MICROSCOPY OF THE DENTAL TISSUES.* 
BY J. H. McQUILLEN, M.D., D.D.S., 
PROFESSOR OF ANATOMY, PHYSIOLOGY, AND HYGIENE IN THE PHILADELPHIA DENTAL COLLEGE. 
An Address before the Connecticut State Dental Association, October 4th, 1865 
Gentlemen:-When accepting the invitation to deliver an address be- 
fore your body, I concluded to make the Microscopy of the Dental 
Tissues the theme for consideration, under the impression that it would 
prove alike interesting, instructive, and a prolific source of investigation 
to all. And although many writers and observers are disposed to re- 
gard the subject as one that has been so thoroughly examined that it is 
impossible to throw additional light upon it by continued research, yet 
all accurate observers and thinkers must admit that there still remains 
"ample scope and verge enough" to whet the appetite of the earnest in- 
quirer in this direction. The celebrated John Hunter, in preparing his 
work on the Natural History of the Human Teeth, no doubt sup- 
posed that he had exhausted the subject; and yet who to-day would agree 
with him in the assertion that "a tooth is composed of two substances, 
enamel and bone;" or be satisfied with the description which he gives of 
the different tissues which enter into the formation of the dental organs ? 
I do not wish to be understood as underrating his valuable contributions 
to our specialty; but, on the contrary, appreciating them most highly, and 
fully recognizing the impulse they gave toward raising our calling from 
a mere handicraft to the position of a liberal profession, I yet cannot 
refrain from expressing the opinion, that had he availed himself of the 
aid of the microscope (that wonderful instrument which has revealed 
during the past half a century so much of the minute structure of organic 
bodies that is hidden from the naked eye), his work, in place of being 
laid on the shelf as a curious and truly wonderful production as far as it 
goes, would still be used as a valuable text-book. It may be said that 
microscopic research in his day was not prosecuted with the same vigor 
that it has been during the past fifty years; while this is true, the fact 
* In the illustrations accompanying this address, from Tomes, Leidy, and Kol- 
liker, the plan in Gray's Anatomy, of placing the names of different parts on or 
near them, has been adopted, in place of the usual numbers and foot-notes. 
1 2 
must not be lost sight of, that Anton van Leeuwenhoek, as early 
as the period between 1632-1723, with the assistance of magnifying 
glasses, powerful though of very simple form, discovered and described the 
dentinal tubuli. Yet, notwithstanding the valuable assistance that was 
here afforded to him by the labors of another, Hunter, evidently owing 
to a want of familiarity with all the literature of the subject, contenting 
himself with the description of the gross anatomy of the jaws and teeth, 
permitted the opportunity to pass by for a thorough account of the 
minute structure of the organs, and thus left the field entirely open for 
the valuable efforts of Retzius, Owen, Nasmyth, Tomes, and Kolliker, 
whose contributions in this field will ever be regarded as inestimable 
additions to the domain of science. Let the remembrance of the fact 
just referred to be to each and all of us a constant incentive to exer- 
tion, not only to become observers, but also familiar with the ertlire 
literature of a subject, and thus avoid falling into the error of supposing 
that having made some advance in any given direction, we have therefore 
mastered and exhausted the theme. 
