^ REESE LIBRARY

OF TIIK

UNIVERSITY OF CALIFORNIA

' / , 188
J

Shelf ATo.

Accessions No.

A MANUAL

OF

DENTAL ANATOMY

. li3f?7'7>v

A MANUAL

OP

DENTAL ANATOMY

HUMAN AND COMPARATIVE

BY

CHARLES S. TOMES, M.A., F.R.S.

WITH 191 ILLUSTRATIONS

SECOND EDITION

LONDON
J. & A. CHURCHILL

NEW BURLINGTON STREET

1882

BIOLOGT
LIBRARY

LONDON :
BBADBURY. AONEW, & CO. PRINTERS, WHITEFRIARS

PEEFACE.

IN introducing a Second Edition of this Manual but
few words are necessary. It has been entirely revised,
some portions, notably the chapter on the dental tissues,
have been in great part rewritten, and many new illus-
trations have been added here and elsewhere in the
book.

My indebtedness to Professor Owen's " Odonto-
graphy," to my father's "Dental Surgery," and to
his " Lectures on Dental Physiology and Surgery," to
Professor Flower's " Lectures on the Teeth " (published
in the British Medical Journal, 1871), to Kolliker's
and to Strieker's Histologies, I will again acknow-
ledge, since it is both impossible and undesirable to
encumber a student's book with references to au-
thorities for every statement. And if I have any-
where failed to give due acknowledgment to another
whose writings I may have made use of, I crave forgive-
ness for my omission. I gladly embrace this opportunity

PREFACE

of expressing my thanks to Professor Hollaender for the
honour he has done to my book in rendering it into
German, and to Dr. Cruet for translating it into
French ; and at the same time I gratefully acknow-
ledge the kindness of many of my friends who have sent
me corrections which they have noted as heing required
in the text of the first edition.

CHARLES S. TOMES.

37, CAVENDISH SQUARE,
December, 1881.

TABLE OF CONTENTS.

CHAPTER I.

PAGE

THE NATURE OP TEETH — DESCRIPTION OF THE TKETH OP MAN . 1

CHAPTER II.
THE MAXILLARY BONES, AND ASSOCIATED PARTS . . . . 23

CHAPTER III.

THE DENTAL TISSUES : ENAMEL, DENTINE, CEMENTUM, TOOTH

PULP, &c. . . . . . . . . . .40

CHAPTER IY.

THE DEVELOPMENT OP THE TEETH — IN FISH — IN REPTILES — IN

MAMMALS — CALCIFICATION OP THE DENTAL TISSUES . . .115

CHAPTER V.

THE DEVELOPMENT OP THE JAWS AND THE ERUPTION AND ATTACH-
MENT OP THE TEETH 176

CHAPTER VI.

THE TEETH OF FISHES . 214

CONTENTS.

CHAPTER VII.

PACK

THE TEETH OF BATRACHIA AND REPTILIA 23!)

CHAPTER VIII.

THE TEETH OF MAMMALS — INTRODUCTORY REMARKS — HOMOLOGIES

OF THE TEETH— MILK DENTITION .... . 264

CHAPTER IX.

THB TEETH OF MONOTREMATA, EDENTATA, AND CETACEA . .301

CHAPTER X.
THE TEETH OF UNGULATA . . . . . , . . 314

CHAPTER XI.
THK TEETH OF SIRENIA, HYRACOIDEA, PROBOSCIDEA, AND RODENTIA 344

CHAPTER XII.
Tns TEETH OF CARNIVORA .' . * 374

CHAPTER XIII.
THE TEETH OF INSECTIVORA, CHIROPTERA, AND PRIMATES . . 394

CHAPTER XIV.
THB TKSTH OF MARSUPIALIA . *•"'.•'• . ,i . 419

MANUAL OF DENTAL ANATOMY

HUMAN AND COMPAKATIVE.

CHAPTER I.

THE TEETH OF MAN.

THE range of the subject of Dental Anatomy turns upon
the meaning which is attached to the word " Tooth ;" but,
although this chapter might most appropriately open with
a definition of this word, it is very much easier to explain
what is ordinarily understood by it, than to frame any
single sentence which shall fulfil the requirements of logical
definition.X Most vertebrate and a great many invertebrate
animals have certain hard masses in or near to the orifice
of the alimentary canal, i.e.y the mouth ; by these hard
masses, sometimes of bony and sometimes of horny nature,
various offices in connection with the prehension or com-
minution of food are performed, and to them the term
" teeth " is applied. In many animals teeth have come to
be used for other purposes, such as for sexual warfare j but
it can hardly be doubted that teeth have primarily to do
with the nourishment of their possessor/^

The subject of the homologies of the teeth cannot be fully
entered upon until the details of their development have

2 A MANUAL OF DENTAL ANATOMY.

been mastered ; still a few words may even at the outset be
devoted to the elucidation of their real nature.
. The mucous membrane which lines the alimentary canal
is continuous with — is, indeed, a part of — the external skin,
with which it blends at the lips. Now if a young dog-fish,
just about to be hatched, be examined, it will be found that
it has no distinct under lip, but that its skin turns in over
its rounded jaw without interruption. The skin outside
carries spines (placoid scales)^1) and these spines are con-
tinued over that part of it which enters the mouth and
bends over the jaws ; only they are a little larger in this
latter position. If the growth of the dog-fish be followed,
these spines of the skin which cover the jaws become deve-
loped to a far greater size than those outside, and the identity
and continuity of the two become to some extent masked.
No one can doubt, whether from the comparison of adult
forms or from a 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. This is what is meant when teeth are called
" dermal appendages," and are said to be perfectly distinct
from the internal bony skeleton of the animal ; the teeth of
the shark (and of many other creatures) remain imbedded in
tough mucous membrane, and never acquire any connection
with the bone. Indeed, all teeth alike are developed from a
part of the mucous membrane, and any connection which
they may ultimately get with the bone is a secondary
matter. As it has been well expressed by Dr. Harrison Allen

(J) "The placoid scale has the structure of dentine ; is covered by
enamel, and is continued at its base into a plate formed of osseous tissue. "
Gegenbaur's Comparative Anatomy, translated by F. Jeffery Bell, p. 424.

THE TEETH OF MAN.

('Anatomy of the Facial Region'), "if the hairs of the
scalp were to be inserted into the skull, or of the moustache
into the upper jaw, we should express great astonishment,
yet such an extreme proposition is no more remarkable than
what is seen to take place in the jaws," again " the feathers of
certain birds making impressions on the radius, the whalebone
pendent from the roof of the mouth, are examples of this
same association of tegumentary appendages with the
bones."

In their simpler forms, then, teeth are met with as very
numerous spines, differing but little from the spines of the
skin except in size, and still less from one another. In
many fish the teeth, though more specialised, are scattered
over almost every one of the numerous bones which form
part of the walls of the mouth and pharynx ; in reptiles
they are much more limited in position, and in mammals
are absolutely confined to the intermaxillary, maxillary,
and mandibular (lower maxillary) bones. In fish and rep-
tiles it is the exception for the teeth in different parts of
the mouth to differ markedly from each other; in mam-
mals it is the rule.

Teeth owe their hardness to an impregnation with salts
of lime ; the organic matrix may be of albuminoid charac-
ter, in which case the tooth is of horny consistence, and is
spoken of as " cornified ; " or the matrix may be, like that
of bone, gelatigenous, in which case the tooth is more richly
impregnated with salts, and is spoken of as " calcified."

The great mass of a calcified tooth is usually made up of
"dentine," which gives to it its characteristic form, and
often practically constitutes the whole tooth : to this may
or may not be added enamel and cementum.

Without further prelude we may pass to a description of
the human teeth, this course appearing to me, after some
little consideration, to afford to the student the most ad-

B 2

4 A MANUAL OF DENTAL ANATOMY.

vantageous introduction to the subject, as he must neces-
sarily already possess some knowledge of their forms, while
to the matters alluded to in the preceding pages more full
reference will be made hereafter.

XIn the human subject no tooth rises above the level of
its fellows, and the teeth are arranged in close contact, with
no interspaces between them. The teeth are ranged around
the margins of the jaws in a parabolic curve, or something
approximating to one ; in the lower races of mankind the
curve tends to a squarish, oblong form, owing to the
prominence of the canines (compare the figure of the denti-
tion of Simia Satyrus), whilst a deviation in the opposite
direction is daily becoming more common in the most
highly civilised races, resulting in a contour to which in
extreme cases the name of V-shaped maxilla is applied.

It may be stated, as generally true, that the teeth are
somewhat larger on their labial than on their lingual aspect,
a result which necessarily follows from their standing with-
out interspaces along a curved line. And as great variations
in size and shape, as well as in colour, are found to exist
between different individuals, it is only possible to give
such a description as shall apply to the generality of teeth.

The teeth of the upper jaw are ranged along a curve of
larger dimensions than those of the lower, the incisors pass-
ing in front of the corresponding lower teeth, and the ex-
ternal cusps of the bicuspids and molars closing outside
those of the lower teeth.

There are, however, some points of detail to be noted in
the relation borne by the upper to the lower teeth, besides,
that comprised in the general statement that the former lie
outside the latter, by which it is brought about that each
tooth is antagonised by portions of two teeth in the other
jaw, and has not only a single opponent.

The upper incisors and canines, when the mouth is closed,
from the larger size of the arch in which they are arranged,

THE TEETH OF MAN.

shut over and in front of the lower teeth, concealing the
upper thirds of their crowns ; while the-

of the bicuspids and molars_j3f_the lower jaw are received
into the depressions between the—external amir-internal
4fee similar teeth in the upper jaw, thus allow-

ing the--^xtHrliaIjLiibe£efeft*^fi the upper teeth to close ex-
ternally to the outer ttlfreTcles^ef the lower row.

From this arrangement of the tubercles, we are enabled
in mastication to use the whole surface of the crowns of the
opposing teeth ; the act of mastication being performed by
bringing tb^ external tubereles of- the under molars opposite
to those of the upper row ; W&&£B§, by the lateral motion
of the under jaw inwards, ffirif QTitaraal tubnrnlrrT^rr^^TTTrn
the inclined surfaces of the external,- and up those of tho
'ntnrnnl tnhnrfiltirirfthn njrpnr tiathj ftrrmftfigrim t1-1'-" n n^ —
any interposed substance.

It will also be observed that, from the difference of width
in the incisors of the two jaws, the central incisors of the
upper extend over the centrals and half of the laterals of
the under row, and that the superior laterals lie over the
remaining half of the inferior laterals and the anterior half
of the canines of the lower jaw. The canines close over the
halves of the canines and first bicuspids, while the first bi-
cuspids impinge on the half of the first and half of the second
bicuspids of the lower row. The second upper bicuspids
close upon the anterior third of the opposing first molars
and the posterior half of the second bicuspids.

The first molars oppose the posterior two thirds of the
first, and one third of the second molars of the lower jaw,
while the second upper molars close upon the unoccupied
posterior third of the second and the anterior third of the
wisdom teeth. The wisdom tooth of the upper being
smaller in size than that of the lower jaw is perfectly
opposed by that portion of the latter left unoccupied by
the second upper molar tooth.

6 A MANUAL OF DENTAL ANATOMY.

By this admirable arrangement no two teeth oppose each
other only, but each tooth in closure of the jaw impinges
upon two, so that should a tooth be lost, or even two alter-
nate teeth, still the corresponding teeth of the opposite jaw
are to some extent opposed, and thus remain useful. For
when a tooth is wholly unopposed, a process is apt to be
set up in the jaw by which the useless organ is gradually
ejected. The direction of the teeth in the upper is verti-
cally downwards and slightly forwards, while those of the
lower jaw are placed vertically, the molars tending slightly
inwards.

It is usual to represent the dentition of any animal by
what is termed a dental formula, which enables the reader
at a glance to see the number of teeth of each variety pos-
sessed by the creature. Vthus, instead of writing out at
length that man has two inc\sors on each side in both upper
and lower jaws, one canine, tw> bicuspids or premolars, &c.,
it is written thus : — \ / SHld 1>O?

T 2 1 z\ 3

I. — c. — prm. — m —

9 1 9 \ ^

or in the deciduous set : — \

I. 2 c. I dm. - ± 20.
21 2 A

For the purpose of description three parts of the tooth, ,
are distinguished by name, viz., the crown, neck, and root.)V

This distinction of parts which we make in describing
human teeth, when we speak of crown, neck, and root, is
applicable to the great majority of mammalian teeth,
though there are some few simple forms of teeth in which
no such differentiation of parts can be seen.

The crown is that portion which is exposed above the
borders of the gum, and is in human teeth coated with
enamel ; the neck is that portion which corresponds to the

THE TEETH OF MAN.

edge of the gum, and intervenes between the edges of the
bony sockets and the edge of the enamel ; the root is that
part which is enclosed within the bony socket, and is
covered by cementum. ^x"

Of these it is to be remarked that the " neck," although
a convenient and necessary term for descriptive purposes,
marks an arbitrary division of less importance than that
expressed by crown and root j also that although this divi-
sion into three parts can be made in the case of socketed
teeth of limited growth, no such distinction of parts can
be made in teeth of perpetual growth.

Special names have been applied to the various surfaces
of the crowns, as, owing to the curvature of the alveolar
border, terms which had reference to front, back, or sides
would, in different parts of the mouth, indicate different
surfaces, and so lead to confusion.

The lips and tongue and the median line of the mouth,
however, are not open to this objection, so the surfaces
which are directed outwards towards the lips are called
" labial ; " and those inwards towards the tongue " lingual ; "
the interstitial surfaces are called " median " and " distal,"
the word median being applied to the surface which would
look towards the middle line of the mouth had the alveolar
border been straightened out. In other words behind the
canine, the " median " is equivalent to anterior, and " distal "
to posterior surface.

Forms of the several Teeth. — It is usual to speak of
the teeth as being modified cones, and to attribute their
variations to deviations from this typical shape. In a broad
sense this may be true of the simplest teeth, such as are met
with in some fish and reptiles and monophyodont mammals,
which are little more than simple cones ; but there are in-
dications which would point to something more complex
than this as the fundamental form of a mammalian tooth,
for even among the monophyodonts, as I have elsewhere

8 A MANUAL OF DENTAL ANATOMY.

pointed out, the armadillo has a bilobed tooth germ, the
one cusp predominating over the other. But I do not think
that we have at present the data upon which to cer-
tainly determine the fundamental form of the mammalian
tooth.

There is evidence that all the teeth in the jaw of a
mammal may have been derived from a single form; in
other words, marked though the distinction between in-
cisors, canines, bicuspids, and molars seems to be at first
sight, a closer inspection reveals various gradational or
transitional characters linking them together, though there
are gaps in the chain not bridged over by forms known to us.
This may be seen by a careful study of the human teeth, as
I shall endeavour to show; but it is much more conspi-
cuously seen in an extinct animal (Homalodontotherium,
an extinct ungulate from Patagonia, described by Professor
Flower, Philos. Trans. 1874), which apparently possessed
the fall typical number of mammalian teeth, viz., forty-four.
The point in which its dentition is chiefly instructive is that
the teeth, in close juxtaposition one with another, present
an exceedingly perfect gradation of form from the front to
the back of the mouth, no tooth differing markedly from
its neighbour, though the difference between, say, the first
incisor and first molar, is exceedingly great. In Professor
Flower's words, " it is only by the analogy of other forms
that they can be separated into the groups convenient for
descriptive purposes, designated as incisors, canines, premo-
lars, and molars."

In viewing the gradational characters which do exist be-
tween the various human teeth, it must not be forgotten
that some links in the chain have dropped out and are
absent. Mention has already been made of the full typical
number of mammalian teeth being 44, i.e.

I. c. Imxn, = 44.

THE TEETH OF MAN.

The human subject does not possess the third incisor, nor
the first two premolars, so that a somewhat abrupt change
of form in passing from the incisors to the canines, and
from the latter to the bicuspids, is no more than might be

/ anticipated.

/N Incisors. — Of these there are four in each jaw ; two cen-
tral, two lateral incisors. Their working surfaces form
wedges, or obtuse and blunt-edged chisels, calculated to
divide food of moderate consistency.

Upper Incisors. — The centrals are very much larger
than the laterals, and viewed either from the back or front
taper with some regularity from the cutting edge to the
point of the root, the neck not being marked by strong con-
striction. The crown of the tooth, as seen from the front,
is squarish, or more strictly, oblong, its length being
greater than its breadth.

The median side, by which it is in contact with its fellow,

FIG. 1 (').

is a little longer than the distal, so that the median angle of
the crown is a little lower, and, as a necessary consequence,
a little more acute than the distal angle of the cutting edge.
Near to its base the crowns narrow rather abruptly, so that
near to the neck a space is left between the contiguous
teeth.

(T) Front and side view of a left upper central incisor.
a Distal surface. & Neck. c Root.

10 A MANUAL OF DENTAL ANATOMY.

The labial surface is slightly convex in each direction,
and often presents slight longitudinal depressions, which end
at the cutting edge in slight notches.

In recently-cut teeth the thin cutting edge is elevated
into three slight cusps, which soon wear down and disappear
after the tooth has been in use.

The edge of an incisor may be regarded as formed by the
bevelling off of the dentine of the lingual surface, which is
nearly flat from side to side, with a slight tendency to con-
cavity, while from above downwards it is distinctly concave,
and often presents longitudinal depressions similar to those
on the labial surface. The lingual surface towards the gum
terminates in a distinct prominence, oftentimes amounting
to a bounding ring of enamel, termed the basal ridye, or, in
the language of comparative anatomy, the cingulum. It is
variable in the extent of its development ; it rarely rises
into a central prominence at the back, but in the angle
where the ridges of the two sides meet a deep pit is often
left in the enamel, which is a favourite site for caries. The
crown, or what amounts to the same thing, the enamel, ter-
minates on the lingual and labial aspect of the tooth in a
curved line, the convexity of the curve being directed
upwards towards the gum; on the insterstitial surfaces,
both median and distal, the curve is less regular, and its
contour would be more correctly described as V-shaped, the
apex of the V being towards the crown of the tooth and
away from the gum. ^ The dentist will do well to remember
the disposition of the enamel in this situation, as it is a
point of some importance in shaping the cervical edge of a
cavity preparatory to filling it.

The transverse indentations of the enamel met with both

on lingual and labial surfaces, though more especially in the

latter, are marks of arrest of development, and, common as

they are, are to be regarded as abnormalities.

ps The central incisors are larger than the laterals, though

THE TEETH OF MAN. 11

not in so great degree as is the case in the . anthropoid
apes.

The pulp cavity bears a general resemblance to the ex-
ternal contour of the tooth ; towards the cutting edge it is
very thin, and is prolonged at its two corners to a slight
extent into "cornua;" at the neck it is cylindrical, and is
also cylindrical in the root, tapering gradually till it ap-
proaches close to the apex, when it becomes suddenly con-
stricted.

Upper lateral incisors are in every dimension some-
what smaller than the centrals. They widen somewhat
abruptly near to the cutting edge, but below this they taper
pretty regularly to the end of the root ; the labial surface

FIQ. 2 1.

is convex in each direction, while the lingual surface is
perhaps rather flatter than that of a central incisor.

The outer (distal) angle of the crown is far more rounded
or sloped away than in the centrals, and the distal surface,
looking towards the canine, is in a slight degree convex;
the median surface may be slightly concave.

The enamel terminates towards the gum in contours pre-
cisely similar to those which obtain in the centrals : but
the basal ridge, or cingulum, is often more strongly pro-
nounced, and the presence of a central tubercle upon it is
less infrequent. From this greater prominence of the cin-

(J) Front and side view of a left upper lateral incisor.

12 A MANUAL OF DENTAL ANATOMY.

gulum and consequent more marked depression in front of
it, caries is more frequent upon the lingual surfaces of upper
lateral than upon those of upper central incisors.

The pulp cavity is, relatively to the whole tooth, perhaps
a little larger than in the central incisors j in other respects
the same description will suffice.

Lower central incisors are very much narrower than
those of the upper jaw j not more than half the width at
their cutting edges, which again are much wider than the
necks of the teeth.

From before backwards they are deep at the neck ; hence
the fangs are very much flattened from side to side, and
rotation is inadmissible in the attempt to extract them.

The enamel contour at the neck is similar to that of the
upper incisors, but there is no well-marked cingulum.

FIG. 3 (J).

Lower lateral incisors are, unlike the upper teeth, dis-
tinctly larger than the centrals in each one of their dimen-
sions, but more especially in the length of their fangs,
which are much flattened, and often present on their sides
a median longitudinal depression, sometimes amounting to
an actual groove.

The distal angle of the crown is rounded off like that of
the upper lateral incisors, though not so markedly.

Canines, Cuspidati, Eye Teeth, are, in all respects,

(*) Front and side view of lower central incisor.

THE TEETH OF MAN. 13

stouter teeth than the incisors ; not only are the crowns
thicker and stronger, but the roots are very much longer.

The crown terminates in a blunt point, which lies in a
straight line with the long axis of the root ; a feebly pro-
nounced line or ridge runs down the outer surface of the
tooth from this point to the neck. The crown slopes away
both before and behind the point or cusp, but as that side
of the tooth which lies next to the bicuspid is convex, and
as it were produced towards that tooth, the slope is longer
on the distal than on the mesial half of the crown. The
crown thus not being perfectly symmetrical renders it easy
to determine at a glance to which side of the mouth the
canine belongs.

