitot, and Legros claim, on the other hand, that it disappears altogether. Most authors believe that the enamel organ is devoid of vascularity, but Beal asserts that there is a vascular network in the stratum intermedium. If it be non-vascular, then it is more than probable that the pulp represents stored-up pabulum from which the requisite formative energy is derived. If vascular, it then probably subserves a mechanical purpose only, as some authorities believe. 364 DENTAL ANATOMY. The Dental Sacculus and Cement Organ.—So far, no mention has been made of the development of the dental sacculus. At an early period in the growth of the dentine papilla a process of the submucous tissue arises from its base and seems to grow upward on the.outside of both dentine and enamel organs, finally coalescing on top, so as to enclose the growing tooth-germ in a shut sac, the dental sacculus. Whether there is an actual growth of processes from the base of the dentine bulb, or whether the adjacent connective tissue is transformed into it, appears not to have been very accurately determined; at all events, the con- nective tissue immediately in contact with the germ soon becomes distinguishable from that external to it by becoming richer in cells, vessels, and fibrillar elements. When the sacculus is fully formed, it is made up of an outer and an inner wall, both richly vascular. The outer wall becomes the dental periosteum, while in the inner wall, especially in the vicinity of the roots, osteoblasts appear and are calci- fied into cementum, as in the formation of ordinary bone-tissue. Its close application to the surface of the enamel, and partial or imperfect calcification in most teeth, give rise to the membrane of Nasmyth. In those animals, however, in which coronal cement is formed, such as the Herbivora, there is developed in connection with the inner wall, between it and the enamel, a fibro-cartilaginous structure containing character- istic cartilage-cells. These undergo calcification in a manner not dif- ferent from that seen in the formation of cartilage bone, and produce the cementum in the teeth of these animals. It is then known as the cementum organ. We have now made clear, we trust, as complete a statement of the anatomy of a single tooth as is consistent with brevity, but which will serve as a basis for the comprehension of the more special part of our subject—viz. the morphology of the teeth in the various subdivisions of the Yertebrata. THE ACCESSORY ORGANS-THE TEETH, THEIR STRUCTURE, DEVEL- OPMENT, REPLACEMENT, AND ATTACHMENT, IN FISHES. It will be impossible to gain anything like a concise understanding of the dental organs of this extensive assemblage of vertebrate forms until we have first briefly outlined their classification. In this I have followed Prof. Gill, believing that his interpretations more nearly coin- cide with a natural arrangement. It is a common practice of naturalists to consider the Yertebrata as divisible into five classes, as follows: Pisces, or fishes; Batrachia, or frogs, salamanders, etc.; Reptilia, or snakes, turtles, lizards, etc.; Aves, or birds; and Mammalia, or mammals; but according to Prof. Gill there are differences quite as great, if not greater, between certain mem- bers of the old class Pisces as there are, for example, between some fishes and frogs. For this reason he divides the permanently gill-bearing ver- tebrates, or those which aerate the blood throughout the entire life of the individual by means of specially adapted organs known as “ gills,” into four classes, which he defines as follows : TEETH OF THE VEETEBBATA. 365 I. Skull undeveloped, with the notochord persistent and extending to the anterior end of the head. Brain not distinctly differentiated. Heart none. Leptocardii. 11. Skull more or less developed, with the notochord not continued forward beyond the pituitary body. Brain differentiated and distinctly developed. Heart developed and divided at least into auricle and ventiicle. A. Skull imperfectly developed, with no lower jaw. Paired fins undeveloped, with no shoulder-girdle nor pelvic elements. Gills purse-shaped. Marsipobranchii. B. Skull well developed, with a lower jaw. Paired fins developed (sometimes absent through atrophy), and with shoulder-girdle (lynform or lurcula- shaped, curved forward, and with its respective sides connected below), and with pelvic elements. Gills not purse-shaped • Lybieeea. a. Skull without membrane bones (“a rudimental opercular bone” in Chimcera); gills not free, the branchial openings slit-like, usually several in numbei, exoskeleton placoid, sometimes obsolete; eggs few and large. Elasmobranchii. b. Skull with membrane bones ; gills free; branchial openings a single slit on each side, sometimes confluent; exoskeleton various, not placoid; eggs compara- tively small and numerous Pisces. The first of these classes, Leptocardii, includes a few small fish-like animals, such as the well-known amphioxns or lancelet occurring on our coast, in which no skull exists. They are in many ways most remark- able forms, being the most primitive of all vertebrates, but as they are devoid of teeth, this class can be dismissed without further consideration. The next, Marsipobranchii, embraces the lampreys, whose “ horny teeth ” have already been alluded to. The relationship as well as examples of each order of the remaining two classes is expressed in the subjoined table (p. 366), which is compiled from Dr. Gill’s papers on the classification of fishes. The Accessory Organs.—A consideration of these organs necessarily involves not only a study of the bones and cartilages taking share in the boundary of the oral cavity, but of all bones and cartilages in connec- tion with which teeth are developed. It would likewise properly include a mention of the muscles which move these parts, together with the vas- cular and nervous supply; but owing to their great range of variation, as well as the limited space at my disposal, these latter will not be con- sidered. This, in my judgment, is best accomplished by describing the normal arrangement in some typical fish and comparing all others with it. For this purpose a gadoid fish, or one of the cod tribe, is most suit- able, since it exhibits the structure which obtains in a large majority of ichthyic forms. If a well-cleaned skull be examined, it will be seen to consist, in the first place, of a cranium or brain-box, or that part which remains intact after the skull has been boiled or macerated a sufficient length of time to cause the soft parts to disappear and the arches and appendages to become disarticulated. This contains the brain, and becomes continuous at its lower back part with the vertebrae or axial pieces of the body skele- ton into which the spinal cord passes. Suspended from either side of its posterior portion there is a chain of bones which extends down beneath the throat and bears the pectoral fins; this is known as the shoulder-gir- dle or scapular arch (see Fig. 190). A short distance in front of this, or at a point about midway between the root of the scapular arch and the eye-socket, another arch springs 366 DENTAL ANATOMY. Class, LEPTOCARDII; example, lancelet. Class, MARSIPOBEANCHII: ex. lamprey s. Sub-class, IlOLOCEPHALI : ex. chimera. Class, ELASMOBEANCHII Sub-class, Plagiostomi Orders, Raice: ex. rays, sawfishes, and torpedos. ' . Squall: ex. sharks. Hyoganoidei f Cycloyanoidei: ex. bowfin. 'B i O ( Rhomboganoidei: ex. bony gars.