The subject is not merely interesting and important to us as profes- 
sional men, but to the naturalist, who, in the classification of animated 
nature from the days of Cuvier, has made the general form of the teeth 
one of the most important guides in determining what position should be 
assigned to different animals in the Zoological series, a thorough knowl- 
edge of this subject is equally indispensable in ascertaining the nature, 
affinities, and position of extinct species, of whose organization, on ac- 
count of the greater durability of the dental tissues, they are frequently 
all that remains in the geological strata. Possessing such knowledge, 
the paleontologist, with the aid of the microscope, has been able to de- 
cide questions which were not only attractive from the scientific interest 
attached to them, but also vastly important on account of large pecuniary 
interests involved in their solution.* 
* In confirmation of the above, the following is appended: "It was in regard to 
Teeth that the possibility of such determinations was first made clear by the laborious 
researches of Prof. Owen; and the following may he given as examples of their 
value: A rocky formation extends over many parts of Russia, whose mineral charac- 
ters might justify its being likened either to the Old or to the New Red Sandstone 
of this country, and whose position relatively to other strata is such, that there is 
great difficulty in obtaining evidence from the usual sources as to its place in the 
series. lienee, the only hope of settling this question (which was one of great 
practical importance, since, if the formation were new red, Coal might be expected 
to underlie it, whilst if old red, no reasonable hope of coal could be entertained) 
lay in the determination of the Organic remains which this stratum might yield; but 
unfortunately these were few and fragmentary, consisting chiefly of teeth, which 
are seldom perfectly preserved. From the gigantic sue of these teeth, together 3 
In inviting your attention to the Microscopy of the Dental Tissues, I 
do not propose to offer to you any discoveries of my own, but rather to 
describe in a plain, and, I trust, comprehensive manner, the characteristic 
peculiarities of the tissues which enter into the composition of the dental 
organs. In doing this, however, I shall present that which I have seen 
myself and shown to you, for as you are well aware a large number of 
valuable sections of the teeth of man and animals and also of bone were 
exhibited to you under the microscope last evening; these, in connection 
with the drawings which are suspended in the room, will, I trust, enable 
me to make the subject more clear and intelligible to you than could have 
been the case without them. The limited period which is afforded me 
only admits, I regret to say, of a hasty survey of the subject; yet I trust 
the time, however, will not be misspent, but may be the means of induc- 
ing you, as already remarked, to examine the subject yourself with the 
aid of good instruments, and thus confirm or correct the statements of 
other observers. In this connection, let me say, it is not absolutely 
necessary that you should invest a large sum of money in the purchase of 
a microscope, for with from sixty to one hundred and twenty-five dollars 
an instrument can be procured adapted to all practical purposes; and 
with such as these some-of the most important and valuable discoveries 
have been made. Every Dental Society should have a microscope. 
With these preliminary remarks, the teeth may be defined as small, 
hard, white bodies found implanted in the alveolar processes of the 
superior and inferior maxillary bones in man and the mammalia generally; 
while in other portions of the animal kingdom supplied with these 
organs they occupy various positions in the alimentary canal, extending 
in some instances from the mouth to the pylorus. In man, as you are 
well aware, the number of permanent teeth is thirty-two, but the typical 
number of the vertebrata is forty-four, and in the series it varies from a 
single tooth to as high as one hundred and forty. Varying thus in num- 
ber and position they also differ in shape, size, and microscopical structure, 
with their form, it was at first inferred that they belonged to Saurian Reptiles, in 
which case the sandstone must have been considered as New Red; but microscopic 
examination of their intimate structure unmistakably proved them to belong to a 
genus of Fishes {Dendrodus}, which is exclusively Palaeozoic, and thus decided that 
the formation must be Old Red. So, again, the microscopic examination of certain 
fragments of teeth found in a Sandstone of Warwickshire, disclosed a most re- 
markable type of tooth-structure, which was also ascertained to exist in certain 
teeth that had been discovered in the 'keuper-sandstein' of Wirtemberg; and the 
identity or close resemblance of the animals to which these teeth belonged having 
been thus established, it became almost certain that the Warwickshire and Wirtem- 
berg sandstones were equivalent formations, a point of much geological importance." 
-Carpenter on the Microscope, page 641. 4 
and in the relations which the different tissues that enter into their com- 
position bear to each other, but having reference under all circumstances, 
however, to the habits and food of the animal; for the teeth, as the primary 
agents in the assimilation of the food, serve to prepare it by mastication 
for the important changes which it undergoes in the rest of the nutritive 
system. 
Although presenting a variety of shapes, the general form of the teeth 
is either conical, wedge-like, or a combination of both. As the time 
allotted does not permit me to describe the shape of the different classes 
of teeth, I will merely state that the body of each tooth is divisible into 
three sections : one part projecting above the mucous membrane or gum, 
the crown; another portion received or implanted in the alveolar socket 
or alveolus, the root; and a line of demarkation, not always strongly 
marked, between these two, the neck. 
If a recent human tooth, an incisor for instance, is divided longitudin- 
ally, a chamber called the pulp cavity, correspond- 
ing with the shape of the tooth and occupied by a 
soft and highly vascular substance, the dental pulp, 
richly supplied with vessels and nerves that have 
entered a foramen at the end of the root, will be 
brought into view. In addition to this, the exterior 
of the root will be found invested by a soft fibrous 
structure, the periodental membrane. These con- 
stitute the soft tissues of the tooth. On examining 
the solid portion of the tooth with respect to its 
component tissues, it will present at least three 
distinct structures: viz., 1, a yellowish-white sub- 
stance constituting the greater portion of the body 
of the tooth, the Dentine ; 2, a hard, vitreous, 
translucent substance investing the crown, the 
Enamel ; 3, a cortical structure generally dis- 
posed in a thin layer around the root, the Ce- 
mentum. In addition to these a fourth substance 
is not unfrequently observable intermediate between 
dentine and bone, the Osteo-Dentine of Owen, 
or the Secondary Dentine of Tomes. 