The internal or lingual surface is not concave like that of
the incisors, but is in a slight degree convex, and a median
ridge runs down it from the apex of the cusp ; this ridge

FIG. 4 (J).

where it meets with the ridge which borders the lingual
surface and corresponds with the cinguluni of the incisor
teeth, is often developed into a well-marked prominence or
cusp.

In transverse section the neck is nearly triangular, the
outer or labial being much wider than the lingual aspect.

(!) Lingual, labial, and distal surfaces of an upper canine, showing the
basal cusp and the three ridges which converge towards it.

14 A MANUAL OF DENTAL ANATOMY.

Lower canines are less pronounced in form than the cor-
responding upper teeth : the point is more blunted, the
fang shorter, the perpendicular labial ridge not being
traceable, and the want of symmetry between the mesial
and distal halves of the crown less marked. The lingual
surface has perhaps a greater tendency to concavity.

Fremolars, Bicuspids, are eight in number, two on each
side of both upper and lower jaws, and they correspond to
the third and fourth premolars of the typical mammalian
dentition, the first and second premolars not being re-
presented in man.

Upper Premolars. — The crown, as seen looking upon its
grinding surface, is roughly quadrilateral, its outer or
lingual border being, however, larger and thicker than its
inner, and the teeth are carried round the curve of the

FIG. 5 0).

alveolar border mainly by means of this difference in size
in the external and internal portions of the canines and the
two bicuspids.

As is implied by its name, the crown has two cusps, of
which the outer is the larger and stouter, and broader.
The outer and inner surfaces (labial and lingual) are convex
and smooth, with no basal ridges at the edge of the gums.
The inner and outer cusps are not joined by a transverse
ridge ; instead of this there is a deep transverse fissure ; in
point of fact the cingulum has been elevated to form the
inner cusp, and forms slight elevations bordering the anterior

(*) Grinding surface of an upper bicuspid.

THE TEETH OF MAN. 15

and posterior (mesial and distal) edges of the grinding
surface.

The root is single, and much compressed from side to
side : very often, however, it is double for the greater part
of its length, and if not so divided is often marked by a
groove upon each side indicating a tendency towards such
division. The outer border of the root is also often marked
by a longitudinal furrow, which may amount to complete
division. In fact a bicuspid may have three perfectly dis-
tinct roots, like a molar, or it may have any form of root
intermediate between this and its typical single laterally-
flattened root. The first bicuspid is more variable in respect
of its roots than the second.

The second upper bicuspid differs from the first in that
the difference in size between its outer and inner cusps is
less, the inner cusp being relatively considerably larger,
and, indeed, often preponderating over the labial cusp in
length.

The pulp cavity in the crown is furnished with distinct
cornua ; at the neck it is very much flattened from side to
side, being often reduced to a mere fissure, which is how-
ever considerably larger at its two extremities than in its
middle. Hence the pulp cavity of an upper bicuspid is
difficult to fill j a difficulty again increased by the impos-
sibility of always discovering what number of fangs it has,
as their division sometimes takes place rather high up.

Lower premolars are smaller teeth than those of the
upper jaw, and are quite distinct in shape. The outer or
labial cusp is bent inwards, and the labial surface of the
crown is very convex. The inner cusp is but feebly deve-
loped, and is connected with the outer by a low ridge ; it is
also narrow.

The root is rounded, a little larger on its outer side than
on its inner, and tapers regularly towards its point ; the
pulp cavity is cylindrical at the neck, and also tapers regu-

16 A MANUAL OF DENTAL ANATOMY.

larly in the root. The cornu of the pulp which corresponds
to the inner cusp is but feebly developed.

The second lower bicuspid differs a good deal from the
first ; its crown is much squarer and larger in all its dimen-
sions. The inner cusp reaches to a higher level and is
stouter, and the greater development of the ridge which
bounds the posterior (distal) border of the grinding surface
makes it attain to such a large size as to make the tendency
towards a transition from the bicuspid type to the quadri-
cuspid type of a true molar very evident.

Having completed the brief description of the forms of
these several teeth, it is worth while to note one or two
general characters of the series. The differences between a
well-marked incisor, canine, or premolar are so strongly
pronounced that the resemblances which underlie them are
apt to be overlooked, and it might be supposed that in shape
they had little in common.

Nevertheless a very distinct gradation may be traced, and

FIG. 6 O.

it is far from uncommon to meet with teeth which possess
in a marked degree transitional characters. If the external
or distal angle of a lateral incisor be sloped off more than
usual, while at the same time its cingulum and basal pro-
minence be well marked, it makes no bad imitation of a

(:) Lower first bicuspid, seen from the inner side, and showing the pre-
ponderance of its outer over its inner cusp.

THE TEETH OF MAN. 17

diminutive canine ; and such laterals are often to be met
with by any who search for such deviations from the normal
form.

Thus the form characteristic of a lateral incisor, if it be a
little exaggerated, very nearly gives us the form of a canine,
and if we look at the teeth of an Orang the lateral incisor is
to all intents a diminutive canine ; and in the present dis-
cussion the great comparative size of the canine, which is
traceable to readily intelligible causes, may be put aside, as
it tends to obscure the point to be here insisted on.

Between the canines and the bicuspids a similar relation-
ship in form exists, and it is more apparent in the lower
than in the upper jaw. The fact that at the base of the
inner or lingual aspect of the canine is to be found an
elevation of the cingulum, in many instances amounting to

FIG.

a low cusp, has been already noted ; and it has already been
pointed out that the inner cusp of the first lower bicuspid is
both smaller and lower than the outer. A longitudinal
section through the crowns of the two teeth will demon-
strate without the necessity of further description that the

Section of a lower canine and first bicuspid, showing the characters
common to the two.

18 A MANUAL OF DENTAL ANATOMY.

basal cusp of the canine and the inner cusp of the bicuspid
are the same thing, differing only in degree, while it is
interesting to note that the pulp chamber in the bicuspid
has hardly any prolongation towards the small inner cusp,
so that the resemblance between the two teeth is thus made
more complete.

This close relationship of canines and bicuspids will be
again considered in the chapter on the Homologies of the
Teeth ; for our present purpose it will suffice to merely point
out its existence. The transition from the bicuspids to the
molars is more abrupt ; at least it is not so easy to point
out exactly how a modification of the one would arrive at
the form of the other. But it merely needs an exaggera-
tion of the differences existing between a canine and a first
bicuspid to make a good imitation of a second bicuspid.

If any one will take the trouble to make mental note of
the deviation in form which he meets with in teeth, he will
find that they almost invariably consist of approaches to-
wards the form of the teeth on either side of them ; and
will infallibly be led to the conclusion that incisors, canines,
and bicuspids are not three patterns of teeth perfectly
distinct, and each sui generis, but that they are modifications
of one and the same pattern. I may add, that comparative
odontology teaches us the same thing, and demonstrates
clearly the substantial identity of the three forms, as also
of the true molars.

Upper molar teeth have crowns of squarish form, the
angles being much rounded off. It may be premised that
the first molar is more constant in shape than the second,
and this latter than the third ; with this proviso the first
and second may be described together.

The masticating surface carries four subequal cusps, two
labial or external and two lingual or internal ; the anterior
internal cusp is distinctly the largest, and it is connected
with the posterior external cusp by a thick oblique ridge

THE TEETH OF MAN. 19

of enamel, the remaining two cusps having no such con
nection.

This oblique ridge on the upper molars is met with in
man, the anthropoid apes, and certain New World monkeys.

The grooves which separate the cusps pass down on to
the labial and lingual surfaces of the crown, but are lost
before reaching the gum ; where they terminate, however,
there is often a pit, which is a very favourite situation for

FIG.

caries, especially on the labial aspect of the teeth. It is
very rare to see the grooves passing down upon the mesial
or distal surfaces of the crown, a raised border of enamel
generally cutting them short in this direction.

The roots are three in number, two external or labial, and
one internal or palatal. The latter is the largest, and runs
in a direction more strongly divergent from the axis of the
crown than the other roots. It is directed obliquely in-
wards towards the roof of the palate, is subcylindrical, and
often curved.

The external roots are less cylindrical, being mutually
compressed, so that their largest diameter is transverse to
the dental arch ; the anterior is rather the larger of the two,
and is more strongly pronounced on the side of the neck of
the tooth. The anterior labial root is occasionally confluent

(*) Masticating surface of a first upper molar of the left side ; the
oblique ridge connects the anterior internal with the posterior external
cusp.

c 2

20 A MANUAL OF DENTAL ANATOMY.

with the palatine root, but still more frequently the pos-
terior labial and palatine roots are confluent : occasionally,
also, four distinct roots may be met with.

Lower molars. — The first lower molar is the most con-
stant in form, and is somewhat the largest; its grinding
surface presents five cusps.

Four cusps are placed regularly at the four corners of a
square, these being divided from one another by a crucial
fissure ; the posterior arm of the crucial fissure bifurcates,
and between its diverging arms is the fifth cusp, which is
thus to be described as median and posterior.

FIG. 9

The transverse fissure passes over the limits of the grind-
ing surface, and on the outside or labial surface of the tooth
ends in a pit, which is a common site for caries ; although it
occasionally passes over the lingual surface, it is here less
pronounced. They are implanted by two fangs, placed
anteriorly and posteriorly ; the roots are much flattened
from before backwards, and they are very usually curved
slightly backwards. In the median line of each root there
is usually a groove, by the deepening of which four fangs
may be produced ; or this may happen with the one root
only, so that a three rooted tooth is the result.

: -(l) Masticating surface of a first lower molar, right side, tlie five cusps
of which are indicated by figures.

THE TEETH OF MAN. 21

The second molar does not greatly differ from the first
save that the roots are more often confluent, and the fifth
cusp less marked, even if it exists at all.

FIG. 10

Third molars, denies sapientice, wisdom teeth, of the upper
jaw, resemble in a general way the first and second molars ;
that is, when they are well developed and placed in a roomy
dental arch. But amongst more civilised races it may
almost be said to be exceptional for the wisdom teeth to be
regular either in form or position, so that extreme variability
prevails among these teeth.

The two inner tubercles are often blended together and
the roots confluent, forming an abruptly tapering cone, the
apex of which is often bent and crooked, so that but little
vestige of the three roots can be traced, the pulp cavity even
being quite single.

Third lower molar. — This tooth is seldom so small as
the corresponding upper tooth, and its crown is often large
even when its roots are very stunted. It has five cusps as
a rule, and bears a more or less close resemblance to the
molars which precede it. It is either two-rooted, or if the
roots be confluent, a groove usually marks a tendency to
division into two fangs.

It is stated by Prof. Owen (" Odontography," page 454)
that although the wisdom tooth is the smallest of the three
molars, the difference is less marked in the Melanian than in

(]) Second lower molar of right side, the four cusps being indicated by
figures.

22 A MANUAL OF DENTAL ANATOMY.

the Caucasian races, adding also that the triple implantation
of the upper and the double implantation of the lower is
constant in the former races. More extended observations
have overthrown this statement as a positive dictum to be
accepted without exceptions, but it may nevertheless be
taken as expressing a general truth.

FIG. 11 0.

The milk teeth differ from the permanent teeth by being
smaller, and having the enamel terminating at the neck
with a thick edge, so that the neck is more distinctly con-
stricted. The incisors and canines are somewhat similar to
their successors, the canines, however, being relatively shorter
and broader than their successors. The first upper molars
have three cusps, two external and one internal : the second
more nearly resemble the permanent molars.

The second lower deciduous molar has four cusps and
resembles a second lower permanent molar. The roots of
the deciduous teeth diverge from the neck at greater angles
than those of permanent teeth, in consequence of their more
or less completely enclosing between them the crypts in
which the latter are developing.

(l) Third lower molar of the left side.

CHAPTER II.

THE MAXILLARY BONES.

THE teeth are implanted in a part of the jaw bones
specially developed for the purpose, the bone being moulded
around the roots of the teeth subsequently to their being-
formed arid moved into position.

The manner of attachment of the human teeth is that
termed " gomphosis," i.e.t an attachment comparable to the
fitting of a peg into a hole ; the bony sockets, however,
allow of a considerable degree of motion, as may be seen by
examining the teeth in a dried skull, the fitting being in
the fresh state completed by the interposition of the dense
periosteum of the socket. This latter, by its elasticity,
allows of a small degree of motion in the tooth, and so
doubtless diminishes the shock which would be occasioned
by mastication were the teeth perfectly immovable and
without a yielding lining within their bony sockets. When
this becomes inflamed and swollen by exudation the tooth is
pushed to a certain extent out of the socket, and so being
to a less extent limited in its range by the bony socket,
acquires an increased mobility.

The teeth are in all mammalia confined to the bones which
carry them in man, namely, the intermaxillary and maxillary
bones and the lower maxillary bone or mandible.

24 A MANUAL OF DENTAL ANATOMY.

While full description of these bones (!) will be found in any
general anatomical work, there are so many points in their
anatomy which directly concern the dental student that a
brief enumeration of some of their relations can hardly be
dispensed with.

Superior maxillary bone. — To facilitate description of
its parts, anatomists divide it into a "body" and "pro-
cesses," of which latter there are four, the nasal, malar,
alveolar, and palatine. As the body of the bone is hollowed
out by an air cavity, the antrum, its shape is similar to that
of that cavity, namely, roughly pyramidal, the base of the
pyramid being inwards towards the nasal chamber.

The nasal process springs directly upwards from the body
in a vertical line with the canine tooth : it is a strong plate
of bone, roughly triangular when viewed from the side.

The malar process forms the apical portion of the pyramid
already alluded to ; it starts out nearly horizontally from
the body just behind and below the nasal process, and is
characterized by its great strength and stoutness. Never-
theless it has been known to be fractured by a blow, and
separated from the body of the bone. The antrum may be
prolonged into it.

The palatine process forms a horizontal table projecting
inwards from the body; as the floor of the nose is nearly flat,
and the palate is arched from before backwards, the front of
the palatine process is necessarily much thicker than the
back, where it is quite a thin plate.

The alveolar process is a strong wide ridge of bone, curved
so as to form with that of the other maxillary bone the
elliptical figure characteristic of the dental arch in the higher
races. It may be described as consisting of two plates, an
outer and an inner, which are connected by numerous trans-

(!) Much that is of great interest, and that is not to be found in text
books, is embodied in a series of papers on " The Facial Region," by Dr.
Harrison Allen (American Dental Cosmos, 1873-74).

THE MAXILLARY BONES.

25

verse septa, the sockets of the teeth being formed by the
interspaces between these septa. The internal alveolar plate
is the stronger, the external the thinner and weaker, a fact
of which we take advantage when we extract a tooth by
bending it slightly outwards. On the outer surface of the
alveolar process are eminences corresponding to the roots
of the teeth, and depressions in their interspaces, apt to be

FIG. 12 (').

especially marked over the canine teeth ; while between
the teeth the alveolar processes attain to a lower level, so
that the margins of the bone are festooned. Looking
down into an empty socket, the bone is seen to be every-
where very porous, and to bo perforated by foramina of
considerable size, while at the bottom there is the larger
foramen admitting the vessels and nerves of the tooth.

The alveolus of each individual tooth consists of a shell of
comparatively dense bone of small thickness, which is im-
bedded in a mass of loose spongy bone; this dense shell
comes into relation with the dense cortical bone of the jaw

(*) Superior maxillary bone of right side. 1. Body. 2. Tuberosity.
7. Malar process. 8. Nasal process. 12. Alveolar process.

26 A MANUAL OF DENTAL ANATOMY.

mainly at its free margin, near to the neck of the tooth.
Over very prominent roots a portion of alveolus is at times
wanting, so that in a macerated skull the root is exposed to
view.

The upper maxilla serves to give form and support to the
soft parts of the face, and also to carry the upper teeth.
These have to be rigidly fixed, while the teeth of the lower
jaw are brought forcibly against them with more or less of
shock. And whilst these blows have to be received, and
resisted, and ultimately borne by the cranium, it is obviously
desirable that they should be distributed over a sufficiently
wide area, so as not to be felt unpleasantly.

The ascending nasal process is very stout, and serves to
connect the maxilla strongly with the frontal bone, which
also in the region in question is powerfully developed ; the
thick malar process gives rigidity and resistance to lateral
movements of the jaws, and carries off the strains to the
lateral walls of the cranium ; it is buttressed at the back
by the pterygoid processes.

Taking next the various surfaces of the bone, there are
four, or, if we include the palatine aspect, five : the external,
forming a large part of the face, the superior or orbital, the
internal or nasal, and the posterior or zygomatic. Upon the
external or facial surface we have to note the eminence
caused by the socket of the canine tooth (" canine emi-
nence"), and immediately behind this a depression, the
canine fossa, through which the antrum is sometimes punc-
tured. The alveolar border, from the situation of the third
molar to that of the second bicuspid, gives attachment to
the buccinator muscle ; while immediately beneath the
margin of the orbit is the infra-orbital foramen, whence
issues the infra-orbital nerve ; hence this is one of the situa
tions to which neuralgic pain really dependent on the teeth
may be referred.

The orbital and nasal surfaces concern us only through

THE MAXILLARY BONES. 27

their relation to the antrum, to be presently described ; in
the zygomatic surface, which is convex and forms part of
the zygomatic fossa, are several orifices transmitting the
posterior dental nerves and vessels ; a groove which, con-
nected by the apposition of the palate bone into a canal,
forms the posterior palatine canal ; and at the bottom, a
rounded eminence, the maxillary tuberosity, which lies
behind the wisdom tooth, and has been occasionally broken
off in extracting that tooth.

The body of the bone is excavated by an air-chamber, the
antrum, which is coated in life by a continuation of the nasal
mucous membrane, and this frequently becomes secondarily
involved in dental disease, so that its anatomical relations
are of great importance to the dentist.

Like the somewhat similar air cavities in the frontal bone
the maxillary sinus does not attain to its full size, relatively
to the rest of the bone, until after the age of puberty,
although it makes its appearance earlier than the other
nasal sinuses, its presence being demonstrable about the
fifth month of foetal life. Hence it follows that its walls are
thicker in the young subject than in the adult; and, ac-
cording to the observations of Mr. Cattlin (*), it is somewhat
larger in the male than in the female.

It is very variable in size, so that out of one hundred
adult specimens the above-mentioned writer found one
which would only contain one drachm of fluid, while in
contrast with that was another which held eight drachms ;
two and a half drachms being the average capacity. Al-
though it is exceedingly variable in form as well as in size,
it tends towards a roughly pyramidal shape, the apex of the
pyramid being directed towards the malar bone, which it
has been seen to encroach upon, and the base towards the
nasal cavity; it is, however, useless to minutely describe

(*) "Transactions of the Odontological Society," vol. ii. 1857.

28 A MANUAL OF DENTAL ANATOMY.

its form, inasmuch as the two antra in the same individual
are sometimes quite dissimilar. The floor of the cavity is
rendered uneven in most specimens by prominences corre-
sponding to the roots of the molar teeth, which ordinarily
are but thinly covered by its bony walls, while it is not by
any means rare to find some of them actually bare.

The cavity is also more or less completely subdivided by
bony partitions springing from its walls, as is well exempli-
fied in the accompanying figure ; these partitions are for
the most part thin, but they occasionally attain to consider-
able thickness, and they are stated to occur most frequently
at the anterior or posterior angles of the base of the
pyramid.

On the base of the pyramid is the orifice by which it
opens into the middle meatus of the nose ; this orifice
being partly closed in by the ethmoid, palate, and inferior
turbinated bones, and also by soft parts, so that in a recent

FIG. 13 0).

subject it will barely admit a goosequill ; and it should be
noted that this orifice opens into the antrum near the top,
so that it does not afford a ready means of egress to fluids
accumulated in the cavity.

Through this orifice the mucous membrane lining the

(*) Section of an antrum of the left side, divided into many pouches,
by bony septa, and extending into the malar bone. Drawn from a
specimen in the collection of Dr. Maynard, in the possession of the Bal-
timore Dental College.

THE MAXILLARY BONES. 29

antrum is continuous with that of the nasal fossee, and,
like that, it is ciliated; but it differs from the latter in
being thinner and less vascular.

The teeth which usually come into the closest relation
with the antrum are the first and second molars, but any of
the teeth situated in the maxillary bone may encroach upon
its walls, and I have seen an abscess, originating at the apex
of the fang of a lateral incisor, pass backwards and perforate
the antrum.

Its walls have four aspects, namely, towards the orbit,
the nose, the zygomatic fossa, and the face, while its floor is
formed by the alveolar border. With the exception only of
the latter, its walls are very thin ; and this exception has
an important practical bearing in the diagnosis of tumors in
this region, as accumulations of fluid or morbid growths
really situated in the antrum bulge any or all of its walls
in preference to the alveolar border, whereas tumors spring-
ing from the base of the sphenoid or elsewhere and encroach-
ing upon the antrum, push down and distort the alveolar
border as easily as any of the other walls of the cavity,
inasmuch as the pressure caused by them is not transmitted
equally in all directions, as is the case when the medium
transmitting the power is a fluid.