As a primary object in this address is to induce 
you to become observers, it is advisable at this 
point to describe how microscopical sections of 
hard structures like bone and the dental tissues are made; for the 
errors into which investigators have frequently fallen have been due 
either to the employment of defective instruments, or the imperfect man- 
Fig. 1. 5 
ner in which the preparations under examination have been made. The 
necessary appliances for the accomplishment of this purpose are, a fine 
watch-spring saw, files, and water of ayr-stones; or, what is still better 
for the purpose, as it will effect the work much more rapidly, a lathe, to 
which can be adjusted a disk of soft iron like that used by the lapidary, 
charged at its end with diamond dust, corundum wheels, and Arkansas 
stones, for the purpose of making and polishing sections of the sub- 
stance to be examined. In addition, oblong slips of plate-glass that 
should be clear, free from veins and bubbles (on which the preparations 
are to be placed, and very thin glass for covering them, which can be 
obtained from opticians), of a standard form and size, agreed upon 
by microscopists, along with Canada balsam and a spirit-lamp, are re- 
quired. 
The first thing to be done in the preparation of a specimen, is to 
obtain a longitudinal or transverse section as thin as it can be safely cut 
with the saw or circular disk. This section is then attached to a slip of 
well-annealed glass with Canada balsam, and filed or ground down until 
it becomes translucent, care being taken to have the pressure applied with 
such equality that the thickness of the section shall be the same in its en- 
tire structure. By occasionally examining it under the microscope during 
this process, any inequalities that may be present can be detected. Hav- 
ing obtained a perfectly smooth and polished surface, by gently warming 
the glass, the section can be detached, and, with the finished surface turned 
downward, may be transferred to the permanent glass slide, to which it is 
to be united by Canada balsam. The upper surface of the section is 
then treated in the same manner as the opposite surface was, until a 
perfect microscopical section is obtained, when it should be covered with 
the thin glass, and the latter secured in position by a black cement sur- 
rounding its edge. In using the balsam, great care should be exercised 
to prevent the formation of air-bubbles, as these injure the appearance 
of the specimen, and the true character of the structure cannot be made 
out with any satisfaction. This can be best effected by placing a drop of 
balsam on the glass slide, and then, holding the latter over the flame of 
the spirit-lamp, when this becomes sufficiently warm, the object can be 
brought in contact with the balsam, and pressed evenly and gently against 
the glass. In placing the thin glass cover in position, it should first be 
warmed on the under surface before it is laid over the specimen: if this 
does not suffice to attach it, the under surface of the slide can be warmed 
so as to soften the balsam. Care should be exercised, however, not to 
apply too high a heat in this operation, as it may cause the specimen to 
curl up, and destroy its usefulness for microscopical examination. 
An easier method of obtaining sections than that described above, is 
VOL. II. 8 6 
to place a tooth in the lathe and make shavings of it with a thin, sharp, 
and well-tempered chisel, or to put a tooth, that has been slightly softened 
by acid, in a vice, and then take off thin slices with a sharp razor. 
Those who may not feel inclined to incur the time and trouble incident 
to the preparation and mounting of specimens, can procure good ones at 
a trifling cost from prominent opticians in our large cities. Those, how- 
ever, who purpose to carry out a line of investigation for themselves, 
cannot depend entirely upon such sources, but must frequently rely on 
their own efforts in the preparation of specimens, and hence the im- 
portance of knowing how to make them. 
Having secured a longitudinal section of a human tooth, it is found to 
be made up of three distinct structures, of which the first is a substance, 
now to be described, named, by Prof. Owen, 
Dentine.-This tissue, which was formerly called ivory or tooth sub- 
stance, constituting the greater part of the crown and root of a tooth, 
giving to it its general form, and inclosing the pulp cavity, is in a per- 
fect human tooth never exposed at any point, but is perfectly protected 
from the influence of external agents by the dense and impermeable 
enamel covering the crown and extending by a thin layer to the neck; 
where this ceases, the cementum commences to invest the root. The 
satiny aspect which a section of dentine presents to the naked eye gives 
it the appearance of a perfectly compact solid structure, but under the 
microscope it is found to consist of a multitude of fine canals, the denti- 
nal tubuli, and a connective matrix or intervening substance, the in- 
tertubular tissue. 
The dentinal tubuli are generally described by microscopists as 
having distinct parietes or walls, formed of harder material than the in- 
tertubiilar tissue, whose thickness is much less than the diameter 
of the tubuli, which, in the largest, according to 
Tomes, is about of an inch. It is diffi- 
cult to distinguish these walls in longitudinal 
sections, but in transverse sections what ap- 
pears to be a narrow ring is readily observable 
surrounding each tubule. Whether this is an 
optical illusion induced by the wavy character 
of the secondary curvatures of the tubuli, is a matter of question with 
many. Be this as it may, there can be no doubt entertained by any one 
at the present time relative to the fallacy of the opinion advanced by 
Nasmyth, that the tubuli are not canals but solid fibres, with brick- 
shaped cells built around them. How this distinguished and laborious 
observer could have been led into such an error is difficult to conceive, 
as the tubular character is readily demonstrable by the passage of colored 
fluids and minute bubbles of air along them. 
Fig. 2. 
Intertubular 
tissue. 
Dentinal tu- 
buli. 7 
The tubuli originate by open mouths on the walls of the pulp cavity, 
and proceed in a slightly wavy and radiated manner through every por- 
tion of the dentine to its periphery, where they generally terminate, 
although in some instances extending beyond and penetrating the enamel 
or the cementum, in the latter of which they communicate with the lacunae 
through the canaliculi. Toward the masticating surface of the crown, 
where the occlusion of an antagonzing tooth has to be received, they 
are vertical, or nearly so, and horizontal where the pressure of contiguous 
teeth has to be resisted; by this arrangement a certain amount of elas- 
ticity is gained, and the shock of occlusion and pressure is more generally 
distributed over the entire structure. The wavy character of the tubuli 
above referred to consists of two or three large or primary curvatures, 
and a number of smaller or secondary curvatures, which follow closely 
upon each other, and are estimated by Retzius as numerous as two hun- 
dred in a single line. 
Fig. 3. 
Dentinal tu- 
buli. 
Lacunae. 
INTERTUBULAE 
TISSUE. 
CaXaliculi 
VERTICAL SECTION OF THE ROOT OF A CANINE TOOTH. (AFTER LEIDY.) 
Starting from the pulp cavity in close proximity to each other, the diverg- 
ence of adjacent tubuli is so slightly apparent, that they seem to run paral- 
lel with each other. In following them out, however, toward the periphery 
of the dentine, the divergence in the crown is readily observable on ac- 
count of the increased distance between them and the ultimate dichotomous 
division, without at first much diminution in the caliber of the tubes. 
These branchings increase in number and decrease in size the nearer they 
approach the periphery of the dentine, and they either anastomose with 
each other, pass into the enamel or cementum, terminate abruptly, or in 
very minute cells. In addition to the large branches just referred to, in- 
numerable fine branches are given off from the sides of the tubuli, which 
penetrate the intertubular tissue and anastomose with each other. In this 
way the most perfect arrangement for the passage of that nutrient fluid, 
the liquor sanguinis, to every portion of the tissue, is effected. 
Up to within the past few years the dentinal tubuli have been invariably 
regarded by histologists as conduits for conveying to the tubular and 
inter-tubular tissues materials inservient-to repair the constant molecular 
disintegration or waste which takes place in dentine as in all other living 8 
organic structures. Of late, however, Mr. Tomes has discovered under 
the microscope a number of fibrils projecting from the dentinal tubuli, 
which he is disposed to regard as continuations of the nerve fibres of the 
dental pulp, and in confirmation of this discovery, Dr. Lionel S. Beale, 
in his work on "The Structure of the Simple Tissues," says that he 
has been able to verify this statement, and, furthermore, that they are not 
"conduits for nutrient fluids which transude through the walls of the 
vessels, and are supposed to pass along the tubes to the outer part of the 
tooth." Last evening, among other microscopical specimens, I showed 
you a section of dentine presented to me by Dr. Allport, of Chicago, 
in which these fibrils were readily seen projecting from the dentinal tubuli. 
Now it is a matter of question with me, whether the fibrils exist as "a soft, 
solid substance " in the Zwfna tooth, or are formed after its extraction. 
You are well aware that when blood is drawn from an artery or vein into 
a basin, or any other vessel, it coagulates or separates into two portions, 
the crassamentum or clot, and the serum or watery part. I think it is 
reasonable to infer that the presence of these fibrils in a microscopical 
section of a recent tooth is due to the coagulation of the organic ele- 
ments present in the liquor sanguinis circulating in the dentinal tubuli. 