The lower maxilla or mandible consists of a body and
two rami, which ascend almost perpendicularly from its
posterior extremity. The horizontal portion or body is
curved somewhat in a parabolic form ; it has a convex ex-
ternal and concave internal surface, and an upper (alveolar)
and a lower border. On the convex facial surface we have
to note the ridge marking the position of the symphysis,
and below this the mental prominence. Externally to this,
below the line of contact of the first and second bicuspids
(or a little before or behind this point) is the mental fora-
men, which constitutes the termination of the inferior
dental canal. Running obliquely upwards, and first visible

30 A MANUAL OF DENTAL ANATOMY.

at a point a little distance from the mental prominence is
the external oblique line, which becomes merged in the base
of the coronoid process. Where it rises as high as the
alveolar border, i.e., opposite to the third and sometimes the

FIG. 14 (l).

second molar, the outer alveolar plate is strengthened by it,
so that it becomes less yielding than the inner plate. The
student should bear this fact in mind when extracting a
lower wisdom tooth.

The buccinator is attached to the alveolar border oppo-
site to the molar teeth ; the platysma myoides to the outer
side of the lower border along a region somewhat further
forward : the masseter over the whole outer face and border
of the ascending ramus and the temporal to the apex and
side of the coronoid process. The other muscles attached
to it are facial muscles of expression.

On the inner surface of the body are four tubercles,
situated in pairs in the median line, about opposite to the
ends of the roots of the incisors, but somewhat variable both

(*) Lower Maxillary Bone. 2. Ramus, where masseter is attached.
3. Symphysis. 5. Mental foramen. 6. External oblique line. 8. Angle
of jaw. 9. Internal oblique line. 10. Coronoid process. 11. Condyle.
12. Sigmoid notch. 13. Inferior dental foramen.

THE MAXILLARY BONES. 31

in position and in size in different individuals. The upper
pair of tubercles give attachment to the genio-hyo-glossus,
the lower to the genio-hyoid muscles ; they are interesting to
the dental student not only as giving attachment to muscles
concerned in deglutition, but as affording convenient fixed
points for measurements of the relative growth of parts of
the jaw. Beneath these genioid tubercles lie the slight
depressions which give attachment to the anterior belly of
the digrastric muscle, while between the two points alluded
to commences the internal oblique line, which runs ob-
liquely upwards and backwards, becoming more pronounced
as it extends backwards, and terminating at the inferior
dental foramen. This internal oblique ridge marks the line
of growth of the condyle (see Development of the Jaws), and
gives attachment to the mylohyoid muscle, which forms the
floor of the mouth, in all its length. Thus the bone above
the ridge belongs strictly to the mouth, that below it has
more relation with cervical structures. The depression for
the sublingual gland is above this line, consequently this
gland is visible from the mouth ; that for the submaxillary
gland is beneath it and further back.

The inner surface of the ascending ramus gives attach-
ment to the following muscles : at the neck of the condyle
to the external pterygoid ; on the inner face of the coro-
noid process, as far down as the level of the top of the
crown of the wisdom tooth, to the temporal ; on the inner
side of the angle, over a large surface, to the internal
pterygoid.

The orifice of the inferior dental canal is rough and
spinous, giving attachment to the internal lateral ligament
of the jaw, while beneath and behind it is the groove for
the mylohyoid vessels and nerves ; the canal runs forward
in the bone a little distance beneath the ends of the roots of
the teeth, and emerges at the mental foramen, turning out-
wards at an angle to reach it, and sending onwards small

A MANUAL OF DENTAL ANATOMY.

canals to the incisors, not traceable far. It is nearer to the
outer than to the inner surface of the jaw in the latter half
of its course, and is apt to be very close to the ends of the
roots of the wisdom teeth, and to those of the bicuspids.
The alveolar processes of the lower jaw, at their posterior
part, diverge more widely than those of the upper jaw, the
relative antagonism between the upper and lower teeth
being preserved in this region b}' the former having an in-
clination outwards, the latter inwards. The ascending rami
join the body at an angle which is very obtuse in the foetus,
nearly a right angle in the adult, and once again obtuse in
advanced old age ; the explanation of this change will be
given under the head of the Development of the Jaw.

The articulation of the human lower jaw is peculiar, and
allows of a degree of play unusual in a joint. The ovoid
condyles, when the jaw is at rest, are lodged in depressions,
the glenoid fossae of the temporal bone, formed partly by the
squamous and partly by the vaginal portions of the bone.
The posterior half of the cavity is rough, and lodges a portion
vof the parotid gland : the anterior is smooth, and is bounded
in front by the eminentia articularis, which is the middle
root of the zygoma, enters into the formation of the joint,
and is coated over by cartilage. Between the condyle of the
lower jaw and the temporal bone lies a moveable inter-
articular Jibro -cartilage, which is an irregular bi-concave oval
plate, the edges of which are united with the capsular liga-
ment, so that the joint is divided into two cavities, furnished
with separate synovial membranes (unless when, as some-
times is the case, the fibro-cartilage is perforated in its
centre).

The joint is described as having four ligaments : the
capsular, stylo-maxillary, internal and external lateral
ligaments.

The capsular ligament is but feebly pronounced, and
hardly deserves the name ; the stylo-maxillary reaches from

THE MAXILLARY BONES. 33

the apex of the styloid process to the angle of the jaw ; the
internal lateral from the spine of the sphenoid to the mar-
gins of the inferior dental foramen; the external lateral,
which alone is a ligament strictly proper to the articulation,
reaches from the outer side and tubercle of the zygoma to
the outer surface of the neck of the condyle.

The form of the articulating surfaces and the compara-
tive absence of retaining ligaments combine to allow of a
variety of movement unusual in any other than a ball and
socket joint. The articulation acts as a simple hinge when
the jaw is simply depressed, and this is the only motion
possible in many animals, as in typical carnivora. When,
however, the mouth is opened to the fullest possible extent,
the condyle leaves the glenoid cavity, slides forward, and
rests on the articular eminence, the interarticular fibro-
cartilage being carried forward with it. The passage of the
condyle on to the articular eminence, although always
taking place when the lower jaw is excessively depressed,
takes place sometimes without any depression of the lower
jaw, which then passes horizontally forward ; or it may take
place on the one side only, giving to the jaw the lateral
movement so useful in mastication. In the mastication of
food the various movements are combined, or succeed one
another with great rapidity ; the lateral movements are not
very extensive, the outer cusps of the lower teeth of one
side being brought to antagonise the outer cusps of the
upper teeth, and then being made to slide forcibly down the
sloping surfaces of the latter till they return to their normal
antagonism ; when one set of muscles is tired the same pro-
cess is gone through on the other side of the mouth.

The closure of the jaw, and the rotatory and oblique
motions, are accomplished by four pairs of very powerful
muscles; these are antagonised by muscles comparatively/
feeble and indirect in their application.

The closure of the jaws is effected by the masseters and

34 A MANUAL OF DENTAL ANATOMY.

the temporals, attached to the outer sides of the jaw ; and
the external and internal pterygoids, attached to its inner
sides.

The masseter, temporal, and internal pterygoid muscles
close the jaws and press the teeth against one another, and

FIG. 15
XT"

! i!f'"////'/

this is their principal action. They are antagonised by the
digastric, the mylohyoid, and the geniohyoid muscles, which,
aided perhaps by the platysma, depress the lower jaw when
the hyoid bone is fixed by its own depressor muscles.

The external pterygoid draws the jaw forward, and so in
some measure tends to open it ; as the two muscles do not
always, or indeed generally, act together, they give a lateral
movement to the jaw. The superficial portions of the
masseter and the internal pterygoid are ordinarily supposed,
as their direction is slightly backwards, to assist in drawing
the jaw forwards, but Langer, one of the most recent inves-
tigators of their action, attaches very little importance to
this, and indeed considers that, when the jaw has been
pulled forwards by the external pterygoid, the combined
action of the internal pterygoid, the temporal, and the mas-
seter, may bring it back again.

(l) Pterygoid muscles. 1. Upper, and 2. Lower heads of external
pterygoid muscle. 3. Internal pterygoid muscle.

THE MAXILLARY BONES. 35

In ordinary mastication the various movements are com-
bined in every possible manner.

When the mouth is widely open the condyles play upon
the articular eminence in front of the glenoid cavity, and
the external pterygoid, which assists in widely opening the
mouth, draws not only the condyle, but also the inter-
articular nbro-cartilage forwards, so that the latter still
intervenes between the condyle and the articular eminence.
The interarticular cartilages do not, however, accompany
the jaw in its extreme movement, but are believed only
to pass forwards as far as that part of the eminence which
is slightly hollowed out. As, however, in dislocation the}7
accompany the condyles, this supposition may be incorrect.

The position of repose is neither complete closure nor
opening of the jaws : in persons with enlarged tonsils the
habitual position is one with the mouth somewhat more
widely open, owing to the difficulty of breathing through the
nose ; a fact which often causes an irregularity in the
disposition of the teeth.

The axis on which the jaw moves is, owing to the bend
of the ramus, far behind the glenoid cavity; it lies very
nearly in a plane formed by prolonging the plane of the
masticating surface of the teeth.

The .motions executed in mastication differ much according
to the nature of the food ; hence it happens that in different
animals the muscles of mastication are very variously
developed.

Thus, in the Herbivora, which move their jaws greatly
from side to side, as any one may observe for himself, the
pterygoids, and especially the external pterygoid, attain to
a very large relative size.

On the other hand, in the Rodents, which move their
jaws backward and forwards in gnawing, the masse ter is
enormously developed, and has a very marked general
backward direction.

D 2

36

A MANUAL OF DENTAL ANATOMY.

Although it is not strictly true, the masseter and temporal
may be said in mammals to be developed in an inverse ratio
to one another : when one is large the other is not.

The masseter is at a maximum in Carnivora, which have

FIG. 16

little lateral movement possible to their jaws ; the temporal
is also highly developed in many of the class.

In the great apes, the temporal becomes enormously
developed only at the period of second dentition ; this fact,
conjoined with its size, which in herbivora seems to have some
relation to the presence or absence of canines, would incline
one to suppose that it was useful in that rapid closure of the
mouth appropriate to biting when animals fight or seize prey.

The form of the glenoid cavity also bears an intimate
relation to the dentition of the animal, and the nature and
extent of the movement of its jaws.

Thus, in a child it is nearly flat, with no well marked
surrounding elevations; its axis is transverse, and little
rotary motion is made use of. In the adult it is deeply

(') Condyle of the lower jaw, and glenoid fossa of a tiger.

THE MAXILLARY BONES. 37

sunk : the axis of the condyle is oblique, and rotary move-
ments are largely made use of in triturating food.

In the Felidse, it is strictly transverse ; their teeth,
adapted for slicing but not grinding, would gain nothing
by lateral motion, which is rendered quite impossible by
the manner in which the long transverse condyles are
locked into the glenoid cavity by strong processes in front
and behind. Curiously enough the interarticular cartilage
is present, but as the condyle never moves forward, the
cartilage is not attached to the external pterygoid muscle.

In Herbivora the condyle is roundish, the ascending ramus
long, the pterygoid muscles large, and the glenoid cavity
shallow ; in the whale, which of course does not masticate at
all, there is no interarticular cartilage, and no synovial mem-
brane; the articulation is reduced to a mere ligamentous
attachment.

The harder a substance is, the farther back between the
molars it is placed ; and as the food escapes from between
the teeth it is constantly being replaced by the lips, cheeks,
and tongue, the buccinator muscle being largely concerned
in this work of preventing morsels of food from escaping
from the teeth during its mastication.

Just as the muscles of mastication vary in their relative
development in accordance with the food to be dealt with,
so also do the salivary glands.

As a rule herbivorous creatures have large parotid glands;
that is to say, those creatures which deal with the driest
food and masticate it the most have this gland largely
developed. For instance it is very large in Ruminants ;
in Herbivorous 'Marsupials it is larger, in the carnivorous
section smaller, than the submaxillaries. When an espe-
cially viscid fluid is required, as, for example, that which
lubricates the tongue of an ant-eater, this is furnished by
exceedingly large submaxillary glands.

The nerves of the teeth are derived from branches of the

38 A MANUAL OF DENTAL ANATOMY.

fifth nerve, the nerve of sensation of the whole side of the
face and head : the lower teeth through the inferior maxil-
lary nerve, the upper through the anterior and posterior
dental branches of the superior maxillary nerve. The
nerves are given off from the nerve trunks in bundles,
corresponding in number to the roots of the teeth for
which they are destined. For the details of the distribu-
tion of the fifth nerve the student must refer to works
treating of anatomy, as it would be out of place to enter
upon the subject at length in these pages, in which merely
one or two matters of special interest to the dental student
will be touched upon.

In the case of the inferior maxillary nerve the roots of
the teeth come into very close proximity with the main
trunk of the nerve ; this is especially the case with the
lower wisdom teeth. Within a few days of writing these
lines I extracted a lower wisdom tooth (with forceps) for
a gentleman, who, immediately after the extraction, in-
quired if he could have bitten his lip, as it felt swollen;
on testing it I found slight but well marked numbness on
that side of the lip and chin, which did not wholly subside
before he left me. In this case a groove upon the under
surface of the much curved roots appeared to indicate that
the nerve trunk was in close contact with the tooth.

No reason is at present known why the tooth pulp should
be so richly supplied with nerves, as no obvious advantage
results therefrom. Teeth with persistent pulps which go 011
growing throughout the life of the animal, have always large
nerves : thus a very large trunk goes to the pulp of a rodent
incisor. But although in this case the rich nervous supply
doubtless has to do with nutrition, and presides over the
great formative activity of the tissue, this does not fully
account for the pulps of the teeth of limited growth being so
amply supplied with nerves.

As has been mentioned in the description of the lower

FIG, 17 I1).

(!) DIAGRAM OF THE DISTRIBUTION OF THE BRANCHKS OF THE FIFTH NERVE.
(From Tomes' "Lectures on Dental Physiology and Surgery" — drawn by Mr. C. De Morgan.)

A. Ophthalmic division :— 1. Frontal. 2. Nasal and long ciliary. 3. Branches to ciliary ganglion.

B. Superior maxillary division :— 4. Orbital j {jjjjjj' lal } 5'. Sphenopalatine (Mcckel's) ganglion

0. Posterior dental, passing down.

' 13. Ptery.
Inferior dental.

U- CrUObCllUi VU II L<llj (mOBJIlK UUTftll I) O» XXUIXJ11VU UCUWUt tf. ailAtorUAWl

C. Inferior maxillary division :— 10. Auriculo-temporal. 11. McUMeteric. 12. Deep temporal.
goid. 14. Buccal to buccinator, &c. 15, Guntatory, 1(5. Mylohyoid branch. 17. Inf
18, Mental,

[To face p. 38.

THE MAXILLAllY NERVES. 39

maxillary bone, the inferior dental nerve emerges from the
bone by the mental foramen, near to the end of the roots of
the bicuspid teeth. Pain due to distant causes is often
referred to the point of emergence of a nerve, as is so
frequently exemplified in supraorbital neuralgia; in the
same way pain due to diseased teeth far back in the lower
jaw (especially to wisdom teeth), is frequently referred to
the bicuspid region. Curiously enough, though there is no
apparent close parallel in the disposition of the nerves, a
similar reference of pain to the bicuspid region is occasionally
observed in the upper jaw. And it may be added that there
is very probably some closer parallel in the minute disposi-
tion of the nerve fibres going to the teeth in the upper and
lower jaws than is recognisable by rough anatomical pro-
cesses, for while, to all appearance, the nerve trunks are
differently arranged, it is a matter of almost everyday ob-
servation to find pain due to one tooth referred with pre-
cision to its fellow in the other jaw.

The lower teeth derive their vascular supply from the
branches given off to each tooth by the inferior dental artery,
itself a branch of the internal maxillary ; the upper teeth
derive their arteries from the superior dental, a part of
the alveolar branch of the internal maxillary, which supplies
the molar and bicuspid teeth ; and the front teeth from the
descending branch of the infraorbital, the vessels thus having
an arrangement somewhat analogous to that of the nerves.

The distribution of the veins corresponds closely to that
of the arteries.

No lymphatics have been traced into the teeth.

TOMES, J. Lectures on Dental Physiology and Surgery. 1848.
HARBISON ALLEN. Anatomy of the Facial Region, Dental Cosmos.

1874.

CATTLIN. Anatomy of Antrum. Trans. Odontological Society,
1857.

CHAPTER III.

THE DENTAL TISSUES.

IT is usual to speak of there being two kinds of teeth,
namely, horny or albuminous, and calcined teeth ; but of the
development of the former nothing is accurately known, and
it is hence impossible to determine in what relation they
really stand to other, or calcined, teeth.

These latter are composed of one or more structures,
which are in great measure peculiar to the teeth (although,
what is to all intents and purposes dentine, is to be found
in the skeletons and in the dermal appendages of some fish,
and other exceptions might be found to the absolute accu-
racy of the statement), and hence are called " dental tissues."
Notwithstanding the existence of certain transitional forms,
it is not possible to doubt the propriety of a general division
of dental tissues into three, viz., Dentine, Enamel, and
Cementum.

The first named of these constitutes the greater part of
all teeth, and so far predominates in mass over the other
constituents that, in very many cases, the tooth would
retain its form and character after the removal of the enamel
and cementum.

This central body of dentine, enclosing the pulp, is very
often covered by a cap of enamel, which forms the surface
of the tooth ; this may be very partial, as in the eel or the
newt, in which animal only this enamel-capped tip of the
tooth projects far above the surface of the mucous membrane ;
or it may cover a much larger proportion of the tooth, as

THE DENTAL TISSUES. 41

in man. Perhaps the most usual condition is that the
enamel invests the whole crown of the tooth, stopping short
at about the level to which the gum reaches, as in the
human and most other mammalian teeth of limited growth.
In teeth of persistent growth the enamel extends down into
the socket as far as the base of the tooth ; in such cases it
may embrace the whole circumference of the dentine, as in
the molar teeth of many rodents, or it may be confined to
one side only, as in their incisor teeth, where by its greater
hardness it serves to constantly preserve a sharp edge as
the tooth is worn away. The enamel is believed to be quite
absent from many teeth ; thus the subclass Edentata compris-
ing sloths, armadillos, and ant-eaters have it not ; the narwal,
certain cetaceans, some reptiles, and many fish have none.

But although it might appear an exceedingly simple
matter to determine whether a tooth is or is not coated with
enamel, as a matter of fact in practice it is not always easy
to be certain upon this point. When the enamel is tolerably
thick there is no difficulty in making sections which show
it satisfactorily, but when it is very thin it is apt to
break off in grinding down the section. And even when it
does not, it is in such cases usually quite transparent and
structureless, and the outermost layer of the dentine being
also clear and structureless, it is very hard to decide whether
the appearance of a double boundary line is a mere optical
effect due to the thickness of the section, or is indicative of
a thin layer of a distinct tissue which might be either
enamel or cement um.

My own investigations upon the development of the teeth
of fish and reptiles have led me to suspect that rudimentary
layers of enamel exist upon many teeth on which their
presence has not been recognised, for I have found that the
formative enamel organs occur universally. Upon the teeth
of snakes, which were stated by Professor Owen to be com-
posed only of dentine and cement, I have endeavoured to

42 A MANUAL OF DENTAL ANATOMY.

show that a thin layer of enamel exists, and that there is no
ceinentum. The frog has an enamel organ as distinct as
that of the snake, but I am hardly positive that there is
enamel upon its teeth, although there is an appearance of a
thin coat of distinct tissue. I have also demonstrated that
the armadillo has an enamel organ, but have failed to discover
any enamel or anything like it upon its teeth, and Professor
Turner has made a similar observation upon the narwal.

At all events we may safely say that in these and many
other creatures no functional development of enamel takes
place : whether it does or does not exist in an extremely
thin and rudimentary layer has become a question of much
less significance, since I have shown the presence of an
enamel organ to be universal at an early stage.

Hence I feel some hesitation in endorsing Professor Owen's
generalisation that the dentine is the most and enamel the
least constant of dental tissues ; it is possible that it may be
so, but recent researches into the development of teeth have
very materially modified the conceptions formed as to the
relations of the dental tissues to one another, and must lead
us to examine carefully into such deductive statements
before accepting them.

The remaining dental tissue is cementum, which clothes.
in a layer of appreciable thickness, the roots of the teeth,
and reaches up as far as the enamel, the edge of which it
overlaps to a slight extent ; when the cementum is present
upon the crown, it occupies a position external to that of
the enamel. Cementum occurs universally upon the teeth
of mammalia, but it is not always confined to the root of
the tooth ; in many teeth of persistent growth it originally
invested the whole crown, and after it has been worn from
the exposed grinding surface, continues to invest the sides
of the tooth. (See the description of the complex teeth of
the elephant, cow, horse, &c.)

It is probably entirely absent from the teeth of snakes, and

THE DENTAL TISSUES. 43

indeed of very many reptiles ; in the reptilian class, at all
events, it appears to me to be confined to those hi which the
teeth are lodged either in sockets or in a deep bony groove,
as I am unacquainted with any tooth anchylosed to the jaw
in which it exists, unless we are inclined to include under the
term cementum the tissue which I have designated " bone
of attachment/' (See " Implantation of Teeth.")

ENAMEL.

Upon the outer surface of the dentine the enamel forms
a cap of a very much harder and denser material. In
its most perfect forms it is very far the hardest of all
tissues met with in the animal body, and at the same
time the poorest in organic matter. In the enamel of a
human adult tooth there is as little as 3 1 to 5 per cent,
of organic matter, and, judging from its brittleness and
transparency, there is probably even less in the enamel of
some lower animals ; the lime salts consist of a large quan-
tity of phosphate, some carbonate, and a trace of fluoride
of calcium ; in addition, there is a little phosphate of
magnesium.

Von Bibra gives two analyses of enamel :

ADULT ADULT
MAN. WOMAN.

Calcium Phosphate and Fluoride . 89*82 81-63

Calcium Carbonate .... 4 -37 8-88

Magnesium Phosphate . . 1*34 2 '55

Other Salts -88 -97

Cartilage . . . . . 3'39 5'97

Fat . -20 a trace

Organic . . . . 3 -5 9 5 -97
Inorganic 96-41 94'03

The cap of enamel is of varying thickness, being thicker
in the neighbourhood of cusps than elsewhere ; in teeth

44 A MANUAL OF DENTAL ANATOMY.

of limited growth it terminates by a thin edge at the
neck of the tooth, where it is overlapped to some slight
extent by the cementum. When a thick coating of cemen-
tum exists over the whole crown, this lies outside the
enamel, the proper place of which is therefore between the
cementum and the dentine.

The external surface of the enamel is finely striated, the
course of the strise being transverse to the long axis of the
crown ; in addition to this very fine striation, there may be
a few deeper and more pronounced grooves or pits, which
are pathological, and are marks of checks in development
more or less complete. The enamel of some animals is, to
all appearance, structureless ; such is the nature of the
little caps which, like spear points, surmount the teeth of
fishes of the eel tribe, cod tribe, or of the newt, and which
from their extreme brittleness are often lost in preparing
sections, so that their very existence has long been over-
looked. But the absence of structure, if such it really be, is
after all a mere question of degree ; in the commonest form
of enamel, such as that of the human teeth, there is a finely
fibrous structure, very apparent in imperfect teeth, but far
less so in well-formed ones, and the enamel of the eel is, in
the manner of its development, fibrous ; so that even though
we cannot distinguish its constituent fibres when it is com-
pleted, this is merely an indication that calcification has
progressed a little farther than in human teeth : if calcifica-
tion only goes far enough, all structure, if not destroyed,
will at all events be masked from sight.

The structure of human enamel has been stated to be
fibrous ; that is to say, it has a cleavage in a definite direc-
tion, and is capable of being broken up into fibres or prisms,
which seem in transverse section to approximate more or
less closely to hexagonal forms brought about by their
mutual apposition. The general direction pursued by the
prisms is one from the dentine towards the surface ; this is,

THE DENTAL TISSUES.

45

however, subject to many minor modifications. The curved
and decussating course of the human enamel prisms renders
them difficult to trace throughout their length, but the
structure of the enamel of many lower animals (especially
the rodents) is more easily intelligible. Enamel such as
that of the Manatee, in which all the prisms pursue a
perfectly straight course, is of comparatively rare occurrence,
but among the rodents the courses pursued by the enamel
prisms are simple, and produce very regular patterns, which
are constant for particular families (J. Tomes). Thus, in
the Sciuridce, a section of the enamel, whether longitudinal

FIG. 18 (*).

or transverse, appears divided into an outer and inner
portion, in which the prisms, although continuous from the
dentine to the free surface, pursue a different direction. As
seen in the longitudinal section, the enamel prisms start
from the dentine at right angles to its surface, and after
passing through about two-thirds of the thickness of the
enamel in this direction, abruptly bend upwards at an angle
of 45 degrees with their original course. In transverse
section the enamel prisms are found to be arranged in hori-

(*) Section of deutine and enamel of a Beaver : in the inner half the
prisme of contiguous layers cross each other at right angles, in the outer
they are parallel.

46 A MANUAL OF DENTAL ANATOMY.

zontal layers, each layer being a single fibre in thickness ;
in alternate layers the prisms pass to the right and to the
left, crossing those of the next layer at right angles, and
thus making a pattern of squares in the inner two-thirds of
the enamel. But in the outer third of the enamel, where
the prisms bend abruptly upwards, those of superimposed
layers no longer pass in opposite directions, but are all
parallel ; in fact no longer admit of distinction into laminae.

Thus each enamel prism passes in a very definite direction
and, seen with those of other layers, forms a very charac-
teristic pattern ; but the enamel prisms are not in any part
of their course curved.

In the beaver, from which the foregoing figure is taken,
the arrangement of the enamel prisms is dissimilar in the
upper and lower teeth, the lamination taking place in
different directions, so that a longitudinal section of the one
might, so far as this is concerned, be mistaken for a trans-
verse section of the other. As regards the decussation of
the prisms of alternate layers, it is similar to that of the
Sciuridce, but it differs in the laminae being slightly flexuous
instead of pursuing perfectly straight lines.

Among the porcupine family very much more complex
patterns are met with, the enamel prisms being individually
flexuous, and their curves not being confined to one plane ;
the individual prisms pursue a serpentine course, and cannot
be followed far in any one section. Near to the surface,
however, they all become parallel, the enamel thus conform-
ing with that of other rodents in being divided into two
portions (at least so far as the course pursued by, and the
pattern traced by, its fibres in its inner and outer parts can
be said to so divide it). The Leporidce, or hares, form an
exception to this ; their enamel has no such lamelliform
arrangement, but is built up merely of slightly flexuous
prisms.

By tracing the courses of enamel prisms from the simple

THE DENTAL TISSUES. 47

pattern found in the Manatee through that of the squirrel
and dormouse, and the porcupine, we see how a very definite
arrangement, at first simple, becomes modified into some-
thing a little more complex, till at last it reaches a degree
of complexity that looks like mere disorder. No one un-
familiar with the enamel of other rodents, looking at the
enamel of the porcupine, would be able to unravel the very-
indefinite looking chaos of prisms before him ; but had he
studied forms in some degree transitional he could not doubt
that the tortuous, curving course which he saw the prism
to be pursuing was nevertheless perfectly definite and precise,
and formed part of a regular pattern.

In perfectly healthy human enamel the fibrillar arrange-
ment is not so very strongly marked ; the prisms are solid,

FIG. 19 x.

are apparently in absolute contact with one another, without
visible intervening substance.

Bat Bodecker, basing his conclusions upon the examination
of thin sections, stained with chloride of gold, holds that
enamel is built up of columns of calcified substance, between
which minute spaces exist. These are filled by a material

(*) Human enamel, from the masticating surface of a molar. The
figure is merely intended to show the general direction of the fibres.

48 A MANUAL OF DENTAL ANATOMY.

which takes the stain deeply, and is probably analogous to
the cement substance of epithelial formations. As seen in
sections, it gives off exceeding fine thorns, which apparently
pierce the prisms at right angles to their length, so that it
forms a close network very intimately mixed up with the
calcined portion of the enamel.

It is not of uniform thickness, but is beaded, and Bodecker
attributes to it a role of far greater importance than that of
a mere cementing substance, for he regards it as being an
active, protoplasmic network, which renders the enamel
much more " alive " than it has hitherto been considered to
be. He believes it to become continuous with the soft
contents of the dentinal tubes through the medium of large
masses of protoplasmic matter found at the margins of the
enamel and dentine.

But although there are various reasons for suspecting that
enamel is not completely out of the pale of nutrition from
the moment that a tooth is cut, yet further observations are
needed before the activity and importance of the cement
substance demonstrated by Bodecker can be held to be fully
established. Klein remarks that "the enamel cells, like all
epithelial cells, being separated from one another by a homo-
geneous interstitial substance, it is clear that the remains of
this substance must occur also between the enamel prisms ;
in the enamel of a developing tooth the interstitial substance
is larger in amount than in the fully formed organ. It is
improbable that nucleated protoplasmic masses are con-
tained in the interstitial substance of the enamel of a fully-
formed tooth, as is maintained quite recently by Bodecker."

The study of the development of marsupial enamel, to be
alluded to at a future page, by showing that the enamel is
penetrated by soft tissue differently placed, also tends to
throw doubt upon Bodecker' s interpretation. W. J. Barkas
(Monthly Review of Dental Surgery, 1874) has also perhaps
had under observation this cementing substance ; he also

THE DENTAL TISSUES. 49

believes that the enamel prisms are tubular, minute canals
running along their axes.

On the whole the prisms are parallel, and run from the sur-
face of the dentine continuously to that of the enamel. Their
paths are not, however, either perfectly straight or perfectly
parallel, for alternate layers appear to be inclined in opposite
directions, while they are also wavy, forming several curves
in their length. The curvature of the enamel prisms is most
marked upon the masticating surface ; while the layers,
alternating in the direction of their inclination as just
described, are in planes transverse to the long axis of the
crown, and correspond to the fine striae on the surface of
the enamel, which appear to be caused by their outcrop.
The curvatures take place in more than one plane ; in other
words, the course of the individual prism is more or less of
a spiral.

Although most prisms run through the whole thickness of
the enamel, yet inasmuch as the area of the outer is much
larger than that of the inner surface of the enamel, and the
individual prisms do not undergo any alteration in size as
they pass outwards, many supplemental fibres are present in
the outer portions of the enamel which do not penetrate far
inwards.

The individual fibres are to all appearance structureless
in perfectly formed human enamel, but a faint transverse
striation, fainter, but otherwise not unlike that of voluntary
muscle, is so general that it cannot be regarded as patho-
logical, although it is most strongly developed in imperfect
brownish enamel. The striation in question may be seen
even in a single isolated fibre, and is not necessarily con-
tinuous over adjacent fibres, though it often is so ; it is
rendered more apparent by the slight action of diluted acids
upon the fibre. Very various interpretations of this appear-
ance have been given. It has been attributed to " an inter-
mittent calcification" of the enamel fibre (Hertz), but is

E

50 A MANUAL OF DENTAL ANATOMY.

with more probability referred to varicosities in the indi-
vidual fibres (Kolliker, Waldeyer) (1). It is very marked in
the enamel of the common rat, which shares with that of
other muridse the peculiarity of having the individual fibres
almost serrated, those of adjacent crossing layers being fitted
to one another with great exactness. In human enamel the
adjacent fibres if united without any intermediate cement-
ing medium, and pursuing courses slightly different, must
of necessity be of slightly irregular form, or else interspaces
would be left, which is not found to be the case. Thus the
"decussation of the fibres " is a plausible explanation of this
appearance of striation ; indeed isolated fibres do present an
appearance of slight varicosities, repeated at regular in-
tervals. The penetration, at regular intervals, of the prisms
by the " thorns " of cement substance (see page 48), affords
another explanation.

Although the perfect enamel fibre appears to be entirely

FIG. 20 (*).

homogeneous, it is not really so, for acids act with far greater
rapidity upon the central or axial portion of the fibre than
upon its periphery. The accompanying figure, taken from

(*) The striation of voluntary muscle has been very recently proved to
be due to this same cause (Dr. Haycraft, " Proceedings of Royal Society,"
Feb. 1881).

(2) From human enamel, softened in chromic acid, until it could be
-readily cut with a knife.

THE DENTAL TISSUES. 51

enamel softened by prolonged maceration in a 1 per cent.
solution of chromic acid shows this well ; the central por-
tions of the fibre are dark, and are stained green by the
reduced chromium sesquioxide, while the clear interspaces
are colourless. Again, if dilute hydrochloric acid be applied
to a section of enamel, the axial parts of the fibres are first
attacked and are dissolved away, so that, if the section be
transverse, a fenestrated mass remains. During the forma-
tion of enamel the hardening salts are deposited first in the
periphery of the enamel cells, so that the youngest layer of
enamel is full of holes, each one of which corresponds to the

FIG. 21

centre of a fibre. Although calcification goes on to obliterate
the visible difference between the centre and the periphery
of the enamel fibre, yet the action of an acid reverses the
order of its formation and once more makes it fenestrated,,
indicating that there is not absolute identity of substance
in the inner part of the fibre. In imperfect enamel, indeed,
a central narrow canal has sometimes been observed in the
interior of an enamel fibre.

In fractured enamel, the line of fracture is said to run.
through the centre of the fibres, and not, as might have been
expected, through their interspaces.

There is also an appearance of striation upon a far larger
scale, consisting of brownish lines, which are never, or almost
never, quite parallel with the outer surface of the enamel,
but which nevertheless preserve some sort of parallelism
with it and the surface of the dentine. These are known

(!) Transverse section of enamel, the axial portion of the prisms having
been removed by dilute hydrochloric acid.

E 2

52

A MANUAL OF DENTAL ANATOMY.

as the " brown striae of lletzius," and as they coincide with
what was at one time the outer surface of the enamel cusp,

FIG. 22 (').

are in some sense marks of its stratification, in its original
deposition.

Pigme.nt is seen in the enamel of many rodents ; it is in

Cavities in human enamel, which communicate with the dentinal

tubes.

THE DENTAL TISSUES. 53

the outer layers of the enamel, but has no sharply denned
boundary, fading away gradually into the colourless tissue
lying within it. Some authors have supposed that the pig-
ment lay in a thin coating of cementum, or in a very dis-
tinct layer of enamel, but as has above been stated, such is
not the case.

Cavities of irregular form sometimes exist in the enamel
near to the surface of the dentine, and when such spaces
exist the dentinal tubes sometimes communicate with them,
but these are perhaps to be regarded as pathological ;
Bodecker regards them as filled up by protoplasm. Irregular
fissures and cavities also occur upon the outer surface of the
enamel, which also have no special significance save as pre-
disposing causes of dental caries.

In man, however, dentinal tubes may occasionally be seen
to enter the enamel, passing across the boundary between
the two tissues, and pursuing their course without being lost
in irregular cavities j though this appearance is seldom to be
found. As was pointed out by my father, the passage of the
dentinal tubes into and through a great part of the thickness
of the enamel takes place in marsupials with such constancy
as to be almost a class characteristic.

The only exception to the rule amongst recent marsupials
occurs in the wombat, in which no dentinal tubes enter the
enamel ; those extinct marsupials which have been examined
present, as might have been expected, a structure in this
respect similar to that of their nearest allies amongst the
recent genera.

The enamel of the wombat is peculiar also in another
respect, being covered by a strong and remarkably uniform
layer of cementum.

The penetration of the enamel by dentinal tubes is not,
however, a peculiarity quite confined to the marsupials, for
it is to be found in some rodents (e.g. the jerboa), and in
some insectivora (e.g., the soricidse).

A MANUAL OF DENTAL ANATOMY.

FIG. 23 (»).

(!) Enamel and dentine of a Kangaroo (Macropus major].
The dentinal tubes in the dentine (A) are furnished with numerous
short branches at the line of juncture with the enamel ; they are dilated,
and a little bent out of their course, while beyond the dilatation they pass
on through about two-thirds of the thickness of the enamel in a straight
course and without branches. Only a part of the whole thickness of the
namel is shown in the figure.

THE DENTAL TISSUES. 55

Waldeyer and Hertz doubt the passage of the tubes of the
dentine into the enamel ; as Kolliker observes, it is difficult
to see how they can doubt it, even after mere observation of
a single specimen ; moreover, it is also capable of experi-
mental demonstration, for if an acid capable of removing the
enamel be applied to one of these sections of marsupial teeth
so as to dissolve away the enamel, the freed tubes are left
hanging out from the edge of the dentine, thus putting the
matter beyond all possibility of doubt, while the develop-
ment of the marsupial enamel makes the nature of the con-
tents of the tubes quite clear.

The most marked variation in the structure of enamel,
which is on the whole a tissue differing but little in various
animals, is met with in the class of fish.

In the Sargus, or sheep's-head fish, for example, the enamel
is penetrated by a system of tubes which are not continued

FIG. 24 (*).

out of or derived from the dentine, but belong to the enamel
itself.

The tubes, as seen in the figure, run at right angles to
the external surface of the enamel, proceed inwards without
branch or bend for some little distance, and then, at about

(*) Enamel and dentine of the Sheep's-head fish (Sargus ovis).
The enamel is penetrated by a system of channels which enter from its
free exposed surface, pass in for a certain distance in straight lines, and
then abruptly bending at an angle cross one another, and produce a com-
plicated pattern in the inner third of the enamel.

56 A MANUAL OF DENTAL ANATOMY.

the same point, bend abruptly at an angle, and give oft
numerous branches. The meshwork produced by the cross-
ing of the tubes at all sorts of angles in the inner part of
the enamel is so complicated as to render it impracticable
to reproduce it in a drawing. That portion of enamel next
to the dentine is without canals. Von Boas (Zeits. f. wissen.
Zoolog. Bd. xxxii.), describing the similarly constructed
enamel of scaroid fishes, says that I was in error in sup-
posing that the canals open upon the outer surface of the
enamel. But I do not understand his reasons for dissenting
from my opinion, which re-examination of many specimens
has tended to confirm. I have not been able to satisfy
myself whether the tubes occupy the interspaces of the
enamel prisms, or their axes.

It would appear also as if these tubes were empty during
life, as in sections they appear to be more or less blocked up
with dirt. The existence of the prisms at all is not certain,
and this led Kolliker to say that true enamel does not
appear to exist in fishes (Mik. Anat. p. 114) ; the enamel of
fish is, however, developed from an enamel organ homologous
with, and exactly like, that of amphibia and reptiles, so
that these anomalous tissues must be regarded as being un-
questionably enamel.

DENTINE.

THE greater part of every tooth is made up of dentine,
which thus, even after the removal of the other tissues
would preserve somewhat its characteristic form. Several
varieties of dentine exist in which those peculiarities of
structure which differentiate it from bone become less
marked, so that a point is sometimes reached at which it
is hard to say whether a particular structure should more
rightly be regarded as dentine, or as bone. It will be most
convenient to commence with the description of that variety

THE DENTAL TISSUES. 57

of dentine which differs most markedly from bone ; in other
words, which has the most typical " dentinal " structure ;
and for that purpose the tissue met with in the teeth of man
and the majority of mammalia, (though it is by no means
confined to that class,) and known under the name "hard"
or " un vascular" dentine may be selected.

Dentine is a hard, highly elastic substance, in colour
white with a slight tinge of yellow, and to some extent
translucent, its transparency being often made more striking
by contrast with the opacity which marks the first advent of
dental caries. When broken a silky lustre is seen upon the
fractured surfaces, which being in the main due to the presence
of air in its tubes, is more apparent in dry than in fresh
dentine ; its fracture is sometimes described as finely fibrous.

The mass of the dentine consists of an organic matrix
richly impregnated with calcareous salts ; this matrix is
everywhere permeated by parallel tubes, which run, with
some deviations, in a direction at right angles to the surface
of the tooth.

The Matrix. — The exact chemical composition of the
matrix is not known ; in man the proportion borne by the
organic to the inorganic constituents varies in different
individuals, and very probably in the same individual at
different ages, so that analyses can only give approximate
results. In a fresh human tooth 62 per cent, of its weight
was found to be inorganic salts, the tooth cartilage being
28 per cent., leaving a residue of 10 per cent, of water.

Von Bibra gives the following analysis of perfectly dried
dentine : —

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 0'83

58 A MANUAL OF DENTAL ANATOMY.

Von Bibra gives another analysis : —

Cartilage . . . . . .2042

Fat . -58

Salts . . . . . .1-00

Magnesium phosphate . . . . 2*49

Calcium phosphate, and fluoride . 67*54

Calcium carbonate . . . . 7' 9 7

And Berzelius gives

Gelatine and water . . . .28-00

Sodium salts . . . /.;*„• 1*50

Magnesium phosphate . . . s- 1 '00

Calcium phosphate . . , . 62-00

Calcium fluoride . . . .2-00

Calcium carbonate . . . . 5 '50

The dentine of many mammals is very much more rich
in magnesium phosphate than human dentine is; even
the latter, it would seem, from the discrepancies existing
between the various analyses, is variable in composition,
but, on the whole, it may be said that, amongst inorganic
constituents of dentine, calcium phosphate largely prepon-
derates ; from 3 J to 8 per cent, consists of calcium carbo-
nate j a much smaller proportion consists of magnesium
phosphate, while calcium fluoride exists in traces only.

The organic basis of the matrix is closely related to that
of bone, with which however it is not identical ; it is of
firmer consistence, and does not really yield gelatine on
boiling, but, according to Kolliker (who quotes Hoppe), the
dentine of the pig yields a substance resembling glutin,
the dentinal globules remaining undissolved. The animal
basis of the dentine is called " dentine cartilage," and is
readily obtainable by submitting a tooth for several days to
the action of diluted acids. The form and most of the

THE DENTAL TISSUES. 59

structural characteristics of a tooth so treated are main-
tained, the dental cartilage forming a tough, flexible, and
elastic semi-transparent mass.

In the matrix of a perfect tooth no trace of cellular
structure can be detected ; it is uniform and perfectly
transparent.

The Deutinal Tubes. — As has been already mentioned,
the matrix is everywhere permeated by tubes, the precise
direction of which varies in different parts of the tooth,
so that the following description of their course must be
taken as merely in a general way descriptive, and not as
of universal or precise application. Each tube starts by an
open circular mouth upon the surface of the pulp cavity ;
thence it runs outwards, in a direction generally per-
pendicular to the surface, towards the periphery of the
dentine, which, however, it does not reach, as it becomes
smaller, and breaks up into branches at a little distance
beneath the surface of the dentine.

Near to the pulp they are so closely packed that there is
little room between them for the matrix, while near to the
outside of the tooth they are more widely separated : their
diameter is also greater near to the pulp cavity.

The dentinal tubes de not pursue a perfectly straight
course, but describe curves both on a larger and a smaller
scale. The longer curves are less abrupt than the others,
and are termed the " primary curvatures ; " they are often
compared to the letter f, to which they bear a certain
amount of resemblance ; the primary curves are more
pronounced in the crown than in the root.