In a series of lectures on the "Anatomy of the Elementary Tissues 
of Man," delivered since the publication of the work referred to, Dr. 
Beale asserts that "there is no evidence of addition and removal of ma- 
terial going on in the enamel and dentine after the completion of their 
formation, and it is probable that the matter upon which the hardness of 
these tissues depends is not removed at all after its deposition." Not- 
withstanding this decided assertion, with the exception of enamel after it 
is fully formed, and the consolidated dentine of very old persons, the uni- 
versally received opinion that there is a constant change taking place in 
all the tissues during the life of a being, is beyond a question of doubt 
correct. It is only necessary to refer to the absorption of the roots of 
the deciduous teeth (a retrograde metamorphosis by which the dentine 
and cementum are removed cell by cell, as in their original construction 
they were formed cell by cell), to disprove the assertion of Dr. Beale. 
The evidence of molecular change in the dentine and cementum is by no 
means, however, confined to the deciduous teeth; for frequent and well- 
marked instances are brought to the notice of the dental practitioner in 
which teeth, originally soft in their structure and cutting readily under 
the instrument, become, in course of time, hard and compact, while on 
the other hand, teeth which are quite hard become very soft and chalky. 
That this change of structure is due to constitutional influences, and is 
brought about by molecular change in the part affected, is more than a 
reasonable supposition, for it bears upon its face an air of positive cer- 
tainty. 9 
Much more might be said in confirmation of this, but I refrain from 
doing so at this time, and now direct 
your attention to what are called the 
CONTOUR LINES of OWEN. You will 
observe them in this drawing, running 
in an arched manner, somewhat parallel 
to each other, and nearly at right angles 
with the dentinal tubuli, in that por- 
tion of the dentine which constitutes the 
greater part of the crown of the tooth; 
although thus represented in this in- 
stance, they are by no means confined 
to this part of the tooth, but may be 
found in the dentine of the root. The 
appearance of these markings, arched 
or contour lines, is attributable to the 
primary curvatures of the dentinal 
tubuli and the formation of the dentine 
in strife or laminae, as with the rings 
observable in the trunk of a tree. These 
markings are not always presented in 
a decided.manner in human dentine, but 
they constitute quite a marked and 
frequently beautiful feature in the teeth 
of some of the lower animals. In addition to the peculiarities just re- 
ferred to, which are in every sense a normal condition, certain cavities or 
spaces, to which the term inter-globular spaces has been applied, are 
also observable. These 
spaces, which are most 
numerous in the vicinity 
of the. enamel, although 
they occur in other por- 
tions of the dentine, are 
said by Kolliker not to 
be filled with fluid during 
life, but by a "soft sub- 
stance, resembling tooth 
cartilage." This dries up 
in microscopical sections, 
and cavities are formed, 
which give the appearance denominated inter-globular spaces. Although 
this cannot always be regarded as an abnormal condition, still in defective 
Fig. 4. 
VVJVP CkVVVY 
Fig. 5. 10 
teeth these cavities are larger and much more numerous than in perfect 
teeth. 
Inter-tubular tissue.-This was spoken of in the preceding part of 
the address as a connective matrix between the dentinal tubuli. It con- 
tains the greater part of the earthy constituents of dentine, and under a 
high power of the microscope presents a granular appearance. The for- 
mation of the contour markings and the inter-globular spaces of course 
involves the inter-tubular tissue as well as the dentinal tubuli. 
According to Von Bibra, the chemical proportions of dentine in man 
are- 
Organic Substance, 28-01. 
Inorganic Substance, ir99. 
Enamel.-This substance, which is the hardest of all organic struc- 
tures, covers and protects the dentine to the neck of the tooth, being 
thicker at the grinding surface, and gradually tapering to a thin edge at 
the neck. A calcified structural substance, called Nasmyth's membrane, 
is found covering the enamel for a short time after the eruption of a tooth, 
but is eventually worn away. The enamel is translucent rather than white, 
and although extremely hard is frequently worn away by protracted use. It 
is composed of solid hexagonal fibres or prisms, 
about of an inch in diameter, arranged in 
close proximity to each other, and fitting at their 
inner extremities into hexagonal depressions upon 
the coronal surface of the dentine. As you ob- 
serve in this drawing, the fibres marked at short 
intervals by dark transverse lines, run in a gently 
waving manner parallel to each other. They are 
nearly vertical when passing toward the grinding 
surface of the tooth, and horizontal on the sides. 