The secondary curvatures are very much more numerous
and are smaller ; the actual course of the dentinal tube is,
in many places at all events, an elongated spiral, as may
be very well seen in thick sections transverse to the tubes ;
by alterations in the focus of the microscope the appearance
of the tube making a spiral turn is made very striking.

60

A MANUAL OF DENTAL ANATOMY.

The effect of an elongated spiral viewed on its side will of
course be only slight undulations, such as are the secondary
curvatures of the tubes. The spiral course of the dentinal
tubes is most strongly marked in the roots of teeth.

When a transverse section of dentine is viewed, bands or
rings, concentric with the pulp cavity are seen, and the same
bands may be seen in longitudinal section. Such a striated
or laminated appearance in the dentine may be due to two
causes ; and some little confusion has arisen in the nomen-
clature, owing to its double origin not having always been
kept in view. Such strise may be due to the presence of
rows of interglobular spaces, or to the coincidence of the
primary curvatures of neighbouring dentinal tubes : that

FIG. 25

i

-V-7- •

is to say, each tube bends at the same distance from the
surface, and the bend makes a difference in the optical pro-
perties of the dentine at that point.

Schreger described these latter: the lines of Schreger,

(*) Dentine and cementuni of a Narwal, showing contour lines due to
rows of interglobulav spaces.

V

THE DENTAL TISSUES.

61

therefore, are markings, ranged parallel with the exterior of
the dentine, which are due to the curvatures of the dentinal
tubes.

The " contour lines " of Owen, even in his own works,
include markings of both classes : i. e., those due to the
curvature of the dentinal tubes, and those due to laminae of
interglobular spaces, such as are met with in the teeth of
Cetacea. Ketzius had seen and described contour markings
due to interglobular spaces, though his name is not usually
associated with them, the " brown striae of Retzius " being
markings in the enamel.

The tubes as they pass outwards often divide into two
equally large branches ; they also give off fine branches,
which anastomose with those of neighbouring tubes. In the

crown of a human tooth these fine branches are compara-
tively few, until the tube has reached nearly to the enamel,
but in the root they are so numerous as to afford a ready
means of distinguishing whence the section has been taken.
The small branches above alluded to are given off at right
angles to the course of the main tube, which, however, itself
frequently divides and subdivides, its divisions pursuing a
nearly parallel course.

The tubes are subject to slight varicosities, and their

(]) Termination of a dentinal tube in the midst of the dentine— human.

62 A MANUAL OF DENTAL ANATOMY.

course is sometimes apparently interrupted by a small inter-
globular space, as is to be seen in an extreme degree in the
dentine of Cetacea.

Owing to their breaking up into minute branches, some
of the tubes become lost as they approach the surface of the
dentine, and apparently end in fine pointed extremities.

Some terminate by anastomosing with terminal branches
of others, forming loops near to the surface of the dentine ;
others terminate far beneath the surface in a similar way.

Some tubes pass into the small interglobular spaces
which constitute the "granular layer" described by my
father, while others again pass out altogether beyond the
boundary of the dentine and anastomose with the canaliculi
of the lacunse in the cementum.

The enamel also may be penetrated by the dentinal
tubes, though this when occurring in the human subject
must be regarded as exceptional and almost pathological
in its nature (see Fig. 22). As has, however, been men-
tioned in speaking of the enamel, in most of the Marsupials
and in certain other animals it is a perfectly normal and
indeed characteristic occurrence, difficult though it be to
see how such a relation of parts is brought about in the
course of development of the two tissues.

Dentinal Sheaths. — If dentine be exposed to the action
of strong acid for some days, a sort of fibrous felt, or if the
action of the acid has gone further, a transparent slime alone
remains. Examined with the microscope this proves to be a
collection of tubes ; it is, in fact, made up of the immediate
walls of the dentinal tubes, the intervening matrix having
been wholly destroyed.

Two facts are thus demonstrated : the one that the tubes
have definite walls, and are not simple channels in the
matrix ; the other, that these walls are composed of some-
thing singularly indestructible. Indeed, the walls of the
dentinal tubes are so indestructible that they may be

THE DENTAL TISSUES.

demonstrated in fossil teeth, in teeth boiled in caustic
alkalis, or in teeth which have been allowed to putrefy.

Although Kolliker was, I believe, the first to describe and
figure these isolated tubes, they are generally known as the
" dentihal sheaths of Neumann," the latter writer having
more fully investigated and described them. The precise
chemical nature of these sheaths will be more conveniently
considered under the head of calcification : similarly inde-
structible tissues are, however, to be met with surrounding
the Haversian canals and the lacunae of bone. It is the
opinion of Neumann, as it was also of Henle, that the
dentinal sheaths are calcified; but the proof of this is.
very difficult, as they cannot be demonstrated, or I should
rather say, isolated, to any extent in dentine, unless -it has,
been decalcified. Their existence has been recently denied
in toto by Magitot.

Transverse sections of dentine present fallacious appear-
ances, owing to the thickness of the section giving to the
tube a double contour which may be easily mistaken for a

FIG. 27 (J).

special wall. Immediately round the opening of the canal,
or " lumen," as it is called, there is however generally a
thin yellowish border, which is the sheath of Neumann.
In the earlier stages of caries, before the dentine is much
softened, the walls of the canals become strikingly appa-

(]) Transverse section of dentine. The appearance of a double contour
is so much exaggerated as to make the figure almost diagrammatic.

434 A MANUAL OF DENTAL ANATOMY.

rent. The canals which everywhere permeate the dentine
are not empty, a fact which might be inferred from the
difference in translucency and general aspect of dry and
fresh dentine, whether seen in mass or in thin section ;
neither are they, as was at one time supposed, tenanted
merely by fluid.

Deutinal Fibrils. — Each canal is occupied by a soft
fibril, which is continuous with an odontoblast cell upon the

FIG. 28 J.

surface of the pulp ; the existence of these soft fibrils was
first demonstrated by my father, who thus, to use the words
of Waldeyer, " opened the way to a correct interpretation
of the nature of the dentine."

Henle, in his "Allgemeine Anatomie," (1841), a transla-
tion of a portion of which is to be found in the " Archives
of Dentistry," (1865), figured and described projections from
the edges of fragments of dentine in continuity with the
dentinal tubes. These he distinctly describes as calcified and
rigid, adding that by the use of acids they may be made
flexible ; he speaks of the tube as empty, save when blocked
by granular calcareous matter, and alludes to fluids entering
it by capillarity ; and lastly, he says nothing whatever of
the connections of the pulp with the tubes.

Miiller, (as translated in Nasmyth on the "Structure of

(J) A fragment of dentine (a), through which run the softer fibrils (c),
which are seen to be continuous with the odontoblast cells (6). (After
Dr. Lionel Beale.)

THE DENTAL TISSUES. 65

the Teeth," 1839), says, "in breaking fine sections of the
teeth perpendicularly to the fibres, he has frequently seen the
latter projecting a little at the fractured edge. In such
cases they are quite straight and not curved, and seem
to be not at all flexible. Hence it follows that the tubes
have an organised basis, a membrane, and that this is stiff
and brittle, and probably saturated with calcareous salts,
but weak and soft in a decalcified tooth."

The whole importance of my father's discovery lay in the
fact that dentine is permeated by soft, uncertified structures ;
and what is yet more significant, that these soft fibrils, per-
meating the hard dentine, proceed from the pulp. In no
sense, therefore, did Henle anticipate this discovery.

In 1854 Lent figured processes from the dentinal cells
(odontoblasts) which he rightly conceived to be concerned
in the formation of dentine ; but in the earlier editions of
the " Histology " of his friend and teacher, Prof. Kolliker,
although Lent's discoveries are described and adopted with-
out reservation, no mention of the real structure of dentine
occurs. But in the last edition, Prof. Kolliker says — " after
Tomes had described a soft fibre in each tube, I fell into the
mistake of supposing that these fibres and the tubes were
one and the same."

The circumstances under which the dentinal fibrils can
or cannot be discovered are as follows, and may be taken as
proofs of the distinction between the dentinal fibrils and the
dentinal sheaths.

If a tooth section be submitted to the action of a caustic
alkali and boiled in it, or be allowed to completely putrefy,
so that the soft parts are entirely destroyed, the dentinal
sheaths can still be demonstrated, but the fibres can in no
way be brought into view (Kolliker). The dentinal sheaths
may be demonstrated also in fossil teeth, as has been shown
by Hoppe (Wurzburg Nat. Zeitschrift, Bd. VI. p. XL) and
others.

66

A MANUAL OF DENTAL ANATOMY.

In fresh dentine every formative cell sends a process into
the dentinal tubes (Tomes, Kolliker, Lent, Waldeyer, Neu-
mann), and it has been found possible to demonstrate both
the sheaths and the fibres in the same sections (Neumann,
Boll).

In transverse and even in longitudinal sections of decalci-
fied dentine the fibrils may be recognised in situ (Kolliker).

The contrast between the dentinal sheaths and the fibrils
is this : — the sheaths are very indestructible, and can be
demonstrated in teeth which have undergone all sorts of

FIG. 29 0).

change ; the soft fibril is no longer demonstrable when the
tooth has been placed in circumstances which would lead
to its soft parts perishing. In dentine, then, we have (i.) a
matrix permeated by tubes ; (ii.) special walls to these tubes
or "dentinal sheaths ;" and (iii.) soft fibrils contained in
these tubes, or " dentinal fibres ; " and it now remains to
consider these in farther detail.

In fortunate sections of small fragments of dentine taken
from the edges of the pulp cavity and including the surface
of the pulp, the dentinal fibrils may be seen stretching from
the cells of the superficial layer of the pulp (odontoblasts)
into the dentinal tubes, as owing to these being extensile
they may be stretched or drawn out from the tubes for some
little distance without being broken across. In the same

(*) Section of dentine from the edge of which hang out the dentinal
sheaths, and beyond these again the fibrils (after Boll).

THE DENTAL TISSUES.

67

way they may be seen stretching across like harp strings
between two pieces of dentine, when this is torn by needles,
and they can be thus shown in fresh fragments just as well
as in those of decalcified dentine. When stretched to a
considerable extent their diameter becomes diminished and
they finally break, a sort of bead sometimes appearing at the
broken end (Tomes). This would seem to indicate that the
substance of the fibril is of colloid consistency, and that its
external portions are in some degree firmer than its axial
portion.

The dentinal fibrils are well seen in the accompanying
figure, in which some hang out from the edge of the dentine,

FIG. 30 *.

while others have been pulled out from the dentine and are
seen attached to the odontoblast cells.

The dentinal fibril is capable of being stained with car-
mine, though with some difficulty; in young dentine it
is more easily stained, especially near the pulp cavity, and
the accompanying drawing is taken from such a section of

(*) Surface of the pulp, with the odontoblast layer in situ. The dentine
fibrils pulled out of the dentinal tube hang like a fringe from the odonto-
blast layer : dentine fibrils are also seen hanging out from the edge of the
dentine, to which, to the right of the figure, a few odontoblasts remain
attached.

F 2

68 A MANUAL OF DENTAL ANATOMY.

dentine from a half-formed human incisor. The matrix is
slightly stained with the carmine, indicating that it has not
yet become fully impregnated with salts, and in the centres
of the clear areas dark spots deeply stained with carmine
are to be seen, the latter being transverse sections of the
dentinal fibrils in situ. I have observed precisely similar
appearances in the thin young dentine of calves' and pigs'
teeth ; Kblliker also mentions that the dentinal fibril may
be recognised in situ in transverse sections of fresh dentine.

FIG. 31 (*).

Bodecker finds that the dentinal fibrils stain darkly with
chloride of gold ; when viewed in transverse sections under
a magnifying power of 2,000 diameters they do not appear
round but somewhat angular, and give off tiny lateral
offshoots which seem to penetrate the dentine. In the
matrix itself there is an appearance of a faint network when
it has been stained with gold, and from this Bodecker infers
that the dentine is penetrated everywhere by a network
of living plasm, derived from, though far finer than, the
dentinal fibrils.

Probably the angularity of the fibril, which, as figured by
him, is much smaller than the canal, is due to its having
shrunk under the action of chromic acid or some such
reagent.

(l) Transverse section of dentine : in four of the dentinal tubes, the
dentinal fibrils deeply stained with carmine, in the preparation from which
this figure was drawn, are seen. The fibrils are somewhat shrunken,
owing to the action of the glycerine in which the section is mounted.

THE DENTAL TISSUES. 69

According to Neumann, in old age the fibrils atrophy or
become calcined ; some observers have failed to detect them
near to the periphery of the dentine, far away from the pulp
cavity. But here they would naturally be more minute,
and it is more probable that the manipulations had failed
to demonstrate them than that they were absent ; for
Bodecker has traced them to the very outside of the
dentine.

Dr. Beale has seen prolongations of the nucleus of the cell
towards the base of the fibril, though in the example which
he figures it does not enter it.

Dentinal fibrils have been demonstrated in the Reptilia
and Amphibia by Santi Sirena and myself; and by myself
in the few fish that I have examined with that purpose.

Of their real nature some doubts are entertained : they
are certainly processes of the formative cells of the dentine,
and their substance seems identical with that of the proto-
plasm of the cell. Nerves, in the ordinary sense of the
word, they are not, and have never been supposed to be ;
but there are many examples of cellular structures which
are connected with the termination of sensory nerve fibres,
such as the goblet cells in the olfactory membrane of the
frog, and it is quite possible that the odontoblast cells may
stand in some such relations to the nerve of the pulp, the
termination of which have never been satisfactorily traced.

Mr. Coleman once suggested that it was possible that
the odontoblasts might have some tactile function ; but
M. Magitot has recently claimed for them a very definite
connection with the nerves of the pulp. According
to his observations and figures the nerves of the pulp
become continuous with a layer of reticulate cells which lie
beneath the odontoblasts ; and these freely communicate
with the processes of the odontoblasts, so that there is a
very direct chain of communication between the dentinal
fibril and the nerves of the pulp. M. Magitot speaks very

70

A MANUAL OF DENTAL ANATOMY.

positively as to the accuracy of his views, which as yet,
however, have not been confirmed by other investigators.

FIG. 32 (1).

Yet another view of the nature of the dentinal fibril is
advocated by Klein ("Atlas of Histology," p. 183), who
holds that the odontoblasts are concerned only in the forma-
tion of the dentine matrix, and that the dentinal fibrils are
long processes of the deeper cells, in the above figure, which
run up between the odontoblasts and enter the dental canals.

In a recent paper (Comptes Rendus, 1880,) Magitot also
impugns the accuracy of the views ordinarily accepted as to
the structure of dentine, denying the existence of any
special walls to the tubes, and further arguing that it is
undesirable to think or speak of the channels in dried dentine
as tubes at all. For, he argues, they are not tubes in the
fresh state, seeing that the fibrils are adherent to the
matrix and form a part of it, and that they were originally

(x) After Magitot. a. Dentinal fibrils. b. Amorphous matrix.
c. Odontoblasts. d. Nuclei of odontoblasts. e. Stellate cells. /. Nerve
extremities which are continuous with the branched cells.

THE DENTAL TISSUES. 71

precisely the same tissue. He would prefer to speak of
dentine as being a fibrillar tissue included in a hard and
homogeneous matrix.

These views, however, do not differ substantially from,
those in the text, save that M. Magitot does not recognise the
existence of that transitional tissue which others believe to
be there, and call the sheaths of Neumann.

No true nerve fibril has ever been seen to enter the den-
tine ; nothing but the dentinal fibril has ever been proved
to pass from the pulp into the hard substance of the
tooth ; nevertheless, the observation of Boll is very sug-
gestive. He found that by treating a perfectly fresh pulp
with £ per cent, solution of chromic acid an immense
number of fine fibres could be demonstrated, a great many
of which projected from above the surface, as though they
had been pulled out of the dentinal tubes ; but although
they pass up from a plexus of dark-bordered nerve fibres
beneath the membrana eboris, between the cells of that
layer, their passage into the dentine remains a mere matter
of inference.

Boll's observations likewise are awaiting confirmation or
disproof, and so far stand alone.

Be that as it may, there can be no question that the
sensitiveness of the dentine is due to the presence of soft
organized tissue in the tubes, and is not a mere transmission
of vibrations to the pulp through a fluid or other inert
conductor. The peripheral sensitiveness of a tooth can be
allayed by local applications which it would be absurd to
suppose were themselves conducted to the pulp ; moreover,
it is within the experience of every operator that after the
removal of a very sensitive layer of caries, you often come
down upon dentine, which, though nearer to the pulp, is
far less sensitive, a condition quite inexplicable, except
upon the supposition of a different local condition of the
contents of the tubes. Irritation applied to the dentinal

72 A MANUAL OF DENTAL ANATOMY.

fibrils may be propagated to the pulp, and irritation of the
pulp set up without any real exposure of the latter.

With reference to the probabilities of actual nerve fibres
entering the dentinal tubes, it must be remembered that, in
those tissues which are naturally so thin as to present
great facilities for examination, nerves of a degree of fine-
ness unknown elsewhere have been demonstrated ; in other
words, the easier the tissue is to investigate, the finer the
nerves which have been seen in it, while dentine is among1
the most difficult substances conceivable for the demonstra-
tion of fine nerve fibrils, if such exist in it.

Inter globular Spaces. — In the layer of dentine which
underlies the cement an immense number of these spaces,
exist, giving to the tissue as seen under a low power an
appearance of granularity. On this account my father gave

to this the name of the " granular layer " of dentine ; on
account of the far greater abundance of the spaces in that
situation it is far more marked beneath the cement than
beneath the enamel, and many of the dentinal tubes end in
these spaces.

Although the name " interglobular spaces " is strictly ap-
plicable to the structures constituting the granular layer
of dentine, it was not to these that it was first applied.
When a dried section of dentine is examined, dark irregular

(J) Dentinal tubes terminating in the spaces of the granular layer.

THE DENTAL TISSUES. 73

spaces, clustered together and usually most abundant at a
little distance below the surface, are often to be seen, parti-
cularly if the section has been made from a brownish, im-
perfectly developed tooth.

These spaces have a ragged outline, furnished with short
pointed processes, and in favourably-prepared sections it
may be seen that their outlines are formed by portions of
the surfaces of closely opposed spheres, and globular con-
tours may often be detected in the solid dentine near to
them, as is seen in the accompanying figure, taken from a
section boiled in wax in order to render it very transparent.

Although these large spaces are very common, they are
perhaps not to be regarded as perfectly normal, but are

Fm. 34

rather indications of an arrested development at that spot.
The occurrence of globular forms during the early stages
of calcification, will again be alluded to in connection with
the development of teeth ; but although the term " inter-

(*) Interglobular spaces in dentine.

A MANUAL OF DENTAL ANATOMY.

globular " is thus strictly applicable, the use of the word
" spaces " is not so correct. In dry dentine it is true that
they are, as Czermak described them, spaces filled with air ;
but that they are so is only due to the fact that their
contents are soft, and shrivel up in drying. In the fresh
condition the interglobular " space " is perfectly full, its
contents often having the structural arrangement of the
rest of the matrix, or else consisting of soft plasm; in
the former case, the dentinal tubes pass across and through
it without any interruption or alteration in their course.
This fact, as well as the soft nature of the contents as com-
pared with the rest of the dentine, is well illustrated by a
section in my possession which was taken from a carious

FIG. 35 C1.

tooth, near to the affected surface. In this the fungus,
leptothrix, had effected an entrance into some of the tubes,
giving to them a varicose beaded appearance, and causing
their enlargement. But when it reached the interglobular
space, the less amount of resistance, or possibly the more
favourable pabulum accessible, led to its more rapid deve-
lopment, so that the tubes within the confines of the space
are many times more enlarged than those outside ; never-
theless the continuity of the tubes across the space is well

(J) Section of carious dentine, in which some of the tubes are beaded
by the ingress of the leptothrix, which has developed with greater freedom
in one or two of the tubes where they cross the interglobular spaces.

THE DENTAL TISSUES. 75

demonstrated by the growth of leptothrix having followed
them with exactitude.

It sometimes happens that indications of spherical forms
and faintly discernible contours resembling those of the
interglobular spaces may be seen in dried sections, in which
no actual spaces occur. The appearances are perhaps pro-
duced by the formation of an interglobular space, the con-
tents of which have subsequently become more or less
perfectly calcined ; and the appearance described as " areolar
dentine" is probably to be explained in this manner.

The exact nature of the contents of the interglobular
spaces is not very certain : they may, with some difficulty,
be tinted by carmine, and it is said that they may, like
the dentinal sheaths, be isolated by the destruction of the
rest of the matrix in acids ; that this may be done I do
not doubt, although I have never succeeded in so isolating
them myself.

Bodecker finds that there is soft living plasm abundantly
distributed in the smaller interglobular spaces which con-
stitute the granular layer, and that this is in very free com-
munication with the soft fibrils in the tubes on the one side,
and with the soft contents of the lacunse and canaliculi of
the cementum on the other.

In the dentine so far described, which is that variety known
as hard or unvascular dentine, some degree of nutrition is
perhaps provided for by the penetration of the whole thick-
ness of the tissue by protoplasmic fibres, the dentinal fibrils,
but this nutrition may be effected in a different way, and
there are other varieties of dentine known in which dentinal
fibrils have never been shown to exist. For descriptive
purposes I would classify dentines as
(i.) Hard or unvascular dentine,
(ii.) Plici-dentine.
(iii.) Vaso-dentme.
(iv.) Osteo-dentine.

76 A MANUAL OF DENTAL ANATOMY.

Ordinary hard dentine has been sufficiently described ; of
it plici-dentine is a variety not very distinct in its essential
nature, though at first sight widely dissimilar.

Plici-dentine. — In ordinary dentine the dentinal tubes
radiate out from a pulp and pulp chamber of simple form ;
render complex that form by foldings of its walls, the den-
tinal tubes still running off at right angles to that portion
of pulp to which they immediately belong, and we have
a " plici-dentine." It is merely an ordinary dentine and its

Fio. 36 (1).

pulp folded up and wrinkled into a greater or less degree
of complexity.

In the teeth of the Varanus (monitor lizard) the process of
calcification of the pulp takes place in such manner that in
the upper half of the tooth a cap of ordinary unvascular
dentine, in which the tubes radiate from a single central

(1) Section of Plici-dentine with the pulp in situ (Lepidosteus).
c. Odontoblasts. p. Connective tissue framework of pulp. d. Dentine.

THE DENTAL TISSUES. 77

pulp cavity, is formed. But in the lower part of the tooth
slight longitudinal furrows appear on the surface, which, on
transverse section, are seen to correspond to dippings in of
the dentine ; and the dentine is, as it were, in folds. The

FIG. 37

pulp on section might be compared to a paddle-wheel, the
floats of which correspond to the thin flat radiating pro-
cesses of pulp ; but as yet the central pulp chamber is
unaltered. A little lower down, as represented in the
accompanying figure, there is no longer a central simple
pulp chamber; the inflections round the periphery have
become relatively much deeper, and the centre of the tooth
is occupied by a tissue irregular, but not otherwise unlike
the dentine of Myliobates ; that is to say, there are a
number of columns of pulp, each of which forms the axis
whence a system of dental tubes radiate.

The outrunning plates of dental pulp, which on section
radiate out like the spokes of a wheel, do not always remain
single ; they may divide simply into two branches, as may
be seen in the section across the base of the tooth of

(a) Transverse section across the crown of the tooth of Varanus, near to
its base. The central pulp cavity is produced out into processes, and it
might be said the dentine is arranged in plates with some little regularity
round its periphery.

78

A MANUAL OF DENTAL ANATOMY.

of Lepidosteus (North American bony pike) ; or sometimes
there are several branches.

FIG. 38

In Lepidosteus oxyurus there are simple inflections, and
a central pulp cavity; in L. spatula the inflections are
branched, and the central pulp cavity all filled up.

In the foregoing figure of the base of a tooth of Lepi-
dosteus some few of the outrunning pulp chambers are

(l) Transverse section across the tooth of Lepidosteus spatula. At the
exterior are regularly disposed radiating plates of dentine, each with its
own pulp cavity, while the central area is composed of more or less cylin-
drical pulp chambers, each of which, forms the starting point for its own
system of dentinal tubes. The pulp chambers are made dark in the figure
for the sake of greater distinctness.

THE DENTAL TISSUES.

seen to be bifurcated, while the central mass of the tooth
is composed of dentine permeated by pulp canals which
pursue a longitudinal course ; a slight further modification
brings us to the structure of the dentine of the Labyrin-
thodon, in which a maximum of complexity is attained,
although a clue to its intimate structure is afforded by the
teeth of Varan us or of Lepidosteus.

The laminse of pulp, with their several systems of den-

FIG. 39

b'

tinal tubes, instead of passing out in straight lines like the
spokes of a wheel, pursue a tortuous course as they run
from the central small pulp chamber towards the surface.
Not only do they undulate, but they also give off lateral

(*) Transverse section of a tooth of Labyrinthodon. (After Owen.)
ft •; The letter a is placed in the centre pulp chamber ; the letter b marks
the lines of separation between the system of dentinal tubes which belong
to each lamina of pulp ; these lines of demarcation were formerly sup-
posed to be occupied by cementum.

80

A MANUAL OF DENTAL ANATOMY.

processes ; and at their terminations near to the surface of
the tooth, the thin laminae of pulp (so thin that the radi-
ating pulp chambers are mere fissures) become dilated ; so
that on section circular canals are seen at these points, as
is also the case at the points where subsidiary processes
branch off.

The wavy course pursued by the radiating plates of den-

tine, and the disposition of the tubes round the dilated
portions of pulp chamber, render the general aspect of the
dentine structure very complicated ; the several "systems" (2)

(1) From tooth of Labyrinthodon, showing the nature of the connection
between the contiguous dentinal systems. (After a drawing of my father's. )

(2) The term " dentinal system " is applied to the portion of dentine in
which all the tubes radiate from a single section of pulp chamber ; thus
the tooth of Labyrinthodon is made up of many dentinal systems ; the
same thing may be said of the tooth of Mylio bates.

THE DENTAL TISSUES. 81

are united to one another by an inosculation of the ter-
minal branches of the tubes in some few places, but more
generally by a clear layer containing radiate spaces, some-
thing like the lacunae of cementum. Hence Professor Owen
has described the tooth as consisting of radiating plates of
dentine, between which pass in equally convoluted plates
of cementum. But, as was pointed out by my father (Phil.
Trans. 1850), the mere presence of lacuna-like spaces is
not sufficient to prove the presence of cementum, inasmuch
as they occur on a small scale in the granular layer of
dentine ; moreover, when cementum and enamel are both
present, the cementum is always outside the enamel, whereas
at the upper part of the tooth of the Labyrinthodon the cha-
racteristic inflections take place within a common investment
of enamel which does not dip in. Thus the whole of the
tissue constituting the very complex pattern of the Laby-
rinthodon tooth is dentine, and the cementum does not, as
was usually supposed, enter into its composition at all.

Another form in which plici-dentine may exist is exempli-
fied in the teeth of Myliobates, a large Ray ; or in the teeth
of the rostrum of the saw-fish (Pristis).

In the Myliobates (Fig. 41) the flat pavement-like
tooth is permeated by a series of equidistant parallel
straight canals, running up at right angles to the surface ;
from the upper end and sides of these channels systems of
dentinal tubes radiate, just as the tubes radiate from the
single pulp chamber of a human tooth, save that they run
for a comparatively short distance. In transverse sections
the tubes are seen radiating from these channels, and at
their terminations sometimes inosculating with the terminal
branches of the tubes of another system. The channels
contain prolongations of the vascular pulp, which are
distinct in the upper part of the tooth, but intimately
united together at its base, where the disposition of the
channels ceases to be regular, and, as a consequence, the

G

82 A MANUAL OF DENTAL ANATOMY.

systems of dentinal tubes pass from them in various direc-
tions without producing the symmetrical patterns which
characterise the upper part of the crown.

FIG. 41 0).

When the tooth comes into use and its immediate surface-
gets worn off, the ends of the perpendicular pulp channels
would be laid open, were it not that they become blocked
"by the deposition of a transparent homogeneous tissue
within them, analogous to the similar tissue which closes
Haversian canals of an antler about to be shed.

Such is an example of plici-dentine in a simple form, in
which the tooth might be said to be built up of a series of
small parallel denticles; and a similar structure is pre-
sented by the rostral teeth of the saw-fish, and by the teeth
of the Orycteropus or Cape ant-eater.

Vaso-dentine. — In the dentine of human teeth it occa-
sionally happens that a larger canal is found, having no
clear relation to the course of the dentinal tubes, which it
crosses at various angles ; this larger canal contained the

(J) Transverse section of the dentine of Myliobates.

THE DENTAL TISSUES. 83

blood-vessel, the remains of which may be found even in
a dried section. But in human dentine vascular canals do
not often occur, and when they do, are to be regarded as
decided abnormalities.

The accompanying figure, representing a canal of large

size, was drawn from a specimen shown to me at the Cam-
bridge (Massachusetts) Museum by Dr. Andrews.

In some mammalian teeth these vascular canals are
disposed with regularity, running out in loops from the
pulp cavity, and lying, for a considerable part of their course,,
at right angles to the dentinal tubes.

In the Manatee for example the dentinal tubes radiate
out with perfect regularity from the central pulp chamber^
and, so to speak, take no notice of the vascular canals,

(!) Vascular canal in dentine. From a human tooth.

G 2

84 A MANUAL OF DENTAL ANATOMY.

which are to be met with (especially in the root) in large
numbers.

Where they are numerous the vascular canals form loops,
so as to constitute a sort of plexus beneath the cementum.

The Tapir, whose teeth in external configuration are not
very dissimilar to those of the Manatee, also has vascular
canals in the dentine ; a curious difference in this respect

FIG. 43 (a).

was pointed out by my father (Proc. Zoolog. Soc. 1851)
between the Indian and the American Tapir, the former
having the canals in the dentine of the crown of the teeth,
the latter having them not. The great extinct Megatherium
possessed dentine very rich in these canals : to the left of
the figure is seen the inner portion of the dentine, rich in
them • in the middle a fine tubed dentine, forming the
external layer of the dentine of the whole tooth, and to the
right cementum, also rich in vascular tubes.

In those teeth in which the whole pulp is converted into
solid material, and no pulp cavity remains, the last portions
of the pulp are often converted into dentine which has not
the same character as that of the rest of the tooth. Thus

(l) Dentine and cementum of Megatherium : the latter to the right of
the figure.

THE DENTAL TISSUES. 85

in teeth of perpetual growth, such as the incisors of rodents,
the axial portion of the tooth is that latest calcined, and
consists of a dentine containing vascular canals, which are
not present in the other part of the tooth. When a change
thus occurs in the character of the tissue formed at a later
time than the rest of the dentine, the name "secondary
dentine " is applied to the resultant tissue.

But secondary dentine may partake of several different
varieties of structure, so that the term must not be taken as

FIG. 44 t1).

denoting anything more than the circumstances under which
it was formed.

It is in the class of Fish, in which vaso-dentine is rather
common, that the most instructive examples of its nature
are to be found.

The conical teeth of the common Flounder, and indeed of
most flat fish (Pleuronectidce) have their pointed tips formed

(T) Tooth of a Flounder, a, Dentinal tubes near apex of tooth ; 6,
Vascular canals ; c, Spear points of enamel.

86 A MANUAL OF DENTAL ANATOMY.

of ordinary hard dentine, surmounted by enamel tips. In
this part of the tooth the dentinal tubes are numerous, and
regular in their disposition, radiating out from the axial pulp
chamber.

Lower down in the teeth the dentinal tubes become less
numerous, and at the same time much larger looped canals
make their appearance, and as these become more numerous
and regular so do the dentinal tubes become less so. These
larger tubes contain blood-vessels, and red blood circulates
through them during the life of the tooth.

We may suppose that the nutrition of the dentine maybe

FIG. 45 (1).

provided for either by protoplasm carried for a long distance
from the pulp by the dentinal tubes, or by blood circulating
through the larger vascular channels, but that both are not
required, and so do not exist together.

And whilst the teeth of the Manatee, the Tapir, and of

f1) Tooth of Ostracion. a, Enamel ; I, Capillary channels ; c, Axial
pulp chamber.

THE DENTAL TISSUES.

87

the Flounder teach that hard dentine and vaso-dentine are
not very dissimilar in their nature, and that the one passes by
imperceptible gradations into the other, the dentine at the
base of the Flounder's tooth provides us with an example of
typical vaso-dentine : that is to say, dentine in which the
dentinal tubes are quite absent, having had their place
taken by a complete system of vascular channels.

The teeth of the Ostracion (Fig. 45), or of the Hake
(Figs. 46 and 86), afford good examples of this form of tissue.

FIG. 46 (l).

The matrix is solid, so far as penetration by fine tubes
goes, but it contains a system of larger canals which carry
only blood, and no pulp tissue, out to near the surface of
the dentine, where they form a plexus.

I have not been able to satisfy myself of the existence of
any definite structure in the matrix; sometimes it looks
granular, and sometimes has a finely reticulated look, re-
calling the appearances described by Bodecker in human
dentine. (See page 68.)

(l) Section of Dentine from a freshly caught Hake (Merlucius). d,
Dentine matrix ; cp, Capillary blood-vessels hanging out from its edge,
containing here and there abundant blood- corpuscles.

88 A MANUAL OF DENTAL ANATOMY.

The arrangement of the vascular canals is regular and
striking, reminding one of the appearance of the capillary
network in an injected intestinal villus. In fact, an intes-
tinal villus petrified, whilst the capillary network remained
pervious and carried red blood circulating through it, would
form no bad representation of a conical vaso-dentine tooth.

For these canals do actually contain capillaries, and blood
actively circulates through them; a section cut from the
fresh, brilliantly red tooth of a Hake often shows the coats
of the capillary hanging out from the edge, and the canals
full of blood-corpuscles (Fig. 46).

In all vaso-dentine teeth with which I am acquainted the
pulp chamber is of simple form, the pulp coated by a distinct
layer of odontoblasts, and no pulp-tissue other than the
capillaries passing out into the dentine, so that each capil-
lary fits and wholly fills its channel in the dentine.

Vaso-dentine is less dense and hard than ordinary dentine,
and consequently generally gets protection by a harder
tissue when exposed to hard work.

The teeth of the Hake, used simply for piercing and
catching fish, are merely tipped with enamel (Fig. 86);
those of Ostracion, put to severer work, are coated with
enamel, while the teeth of the Wrasse (Labrus), which are
composed of ordinary dentine are, though very hard worked,
unprotected by enamel.

Osteo-dentine. — This is a tissue far more sharply marked
off from hard dentine, plici-dentine and vaso-dentine, than,
these are from one another, and approaches closely to bone,
from which it has few points of essential difference.

The distinction can hardly be fully emphasized until the
development of dentine has been described, but it may be
mentioned that it is not developed on the surface of the pulp,
from an odontoblast layer, but within its whole substance.
Consequently in a completed osteo-dentine tooth there is no-
single simple pulp, which can be withdrawn from the

THE DENTAL TISSUES.

tooth, but pulp and calcified tissue are quite inextricably
mixed up.

And though there are numerous large channels, often
much larger than those of vaso-dentine, they are less regular,

FIG. 47

do not in their arrangement suggest the idea of capillary
loops, and in a fresh tooth contain masses of pulp-structure
as well as blood-vessels.

The Pike's tooth affords a good example of osteo-dentine.
Its surface is formed of a layer of fine tubed tissue, almost

0) Tooth of Common Pike,
inner mass of osteo-dentine.

a, Outer layer of fine tubed dentine ;

90 A MANUAL OF DENTAL ANATOMY.

like ordinary dentine, but this soon gives place to a coarsely
channeled tissue, containing elongated spaces filled with pulp,
from which canaliculi, like those of a bone lacuna, branch
off in all directions, but do not run far.

Very many sharks have teeth composed of osteo-dentine,

FIG. 48 (»).

with an outer dense layer : the tooth of Lamna here figured
shows a central core of osteo-dentine, which constitutes the
bulk of the tooth j external to this a somewhat thin layer of
hard dentine, in which all the dentinal tubes run out at
right angles to the surface, but are derived from the channels
of the osteo-dentine and not from any single pulp chamber ;
while the outermost layer, which is clear and structureless,

(l) Tooth of a species of Lamna, consisting of a central mass of vaso-
dentine, passing towards its surface into a fine-tubed unvascular dentine.
The clear structureless layer on the surface may probably be regarded as
enamel.

THE DENTAL TISSUES. 91

may be merely the outer part of the hard dentine, or may
be a thin layer of enamel. It is to be regretted that special
names have been given to this layer ; it is sometimes called
vitro-dentine, sometimes ganoin or fish-enamel ; but there is
no reason why it should have a special name at all. The
similarity of the channels of pulp in osteo-dentine to Haver-
sian canals in bone is very close ; in fact, when teeth con-
sisting of osteo-dentine become, as in many fish they do,
anchylosed to the subjacent bone, it becomes impossible to
say at what point the dentine ends and the bone commences ;
and this difficulty is intensified by the fact that the bone of
many fishes lacks lacunse, and is almost exactly like dentine.

Osteo-dentine was defined by Professor Owen as dentine
in which the matrix was arranged in concentric rings around
the vascular canals, and in which lacunae similar to those of
bone were found.

But neither of these characters are to be found in many
teeth, which, if the manner of their development is to be
taken into account, are unquestionably made of osteo-den-
tine ; and so they cannot be made use of for purposes of
definition, although lacunie and lamination of the matrix
are far more often present in osteo-dentine than in the other
varieties of dcntinal structure.

The varieties of dentine may be grouped thus : —

(A.) Dentines developed upon the surface of a pulp, by
calcification of a specialised layer of odontoblast
cells.

(i.) Hard, unvascular dentine, thoroughly per-
meated with dentinal tubes, which radiate
from a simple central pulp chamber.
Example — Human dentine,
(ii.) Plici-dentine, permeated with dentinal
tubes, which radiate from a pulp chamber
rendered complex in form by foldings in

92 A MANUAL OF DENTAL ANATOMY.

of its walls. Example — Lepidosteus,.
Labyrinthodon.

(iii.) Vaso-dentine, dentinal tubes few or
absent, but capillary channels with
blood circulating through them abun-
dant. Example — Hake.

(B.) Dentines developed by calcifications shooting-
through the interior of a pulp, not by calcifi-
cation of a specialised surface layer of cells,
(iv.) Osteo-dentine ; with no true dentinal
tubes, but minute tubes analogous to
bone canaliculi, and large irregular
channels containing pulp-tissue (not
blood-vessels only). Example — Tooth
.of Pike.

It remains to be added that the same pulp may undergo
a change in the manner of its calcification ; that is to say,
that after having gone on with surface calcification from
an odontoblast layer for a certain length of time, this may
give place to a more irregular internal calcification into an
osteo-dentine.

This is especially prone to happen after injury, and is
often exemplified upon a large scale in Elephants' tusks ;
the pulp of which, normally engaged in calcifying the
odontoblast layers into ivory, may after an injury calcify
irregularly, and solidify into a coarse osteo-dentine.

It will then be easy to understand that so-called secon-
dary dentine, produced in a pulp which ordinarily forms
hard dentine, may partake of the character of vaso- or of
osteo-dentine.

Thus the pulp of a sperm whale's tooth becomes oblite-
rated by a development of secondary dentine, which some-
times forms irregular masses loose in the pulp chamber, and
sometimes is adherent to and continuous with the dentine

THE DENTAL TISSUES.

previously formed. The structure of these masses is very
confused. Tubes, of about the same diameter as dentinal
tubes, abound ; but they are often arranged in tufts or in

FIG. 49

bundles, and without any apparent reference to any com-
mon points of radiation. Irregular spaces, partaking of the
character of interglobular spaces or of bone lacunae, abound;
and vascular canals are also common.

In the human tooth secondary dentine occurs in the teeth
of aged persons, in which the pulp cavity is much contracted
in size, and is also very frequently formed as a protection to
the pulp when threatened by the approach of dental caries,
or by the thinning of the walls of the pulp cavity through
excessive wear. The accompanying figure, representing one
of the cornua of the pulp chamber from a molar tooth
affected by caries, is a good- example of secondary dentine.

Section of a mass of secondary dentine from the tooth of a sperm

whale.

94

A MANUAL OF DENTAL ANATOMY.

It occasionally happens that the pulp resumes its formative
activity, and new dentine is developed which, with the
exception of a slight break or bend in the continuity of the
tubes, is almost exactly like normal dentine. More often,

FIG. 50 (').

however, the boundary line between the old and the new is
marked by an abundance of irregular spaces and globular
contours, whilst further in the mass of new secondary dentine,
the tubular structure again asserts itself more strongly : this,
is well seen in the specimen figured.

(*) Secondary dentine filling up one of the comua of the pulp cavity.
From a human molar affected by caries.

THE DENTAL TISSUES.

95

CEMENTUM.

The cement forms a coating of variable thickness over
the roots of the teeth, sometimes, when the several roots are
very close to another, or the cement is thickened by disease,
uniting the several roots into One.

The cement is ordinarily said to be absent from the
crowns of the teeth of man, the carnivora, &c., and to com-
mence by a thin edge just at the neck of the tooth, over-
lapping the enamel to a slight extent ; it is, in the healthy
state, thickest in the interspaces between the roots of molar
or bicuspid teeth : it is, however, often thickened at the end

Fia. 51 I1).

of a root by a dental exostosis. In compound teeth, the
cementum forms the connecting substance between the den-
ticles (see the figures of the tooth of the Capybara, the
Elephant, <fec.), and, before the tooth has been subject to
wear, forms a complete investment over the top of the
crown. The cementum also covers the crowns of the com-

(l) Thick laminated cementum from the root of a human tootK

96 A MANUAL OF DENTAL ANATOMY.

plex-patterned crowns of the teeth of ruminants ; and, in
my opinion, is present in a rudimentary condition upon the
teeth of man, &c., as Nasmyth's membrane. The cementum
is the most external of the dental tissues : a fact which ne-
cessarily follows from its being derived more or less directly
from the tooth follicle.

Both physically and chemically, and also in respect of the
manner of its development, the cementum is closely allied
to bone. It consists of a calcined matrix or basal substance,
to a slight extent laminated, and lacunas. Vascular canals
corresponding to the Haversian canals of bone, are met with,
but it is only in thick cementum that they exist ; and, in
man, perhaps in exostosis more often than in the thick
healthy tissue.

The matrix is a calcined substance, which, when boiled

yields gelatine, and if decalcified retains its form and struc-
ture : it is, in fact, practically identical with the matrix of
bone. It is sometimes apparently structureless, at others
finely granular, or interspersed with small globules.