To compensate for the increased extent of surface 
at the periphery of the enamel, the diameter of the 
outer ends of the fibres is somewhat larger than 
the inner, and still further to secure a perfect struc- 
ture, short enamel fibres are interposed in what 
would otherwise be vacant spaces. In young teeth, 
Tomes states that canals exist in the enamel prisms. 
In addition to this, fissures frequently occur in the 
enamel, particularly in the depressions or crevices 
between the cusps of the bicuspids and molars, and 
extend down to the dentine. When existing, these 
fissures constitute a decided predisposing cause of 
Fig. 6. 
NhSNWTH'S VAENV&RANt 11 
decay in the teeth. The dentinal canals sometimes extend into the enamel, 
and terminate in somewhat expanded cavities. 
Concentric ridges or furrows may be readily distinguished, and fre- 
quently even without the aid of a magnifying glass, upon the external 
surface of the enamel, particularly in deciduous teeth. Their presence is 
to be accounted for on the same principle as the contour lines of dentine. 
When subjected to the action of dilute acids, the enamel is almost en- 
tirely dissolved, and scarcely a trace of animal matter is discernible. 
According to Von Bibra, the chemical composition of enamel is- 
Organic Substances, 3'5. 
Inorganic Substances, 96'5. 
Cementum.-This substance, which invests the roots of human teeth 
from the termination of the enamel to the apex of the root, where it is 
usually very thick, approximates to bone, not only in microscopical struc- 
ture, but also in chemical composition. The Haversian canals, peculiar 
to bone, are rarely seen in cementum,' except in hypertrophied con- 
ditions of the structure, but lacunae and canaliculi are always present 
(though sparingly), and well marked, as in the drawing (see Fig. 3, page 
7), and resemble in every particular the lacunae and canaliculi of bone. 
They are placed lengthwise around the fang, those in proximity to the 
dentine joining with the terminal branches of the dentinal tubuli, while 
those upon the external surface radiate toward the periodontal membrane. 
By this arrangement, even after the extirpation of the dental pulp, the 
vitality not only of the cementum but also of the dentine of the root is 
maintained. As age advances, or from exciting causes in adult life, the 
cementum frequently becomes hypertrophied, and constitutes what is 
called ezrosiosis of the roots. The chemical composition of human ce- 
mentum, according to Von Bibra, is- 
Organic Substances, 29'42. 
Inorganic Substances, 70'58. 
OSTEO-DENTINE of OWEN, OF the SECONDARY DENTINE OF TOMES.- 
This structure is frequently found in the pulp cavities of human teeth, 
either coalescing with the previously formed dentine, or as separate and 
distinct nodules; and in some instances the pulp cavity is occupied by 
an entirely calcified pulp, which has no connection with the walls of the 
cavity, and with proper management can be removed entire from its posi- 
tion. Under the microscope, this substance presents the lacunae and 
canaliculi of bone and cementum, and in some rare instances the Haver- 
sian canals are also observable. 
The different microscopical structures which have engaged our atten- 12 
tion as presented in the human teeth are arranged upon a different plan 
in many of the lower animals. We have found, for instance, in making a 
vertical section of a human tooth, that the dentine of the crown is covered 
entirely by enamel, while that of the root is invested by the cementum, 
and this is true of the Quadrumana, and nearly all the Carnivora; but in 
the Herbivora, the Pachydermata, Ruminantia, Rodentia, etc., on making 
a transverse section of the crown of their teeth, the enamel, cementum, 
and dentine are found upon the same plane. By this arrangement an 
admirable provision is made for the thorough comminution of the food 
upon which such animals subsist. Consisting as it does of grass, hay, 
or grain, considerable trituration or grinding is necessary, and owing to 
the presence of large quantities of silica, which is extremely hard, the 
teeth wear away rapidly, and on account of the different degrees of den- 
sity of the various tissues, do so in an unequal manner; the cementum 
being the softest, wears most rapidly, the dentine next, and enamel the 
slowest of all. In this way an uneven and roughened exterior is presented 
by the masticating surface of the molars, like that in the opposing sur- 
faces of the upper and nether stones in a grist-mill. 
Much more could be said in connection with this interesting subject, 
for it has only been touched upon by me in the most general and element- 
ary manner, but I will not trespass further upon your patience, as a con- 
siderable portion of your time has already been occupied.