The lacunae of cementum share with those of bone the
following characters: in dried sections they are irregular

(l) Lacuna of cementum which communicates with the terminations of
the dentinal tubes.

THE DENTAL TISSUES. 97

•cavities, elongated in the direction of the lamellae of the
matrix, and furnished with a large number of processes.
The processes of the lacunae (known as canaliculi) are most
abundantly given off" at right angles to the lamellae (see
Fig. 51), and, again, in cementum, are more abundantly
•directed towards the exterior of the root than towards the
dentine. The lacunae of cementum differ from those of
bone in being far more variable in size, in form, and in the
excessive number and length of their canaliculi ; in this
latter respect the lacunae of the cement of Cetacean teeth are
very remarkable.

Many of the lacunae in cementum are connected, by means
of their canaliculi, with the terminations of the dentinal
tubes (Fig. 52) ; they, by the same means, freely intercom-
municate with one another, while others of their processes
are directed towards the surface, which, however, in most
instances, they do not appear to actually reach.

The lacunae assume all sorts of peculiar forms, especially
In the thicker portion of the cement.

Here and there lacunae are to be found which are fur-
nished with comparatively short processes, and are contained
within well-defined contours. Sometimes such a line is to
be seen surrounding a single lacuna, sometimes several
lacunae are enclosed within it ; lacunae so circumscribed are
called " encapsuled lacunae," and were first observed by
Gerber in the cement of the teeth of the horse (they are
specially abundant in the teeth of the solidungulata). By
cautious disintegration of the cementum in acids these en-
capsuled lacunae may be isolated ; the immediate walls of
the lacunae and canaliculi, just as in bone, being composed
of a material which has more power of resisting chemical
re-agents than the rest of the matrix.

The encapsuled lacunae are to be regarded as individual
osteoblasts, or nests of osteoblasts, with a common connective

H

98 A MANUAL OF DENTAL ANATOMY.

tissue investment, which have to some extent preserved their
individuality during calcification.

In the fresh condition it appears probable that the lacunae
are filled up by soft matrix, which shrinks up, and so leaves.
them as cavities in dried sections. It can hardly as yet be
said that the question of the contents of lacunae has been
finally settled, though the researches of Bodecker and Heitz-
mann have gone far towards doing so.

According to them each lacuna contains a protoplasmic-
body, which they term the cement corpuscle, with a central
nucleus.

This nucleus may be large and surrounded by but little-
protoplasm, or it may be small ; or there may be many
nuclei.

The cement corpuscles communicate freely with one another
by offshoots, those of large size occupying the conspicuously
visible canaliculi of the lacunae, whilst the finer offshoots are
believed by them to form a delicate network through the
whole basis substance or matrix. The cement corpuscles near
to the external surface give off numerous offshoots which
communicate with protoplasmic bodies in the periosteum.
By this means the cementum can remain alive even when
the pulp of the tooth is dead, and thus the tooth is in no-
way a mere foreign body, dead and inert.

Like bone, cementum is also sometimes found to contain
Sharpey's fibres ; that is to say, rods running through it at
right angles to its own lamination, and, as it were, perforat-
ing it. These are probably calcified bundles of connective
tissue.

Where the cementum is very thin, as, for instance, where
it commences at the neck of a human tooth, it is to all
appearance structureless, and does not contain any lacuna),
and therefore no protoplasmic bodies : nevertheless lacuna*
may be sometimes found in thin cementum, as, for example,

THE DENTAL TISSUES. 99

in that thin layer which invests the front of the enamel of
the rodent-like tooth of a wombat.

The cementum at the neck is also devoid of lamellae ; it
appears to be built up by direct ossification of osteoblasts,
the prismatic shape of which may be traced in it : Bodecker
describes it as permeated by a fine but abundant network
of soft living matter. The larger dentinal tubes fall short
of the boundary line at the neck, but a fine protoplasmic-
network crosses it. Bodecker states that it has a covering
of epithelial elements, like those of the gum.

The outermost layer of thick cementum is a glassy film,,
denser apparently than the subjacent portions, and quite
devoid of lacunae ; on the surface it is slightly nodular, and
might be described as built up of an infinite number of
very minute and perfectly fused globules ; this is, in fact,
the youngest layer of cement, and is closely similar to that
globular formation which characterizes dentine at an early
stage of its development.

The cementum is very closely, indeed inseparably, con-
nected with the dentine, through the medium of the
" granular " layer of the latter ; the fusion of the two tissues
being so intimate, that it is often difficult to say precisely
at what point the one may be said to have merged into the
other. And in this region there is an abundant passage of
protoplasmic filaments across from the one to the other.

Nasmyth's membrane. — Under the names of Nasmythrs
membrane, enamel cuticle, or persistent dental capsule, a
structure is described about which much difference of
opinion has been, and indeed still is, expressed. Over the
enamel of the crown of a human or other mammalian tooth,
the crown of which is not coated by a thick layer of cemen-
tum, there is an exceedingly thin membrane, the existence
of which can only be demonstrated by the use of acids,
which cause it to become detached from the surface of the
enamel. When thus isolated it is found to form a continu-

n 2

100

A MANUAL OF DENTAL ANATOMY.

cms transparent sheet, upon which, by staining with nitrate
of silver, a reticulated pattern may be brought out, as
though it were made up of epithelial cells. The inner
surface of Nasmyth's membrane is, however, pitted for the
reception of the ends of the enamel prisms, which may have
something to do with this reticulate appearance. It is
exceedingly thin, Kblliker attributing to it a thickness of
only one twenty-thousandth of an inch, but, nevertheless,
it is very indestructible, resisting the action of strong nitric
or hydrochloric acid, and only swelling slightly when boiled
in caustic potash. Notwithstanding, however, that it resists
the action of chemicals, it is not so hard as the enamel, and
becomes worn off tolerably speedily, so that to see it well a
young and unworn tooth should be selected.

Observations upon the presence or absence of Nasmyth's

Fia. 53 O.

membrane in fish and reptiles are very much needed ; my
own recent investigations upon the development of the

(*) From a section of a bicuspid tooth in which the cementum, c, is con-
tinued over the outside of the enamel, a ; the dentine is indicated by the
letter 6.

THE DENTAL TISSUES.

101

teeth in these classes make me doubt whether the ct priori
conclusion of Waldeyer, who believes that the cuticle will
be found on all teeth, is not based upon an interpretation of
its nature which is incorrect.

The observation of Professor Huxley, who believed that
he found it upon the teeth of the frog, &c., may be suscep-
tible of another explanation, to which I shall have to recur,
merely premising here that its presence is only certain in
Primates, Carnivora, and Insectivora.

The singular power of resistance to re-agents which
characterises it proves nothing more than that it is a tissue,
imperfectly calcined, on the border-land of calcification, so
to speak, since similarly resistant structures are to be found
lining the Haversian canals, the dentiual tubes, the surface
of developing enamel, the lacunae, &c.

In my father's opinion (Dental Surgery, 1859) it is to be

FIG. 54 (').

regarded as a thin covering of cementum, and I have given
additional evidence in support of the view in a paper
referred to already in the list of works which heads this
chapter.

It now and then happens that the cementum upon a
more or less abnormal tooth, instead of ceasing at the neck,
(*) Encapsuled lacuna occupying a pit in the enamel.

102

A MANUAL OF DENTAL ANATOMY.

is continued up over the exterior of the enamel. This has
occurred less uncommonly than is generally imagined, and
the accompanying figure represents a portion of the crown
of such a tooth.

If the section be made of the grinding surfaces of such
teeth as present rather deep fissures in these situations,
well marked and unmistakeable lacunal cells, or encapsuled
lacunae, will be met with with considerable frequency. Now

FIG. 55 f1).

and then an encapsuled lacuna may be found occupying a
shallow depression in the enamel which it just fits, but
more commonly a dozen or more are crowded together in a
pit in the enamel, where they are usually stained of a brownish

(l) Nasmyth's membrane, set free by the partial solution of the enamel.
a. Nasmyth's membrane, b. Dentine, d. Mass occupying a pit in the
enamel, e. Enamel, a'. Torn end of Nasmyth's membrane.

THE DENTAL TISSUES. 103

colour. The occurrence of lacunee in these situations is far
from rare : my father's collection contains more than a dozen
good examples of them in these positions.

Nasmyth's membrane, thin though it is over the exterior
of the enamel, is thickened when it covers over a pit or
fissure, and when isolated by an acid is seen to have entirely
filled up such spots. (Fig. 55).

In these places, then, where the encapsuled lacunae are to
be found, Nasmyth's membrane also exists, a fact which
•alone would lend some probability to the view that it is
•eementum.

The general absence of lacunae in Nasmyth's membrane
is due to the fact that it is not thick enough to contain them ;
just as the thinnest layers of unquestionable cementum also
are without lacunae.

In sections of an unworn bicuspid which was treated with
acid subsequently to its having been ground thin and placed
upon the slide, I have several times been fortunate enough
to get a view of the membrane in situ ; it then appears to
be continuous with an exterior layer of cementum, which
becomes a little discoloured by the acid employed to detach
Nasmyth's membrane from the enamel. I am therefore
inclined to regard it as young and incomplete cementum,
and to consider it as representing (upon the human tooth)
the thick cementum which covers the crowns of the teeth of
Herbivbra ; and I am very glad to learn from my friend
Dr. Magitot, who has made many as yet unpublished re-
searches upon this subject, that he entirely concurs in this
view, which has also the support of Professor Wedl.

The evidence offered that Nasmyth's membrane is cementum,
although strong, does not amount to absolute proof ; it is therefore
desirable to briefly recapitulate the other explanations of its nature
which have been offered.

Nasmyth, who first called attention to its existence, regarded it
as " persistent dental capsule ; " a view of its nature not -very ma-
terially differing from that advocated in these pages.

104 A MANUAL OF DENTAL ANATOMY.

Professor Huxley described it as being identical with the mem-
brana performativa ; that is to say, with a membrane which covered
the dentine papilla prior to the occurrence of calcification, and
which afterwards came to intervene between the formed enamel
and the enamel organ. The objections to the acceptance of this,
view of its nature are so inextricably wrapped up with other ob-
jections to Professor Huxley's theory of the development of the
teeth, that they cannot profitably be detailed in this place ; it will
suffice to say, that evidence and the weight of authority alike point
to there being no such true membrane as this membrana performa-
tiva in the place in question.

Waldeyer holds that it (i.e., Nasmyth's membrane) is a product of
a part of the enamel organ. After the completion of the formation
of the enamel he believes that the cells of the external epithelium
of the enamel organ become applied to the surface of the enamel
and there cornified ; in this way he accounts for its resistance to
reagents, and for its peculiar smell when it is burnt.

Its extreme thinness, so far as it goes, is an objection to this
supposition : a more weighty argument against it is the absence of
analogy for such a peculiar change, by which one portion of the
same organ is calcified, and the rest cornified ; and again, what be-
comes of these cells in those teeth in which cementum is deposited
in bulk over the surface of the enamel ? According to the statement
of Dr. Magitot, the layer of cells in question (external epithelium
of the enamel organ) is atrophied before the time of the completion
of the enamel ; a fact which, if confirmed, is fatal to Waldeyer' a
explanation. And Dr. Magitot, in his most recent paper on the
subject (Journal de 1'anatomie, &c., 1881), gives his adherence to
the view that it is cementum.

Kolliker, who dissents strongly from the views of Waldeyer, and
admits some uncertainty as to its nature, provisionally regards it aa
a continuous and structureless layer furnished by the enamel cells
after their work of forming the fibrous enamel was complete ; a
sort of varnish over the surface, as it were.

This would not account for the occurrence of lacunae in it.

THE TOOTH PULP.

The Tooth Pulp, occupying the central chamber, or pulp
cavity, is the formative organ of the tooth, and consequently
varies to some extent in its anatomical character according
to its age. As well as being what remains of a formative
organ, it is the vascular and nervous source of supply
whence the dentine mainly derives its vitality.

THE DENTAL TISSUES. 105

The pulp may be described as being made up of a mucoid,
gelatinous matrix, containing cells in abundance, which are
especially numerous near to its periphery. In it some
fibrous connective tissue is discoverable, though this is not
abundant until the period of degeneration has set in. Nerves
and vessels also ramify abundantly in it.

The cellular elements of the pulp are arranged, as seen.
in transverse sections, in a direction radiating outwards from
the centre ; this is most marked in the highly specialised
layer of cells which form the surface of the pulp, and are
termed odontoUasts.

The odontoblast layer, sometimes called the membrana
eboris, because it usually adheres more strongly to the
dentine than to the rest of the pulp, and is therefore often
left behind upon the dentine when the pulp is torn away,
consists of a single row of large elongated cells, of darkish
granular appearance, with a large and conspicuous nucleus-
near to the end farthest from the dentine.

The sharp contours which the odontoblasts possess in
pulps which have been acted on by chromic acid, alcohol,
or even water, are absent in the perfectly fresh and unaltered
condition, and it is believed that they have no special in-
vesting membrane. They are furnished with three sets of
processes. The dentinal process (which is equivalent to the
dentinal fibre) enters the canal in the dentine, and the in-
dividual odontoblast may be furnished with several dentinal
processes. By means of lateral processes the cells communi-
cate with those on either side of them, and by means of
their pulp processes with cells lying more deeply; these
deeper cells again are to some extent intermediate in size
between the odontoblasts and the internal cells of the pulp.
The membrana eboris covers the surface of the pulp like
an epithelium. The odontoblasts vary much in form at
different periods ; in the youngest pulps, prior to the forma-
tion of dentine, they are roundish, or rather pyriform j

106 A MANUAL OF DENTAL ANATOMY.

during the period of their greatest functional activity the
end directed towards the dentine is squarish, though tapering
to a slight extent into the dentinal process ; while in old
age they become comparatively inconspicuous, and assume
a rounded or ovoid shape. The general matrix of the pulp,
as has been before noted, is of firm, gelatinous consistency :
it is a little more dense upon the surface, whence has
perhaps arisen the erroneous idea that the pulp is bounded
by a definite membrane.

The vessels of the pulp are very numerous ; three or
more arteries enter at the apical foramen, and breaking up
into branches which are at first parallel with the long axis
of the pulp, finally form a capillary plexus immediately
beneath the cells of the membrana eboris.

No lymphatics are known to occur in the tooth pulp.

The nerves enter usually by one largish trunk and three
or four minute ones : after pursuing a parallel course, and
giving off some branches which anastomose but little, in the
expanded portion of the pulp they form a rich plexus beneath
the membrana eboris, as has been described by Raschkow
and many subsequent writers.

But here our exact knowledge ends, for the nature of the
terminations of the nerve fibres in the pulp is not with
certainty known : the primitive fibrils, which are extra-
ordinarily abundant near to the surface of the pulp, often
form meshes, but this does not appear to be their real
termination.

Boll, as has been mentioned at a previous page, investi-
gated this point, and found that if a pulp be treated for an
hour with very dilute chromic acid solution, an immense
number of fine non-medullated nerve fibres, which he suc-
ceeded in tracing into continuity with the larger medullated
fibres, may be discerned near to the surface of the pulp.
The ultimate destination of these nerve fibres is un-
certain ; but he has seen them passing through the mem-

THE DENTAL TISSUES. 107

brana eboris, and taking a direction parallel to that of the
dentinal fibrils in such numbers that he infers that they
have been pulled out from the canals of the dentine. Still,
whatever may be the probabilities of the case, he has not
seen a nerve fibre definitely to pass into a dentinal canal,
nor has any other observer been more fortunate.

Boll's observations have not however been fully confirmed
by any subsequent 'worker in the field, nor have they been
definitely controverted until Magitot recently stated that he
had fully satisfied himself that the nerves become continuous
with the branched somewhat stellate cells which form a layer
beneath the odontoblasts, and through the medium of these
cells, with the odontoblasts themselves. (See Fig. 32.)

If this view of their relation to the nerves be correct the
sensitiveness of the dentine would be fully accounted for
without the necessity for the supposition that actual nerve
fibres enter it, for the dentinal fibrils would be in a
measure themselves prolongations of the nerves.

It has already been mentioned that the pulp undergoes
alterations in advanced age, its diminution in size by its
progressive calcification and the addition thus made to the
walls of the pulp cavity being the most conspicuous change
which occurs. In pulps which have undergone a little
further degeneration, the odontoblast layer becomes atro-
phied ; fibrillar connective tissue becomes more abundant,
€oiiicidently with the diminution in the quantity of the cel-
lular elements. Finally, the capillary system becomes
obliterated by the occurrence of thrombosis in the larger
vessels, the nerves undergo fatty degeneration, and the pulp
becomes reduced to a shrivelled, unvascular, insensitive
mass. These changes may go on without leading to actual
putrefactive decomposition of the pulp, and are hence not
attended by aveolar abscess ; but a tooth in which the pulp
has undergone senile atrophy is seldom fast in its socket.

The pulps of the teeth of some animals become eventually

108 A MANUAL OF DENTAL ANATOMY.

entirely converted into secondary dentine, but it would seem
to be very generally the case that those teeth which exercise
very active functions and last throughout the life of the
creature retain their pulp in an active and vascular condition.

THE GUM.

The gum is continuous with the mucous membrane of
the inside of the lips, of the floor of the mouth, and of the
palate, and differs from it principally by its greater density.
Its hardness is in part due to the abundant tendinous fasci-
culi which it itself contains, in part to its being closely
bound down to the bone by the blending of the dense fibrous,
fasciculi of the periosteum with its own. The fasciculi
springing from the periosteum spread out in fan-like shape
as they approach the epithelial surface. There is thus no
very sharp line of demarcation between the gum and the
periosteum when these are seen in section in situ.

The gum is beset with rather large, broad-based papillae,
which are sometimes single, sometimes compound; the
epithelium is composed of laminee of tesselated cells, much
flattened near to the surface ; but cylindrical cells form the
deepest layer of the epithelium, the rete malpighi.

Small round aggregations of pavement epithelium are
met with at a little depth, or even bedded in the surface ;
these, the "glands" of Serres, have no known significance.
In the neighbourhood of developing tooth-sacs epithelial
aggregations of similar appearance are to be met with, and
in such spots are remains of the neck of the enamel organ
(cf. page 137), which has undergone this curious change
subsequently to the completion of its original function.
The gums are rich in vessels, but remarkably scantily sup-
plied with nerves.

THE DENTAL TISSUES. 109

At the necks of the teeth the gum becomes continuous
with the periosteum of the internal surface of the alveoli,
into which it passes without any line of demarcation.

THE ALVEOLO-DENTAL MEMBRANE.

The Alveolo-dental 'Periosteum, or Eoot membrane, is a
connective tissue of moderate density, devoid of elastic
fibres, and richly supplied with nerves and vessels.

It is thicker near to the neck of the tooth, where it
passes by imperceptible gradations into the gum and peri-
osteum of the alveolar process, and near to the apex of the
root. The general direction of the fibres is transverse ; that
is to say, they run across from the alveolus to the cemen-
tum, without break of continuity, as do also many capillary
vessels ; a mere inspection of the connective tissue bundles,
as seen in a transverse section of a decalcified tooth in its
socket, will suffice to demonstrate that there is but a single
" membrane," and that no such thing as a membrane proper
to the root and another proper to the alveolus can be dis-
tinguished j and the study of its development alike proves
that the soft tissue investing the root, and that lining the
socket, are one and the same thing : that there is but one
" membrane," namely, the alveolo-dental periosteum.

At that part which is nearest to the bone the fibres are
grouped together into conspicuous bundles ; it is, in fact,
much like any ordinary fibrous membrane. On its inner
aspect, where it becomes continuous with the cementum, it
consists of a fine network of interlacing bands, many of
which lose themselves in the surface of the cementum.

But although there is a marked difference in histological
character between the extreme parts of the membrane, yet
the markedly fibrous elements of the outer blend and pass

110

A MANUAL OF DENTAL ANATOMY.

insensibly into the bands of the fine network of the inner
part, and there is no break of continuity whatever.

At the surface of the cementum it is more richly cellular,

FIG. 56 (').

and here occur abundantly large soft nucleated plasm
masses, which are the osteoblasts concerned in making
cementum, and which by their offshoots communicate with
plasm masses imprisoned within the cementum.

(*) Portion of the side of the root of a tooth, the gum and alveolar
dental membrane, and the edge of the bone of the alveolus.

A band of fibres is seen passing over the surface of the alveolus and
dividing, some to pass upwards into the gum, others to pass more directly
across to the cementum. Numerous orifices of vessels cut across trans-
versely are seen between the tooth and the bone.

THE DENTAL TISSUES. Ill

I have never seen the fibres, whether in longitudinal or
in transverse sections, pass straight in the shortest possible
line from the bone to the cementum, but they invariably
pursue an oblique course, which probably serves to allow for
slight mobility of the tooth without the fibres being stretched
or torn.

The vascular supply of the root membrane is, according
to Wedl, derived from three sources ; the gums, the vessels
of the bone, and the vessels destined for the pulp of the
tooth, the last being the most important.

The nerve supply also is largely derived from the dental
nerves running to the dental pulps ; other filaments come
from the inter-alveolar canals (canals in the bone, contain-
ing nerves and vessels, which are situated in the septa
separating the alveoli of contiguous teeth).

It should be borne in mind that the tooth pulp and the
tissue which becomes the root membrane have sprung from
the same source, and were once continuous over the whole
base of the pulp. A recognition of this fact makes it easier
to realise how it comes about that their vascular and nervous
supplies are so nearly identical.

The human tooth is, accepting as correct the researches
of Bodecker, which appear in every way deserving of credence,
connected with the living organism very intimately, even
though its special tissues are extra-vascular.

For , blood vessels and nerves enter the tooth pulp in
abundance ; the dentine is organically connected with the
pulp by the dentinal fibrils ; these are connected with the
soft cement corpuscles, which again are brought by their
processes into intimate relation with similar bodies in the
highly vascular periosteum.

So that between pulp inside, and periosteum outside, there
is a continuous chain of living plasm.

112 A MANUAL OF DENTAL ANATOMY.

KOLLIKER. Gewebelehre.

Manual of Histology, annotated by Messrs. Busk and

Huxley, 1853.

WALDEYER. Strieker's Histology. 1870.
FREY. Manual of Histology. 1874.
OWEN. Odontography.

CZERMAK. Zeitschrift f. Wiss. Zoologie. 1850.
NEUMANN. Zur Kentniss der Normalen Zahngeweber. 1853.
BOLL. Untersuchungen iiber die Zahnpulpa. Archiv. f. Mikros.

Anatom. 1868.

KLEIN. Atlas of Histology. 1880.
SALTER. Dental Pathology. 1874.
TOMES, J. Lectures on Dental Physiology and Surgery. 1848.

On the Dental Tissues of Rodentia and Marsupialia.

Philos. Transac. 1849, 1850.
On the Presence of Soft Fibrils in Dentine. Philos.

Transac. 1853.
TOMES, CHARLES S. On Vascular Dentine. Phil. Trans. 1878.

On the Implantation of Teeth. Proc. Odontol.

Soc. 1874—1876.
On Nasmyth's Membrane. Quart. Jour. Micros.

Science, 1872.
MAGITOT ET LEGROS. Journal de 1'anatomie de M. Ch. Robin.

1881.

RETZIUS. Mikrosk. Undersok, &c. Stockholm, 1837, and Transla-
tion in Nasmyth on the Teeth, 1839.
NASMYTH. On the Teeth. 1839.

HERTWIG. Ueber der Bau der Placoidschuppen Jenaische Zeit-
schrift, B. viii.

YON BOAS. Zahne der Scaroiden, Zeits. f. Wiss. Zoologie, B. xxxii.
BODECKER. Dental Cosmos. 1878.

LANKESTER, RAY. On the Teeth of Micropteron. Quart. Journal
Micros. Science, 1857.

CHAPTER IV.

THE DEVELOPMENT OF THE TEETH.

THE development of the teeth is a process which, while
subject to modifications in the different groups of verte-
brates, retains nevertheless in all certain essential charac-
ters, so that it becomes possible to embody its main features
in a general account.

Prior to the commencement of any calcification there is
always a special disposition of the soft tissues at the spot
where a tooth is destined to be formed ; and the name of
" tooth germ " is given to those portions of the soft tissue
which are thus specially arranged. All, or a part only, of
the soft structures making up a tooth germ, become con-
verted into the dental tissues by a deposition of salts of lime
within their own substance, so that an actual conversion of
&t least some portions of the tooth germ into tooth takes
place. The tooth is not secreted or excreted by the tooth
germ, but an actual metamorphosis of the latter takes
place. The details of this conversion can be better dis-
cussed at a later page ; for the present it will suffice to say
that the three principal tissues, namely, dentine, enamel,
and cementum are formed from distinct parts of the tooth
germ, and that we are hence accustomed to speak of the
enamel germ and the dentine germ ; the existence of a
special cement germ is asserted by Magitot, but as yet his
descriptions await confirmation.

In many anatomical works which the student may have
occasion to consult, the process of tooth development may

i

314 A MANUAL OF DENTAL ANATOMY.

still be found to be divided into periods, under the names
of " papillary," " follicular," and " eruptive " stages.

These stages are based upon a false conception, upon
theories now known to be incorrect, and may advantageously
be absolutely abandoned. The account of the development
of the teeth given in the following pages (based in the
case of man and mammals upon the researches of Kolliker,
Thiersch, and Waldeyer ; in the case of reptiles and fishes,
upon those of Huxley and Santi Sirena, and upon Hertwig's
and my own), will be found to conflict with the accounts,
published by a deservedly great authority, Professor Owen.
I cannot reconcile these discrepancies, except upon the as-
sumption that modern methods of research have disclosed
facts heretofore not demonstrable ; yet twenty years ago
Professor Huxley demonstrated in a remarkable paper the
incorrectness of certain of the theories then promulgated.
Of the general accuracy of the following description I am
however fully satisfied, and most of the facts may be easily
verified by any one desirous of so doing.

True tooth germs are never formed quite upon the surface (%
but are always situated at a little distance beneath it, lying in
some creatures at a considerable depth. Every known tooth
germ consists in the first instance of two portions, and two-
only, the enamel germ and the dentine germ ; and these are
derived from distinct sources, the former being a special
development from the epithelium of the mouth, the latter
from the more deeply lying parts of the mucous membrane.
Other things, such as a tooth capsule, may be subsequently
and secondarily formed, but in the first instance, every
tooth germ consists of an enamel germ and a dentine germ-
only, and the simplest tooth germs never develop any addi-
tional parts. The existence of an enamel organ in an early
stage is therefore perfectly independent of any subsequent

0) The placoid scales of embryonic sharks are, however, formed on the
surface, and the "germs " covered in by epithelium only (Hertwig).

THE DEVELOPMENT OF THE TEETH. 115

formation of enamel by its own conversion into a calcined
tissue, for I have shown that it is to be found in the germs
of teeth which have no enamel ; in fact, in all tooth germs
whatever.

That part of the tooth germ destined to become dentine
is often called the dentine papilla, having acquired this
name from its papilliform shape ; and in a certain sense it
is true that the enamel organ is the epithelium of the den-
tine papilla. Yet, although not absolutely untrue, such an
expression might mislead by implying that the enamel
organ is a secondary development, whereas its appearance
is contemporaneous with, if not antecedent to, that of the
dentine germ. The most general account that I am able
to give of the process is, that the deeper layer of the oral
epithelium sends down into the subjacent tissue a process,
the shape and structure of which is, in most animals, dis-
tinguishable and characteristic before the dentine germ has
taken any definite form. This process enlarges at its end,
and, as seen in section, becomes divaricated, so that it bears
some resemblance to an inverted letter Y ; or it might more
truthfully be compared to a bell jar with a handle ; this
constitutes the early stage of an enamel germ (see Fig. 64),
while beneath it in the mucous tissue, the dentine germ
assumes its papilliform shape. The details of the process
varying in different creatures, I will at once proceed to the
description of the development of teeth in the various
groups.

In Elasmobraiich Fishes. — If a transverse section
through the jaw of a dog-fish (Scyllium canicula) be ex-
amined, we shall find that the forming teeth lie upon the
inside of the semi-ossified jaw-bones, the youngest being at
the bottom (Fig. 57) ; progressing upwards, each tooth is-
more fully calcified till, on passing over the border of the
jaw, we come to those teeth whose period of greatest useful-
ness is passed and which are about to be cast off in the

i 2

116 A MANUAL OF DENTAL ANATOMY.

course of that slow rotation of the whole tooth-bearing
mucous membrane over the border of the jaw, which is
constantly going on.

In the section figured there are four teeth advanced in
calcification, while beneath them are four tooth germs in
earlier stages ; of the former two only are fully protruded
through the epithelium, the third being in part covered
in ; the remaining teeth are altogether beneath the surface
of the epithelium, and therefore shut off from the cavity of
the mouth, if the soft parts be all in situ.

All the teeth not fully calcified are covered in and pro-
tected by a reflexion upwards of the mucous membrane (c in
the figure), which serves to protect them during their calci-
fication.

But although this may be termed a fold reflected upwards, it is
not, as was supposed by Professor Owen, a free flap, detached from
the opposite surface on which the teeth are developing ; there is no
deep open fissure or pouch running round inside the jaw, as would
in that case exist, and the epithelium does not pass down on the
one side to the bottom of such fissure, and then ascend upon the
other as a distinct layer. Although the fold is very easily torn
away from the tooth germs which it covers in, yet in the natural
condition it is attached, and there is no breach of surface ; the
epithelium passing across from the jaw to cover it is well seen in
the figure, in which the epithelial layer is represented as broken
just at the point (between the third and fourth teeth) where it
leaves the jaw to cross over on to the surface of the flap.

The conditions met with in the Elasmobranch fishes are
peculiarly favourable for the determination of the homolo-
gies of the several parts of the tooth germ and of the formed
tooth (1). At the base of the jaw, where the youngest
tooth germs are to be found, the tissue whence the dentine
papillae arise blends insensibly with that making up the
substance of the thecal fold on the one hand, and on the

(!) Compare the description of the placoid dermal spine (page 2).

THE DEVELOPMENT OF THE TEETH.

117

other, with that clothing the convexity of the jaw and giving
attachment to the teeth.

No sharp line of demarcation at any time marks off the
base of the dentine papilla from the tissue which surrounds
it, and from which it springs up, as would be the case in
mammalian or reptilian tooth germs ; all that can be said
is, that the dentine germs are cellular, the cells being large

FIG. 57 (!).

and rounded, while in the rest of the mucous membrane
the fibrillar elements preponderate, so that it passes by
imperceptible gradations into the densely fibrous gum, found
on the exposed border of the jaw.

(!) Transverse section of lower jaw of a Dog-fish, a. Oral epithelium.
6. Oral epithelium passing on to flap. c. Protecting flap of mucous mem-
brane (thecal fold), d. Youngest dentine pulp. e. Youngest enamel
organ. /. Tooth about to be shed. y. Calcified crust of jaw.

118 A MANUAL OF DENTAL ANATOMY.

The dentine germs, and consequently the dentine, are
indisputably derived from the connective tissue of the
mucous membrane immediately subjacent to the epithe-
lium, nor can it be doubted that the enamel organs are
simply the modified epithelium of that same mucous
membrane.

Of course there is nothing new in this conclusion, which
had been already arrived at by the study of other creatures,
but the sharks happen to demonstrate it with more clear-
ness than those other animals in whom the original nature
of the process is more or less masked by the introduction of
further complexities.

Hence it is worth while to study carefully the relations
of the epithelium constituting the enamel organs with that
of the surface of the mouth. As has been already mentioned,
in the normal condition of the part there is no deep fissure
on the inner side of the jaw, but the epithelium passes
across (from the interspace between the third and fourth
teeth in the figure) on to the protecting fold of mucous
membrane (c in fig.) But although the epithelium is re-
flected across on to the thecal fold, it is also continued
downwards along the inner side of the developing teeth and
tooth germs, giving to each a complete investment, and
filling up the whole interval between the tooth germ and the
thecal fold. The epithelium in this situation does not, then,
consist simply of one layer going down on the one side and
covering the tooth germs, and then reflected up at the
bottom to coat the inner side of the thecal fold, but it is so
arranged as to have relation only to the tooth germs ; it is
termed " enamel organs " because over the tooth germs these
epithelial cells assume a marked columnar character, and
are very different in appearance from the epithelium else-
where.

The terminal portion of this epithelium, or, in other
words, the youngest enamel germ, forms a bell-like cap over

THE DEVELOPMENT OF THE TEETH. 119

the eminence of mucous membrane connective tissue which
constitutes the earliest dentine germ, and in section is of
the form shown in the figure. The surface next to the
dentine papilla consists of elongated columnar cells, with
nuclei near to their attached extremities, while the rest of
its substance is made up of much smaller cells, some of
which have inosculating processes, so that they constitute
a sort of finely cellular connective tissue, very different in
appearance from anything met with in mammalian enamel
organs. It is sufficiently consistent to keep up the con-
tinuity of all the enamel organs, even when displaced in
cutting sections, so that the whole might be described as
forming one composite enamel organ. The columnar cells
already alluded to invest the whole surface which is
directed towards the forming teeth, but they atrophy some-
what in the interspaces of the tooth germs.

Before proceeding further in the description of the deve-
lopment of the tooth germs, it will be well to refer to a
somewhat earlier stage in the growth of the Dog-fish, in
which the relation subsisting between the teeth and the
dermal spines is still well seen.

On the lower jaw of the young dog-fish there is no lip ;
hence, as is seen in the figure, the spines which clothe the
skin come close to the dentigerous surface of the jaw.

Although there are differences in form and size, a glance
at the figure will demonstrate the homological identity of the
teeth and the dermal spines. As the dog-fish increases in
size, this continuity of the teeth with the dermal spines on
the outside of the head becomes interrupted by an extension
of the skin to form a lip ; this happens earlier in the upper
jaw than in the lower, and at first the spines are continued
over the edge and the inside of the newly formed lip — from
these situations, however, they soon disappear. In structure,
the teeth and the dermal spines are, in many species, very
closely similar ; the latter are, however, much less often

120 A MANUAL OF DENTAL ANATOMY.

shed and reproduced, so that it is less easy to find them in
all stages of their growth ; I believe, however, that they
follow a course essentially similar to that of the teeth.

FIG. 58 0).

It is stated by Gegenbaur that in Selachia the mucous
membrane of the mouth is clothed with spines of a structure-
similar to that of the teeth, and that these spines are often
limited to particular regions, extending back as far as the
pharynx — these same regions in Ganoids and Osseous fishes,
being occupied by conspicuous teeth; and Hertwig has.
shown that the dermal spines are developed in a manner
precisely analogous to that described in the teeth, save that
the germs are even less specialised.

In Teleostei or Osseous Fishes. — In passing from
the consideration of the development of the tooth germs of
Elasmobranch to those of Osseous fishes, the first difference
to be noted is this : whereas in the former each tooth germ
was, so far as the enamel germ is concerned, derived from that
of the next older tooth, in the latter each enamel germ often,
arises independently and, as it were, de now. At all events,

(*) Section of lower jaw of young Dog-fish, showing the continuity of
the dermal spines of the skin under the jaw, with the teeth which lie
above and over its end.

THE DEVELOPMENT OP THE TEETH. 121

so far as my own investigations go, no connection has been
traced between the germs of teeth of different ages ; but
Heincke says that in the Pike new enamel organs may be
derived from older ones.

This independent origin of an indefinite number of teeth,
having no relation to their predecessors, is only certainly
known to occur in the osseous fish : of the development of
the teeth of Ganoid fish nothing is known.

The oral epithelium, which varies much in its thickness
and in other characters in different fishes, sends down a
process which goes to form an enamel organ, whilst a dentine
papilla in rising up to meet it, comes to be invested by it as
with a cap. The after- history of the process depends much
on the character of tooth which is to be formed. If no
enamel, or but a rudimentary coat of enamel, is to be formed,

FIG. 59 (l).

the cells of the enamel organ remain small and insignificant,
as in the mackerel. If, on the other hand, a partial invest-
ment of enamel is found upon the perfected tooth, such
for instance, as the little enamel tips upon the teeth of the
eel (see Fig. 90), then the after-development of the enamel
organ is very instructive.

(l) Tooth-germ of an eel. d. Neck of enamel organ, e. Enamel cells.
a. Cap of enamel, b. Cap of dentine, e. Rudimentary enamel cells
opposite to that part of the dentine germ where no enamel will be formed.

122 A MANUAL OF DENTAL ANATOMY.

Opposite to the apex of the dentine papilla, where the
enamel cap is to be, the cells of the enamel organ attain to
a very considerable size, measuring about T^ of an inch in
length ; below this the investing cap of enamel organ does
not cease, but it is continued in a sort of rudimentary con-
dition. Thus, although the enamel organ invests the whole
length of the dentine papilla, its cells only attain to any
considerable size opposite to the point where the enamel is
to be formed. The knowledge of this fact often enables an
observer to say, from an inspection of the tooth germ,
whether it is probable that the perfected tooth will be coated
with enamel or not. In any case an enamel organ will be
there, but if no enamel is to be formed, the individual cells
do not attain to any considerable degree of differentiation
from the epithelium elsewhere ; in other words, the whole
enamel organ will partake of the character of the lower
portion of that represented in the figure of the tooth germ
of the eel.

Although of course there are many differences of detail
arising from the very various situations in wrhich teeth are
developed in fish, so great uniformity pervades all which
I have examined, that we may at once pass on to the con-
sideration of the development of the teeth of reptiles, merely
adding that it is not altogether true to say that the teeth of
fish in their development exemplify transitory stages in the
development of mammalian teeth.

In Reptiles. — So far as the appearances presented by
the individual germs go, there are few differences worthy
of note to be found in the present class by which they are
distinguishable from those of either fish or mammals. The
enamel organ is derived from the oral epithelium, the den-
tine organ from the submucous tissue in a manner very
similar ; nevertheless, there are points in the relation which
the successional tooth germs bear to one another, arid to the
teeth already in situ, which are of some little interest. The

THE DEVELOPMENT OF THE TEETH.

123

constant succession of new teeth met with amongst almost
all reptiles renders it easy to obtain sections showing the
teeth in all stages of growth : upon the inner side of the jaw
there will be found a region occupied by these forming teeth
and by nothing else, which may be called " area of tooth deve-
lopment ; " this is bounded on the one side by the bone and

FIG. 60

teeth which it carries, and on the other by a more or
less sharply denned wall of fibrous connective tissue. In
the newt, for example (Fig. 60), to the left of the tooth hi
use are seen four tooth sacs, in serial order, the youngest
being nearest to the median line of the mouth. As the sacs
increase in size they appear to undergo a sort of migration
towards the edge of the jaw, while simultaneously new ones
are constantly being developed beyond them. In the newt,

(*) Section of upper jaw of Triton cristatus (newt). To the inner side
of the tooth attached to the bone are three younger tooth germs.

124 A MANUAL OF DENTAL ANATOMY.

the ingrowth of the epithelium is obviously the first step
apparent; this ingrowth of a process of epithelium takes-,
place in close relation with the " neck " of an older enamel
organ (i.e., the contracted band of epithelium which remains
for some time connecting the new enamel organ with the
epithelium whence it was derived). New enamel organs are
therefore not derived directly from the epithelium of the
surface, but from the necks of the enamel organs of their
predecessors.

In the newt, the developing teeth spread out for a con-
siderable distance towards the palate, and thus, being free
from crowding, the relations of the enamel organs of three
or four successional teeth of serial ages may be studied in a
single section j and the arrangement so disclosed may be
advantageously compared with that seen in the dog-fish (see
Fig. 57).

The tooth sac of the newt is a good example of the sim-
plest form of tooth sac, consisting solely of an enamel organ
and a dentine germ, without any especial investment. The
" sac" is wholly cellular, and on pressure breaks up, leaving:
nothing but cells behind it. The cells of the enamel organ
are large, and resemble those of the eel ; the teeth of newts,
have a partial enamel tip, like those of the fish referred to,
but differing from them in being bifurcated, as is very
early indicated by the configuration of the enamel organ.

In the frog there is a peculiarity in the manner in which
the two jaws meet, the edentulous lower jaw, which has no-
lip, passing altogether inside the upper jaw and its sup-
ported teeth, and so confining the area of tooth develop-
ment within very narrow limits. Consequently I have been
unable to satisfy myself whether the new tooth germs, or
rather their enamel organs, are derived from those of their
predecessors, or spring up de novo — Analogy would indicate
the former, but appearances tend towards the latter sup-
position.

THE DEVELOPMENT OF THE TEETH. 125

In the lizards the new tooth germs are formed a very long
way beneath the surface, so that the neck of the enamel
organ becomes enormously elongated, for the dentine papilla
is, just as in the newt, situated at first quite at the level of
the floor of the area of tooth development. The teeth of
the lizards have a more complete investment of enamel,
hence the enamel cells are developed upon the side of the
dentine germ to a much lower point than in the newt. The

germs also acquire an adventitious capsule, mainly derived
from the condensation of the connective tissue around

(]) Transverse section of the lower jaw of common English Snake, e.
Inward dipping process of epithelium. /. Oral epithelium. 1, 2, 3, &c.
Tooth germs of various ages. 8. Tooth in place, cut somewhat obliquely,
so that its tip apparently falls short of its surface, and does not project
above the mucous membrane.

126 A MANUAL OF DENTAL ANATOMY.

them, which is pushed out of the way as they grow larger.
The further progress of the tooth germ being identical with
that of mammalia, its description may be for the present
deferred.

In ophidian reptiles (snakes) several peculiarities are met
with which are very characteristic of the order. A snake's
method of swallowing its food would seem to render the
renewal of its teeth frequently necessary; although I da
not know of any data by which the probable durability of
an individual tooth could be estimated, the large number
of teeth which are developing in reserve, all destined ta
succeed to the same spot upon the jaws, would indicate that
it is short.

I have seen as many as seven successional teeth in a single
section, and their arrangement, particularly in the lower
jaw, which undergoes great displacement while food is being
swallowed, is very peculiar. The numerous successional
tooth sacs, instead of being spread out side by side, as in
the newt, are placed almost vertically, and in a direction
parallel with the surface of the jaw-bone ; they are, more-
over, contained in a sort of general investment of connective
tissue ; a species of bag to keep them from displacement
during the expansion of the mouth.

The inward growing process of oral epithelium enters this
case of tooth sacs at its top j and may be caught sight of
here and there as its prolongations wind their way by the
sides of the tooth sacs to the bottom of the areat Here
the familiar process of the formation of an enamel organ and
dentine papilla may be observed, in no essential point differ-
ing from that which is to be seen in other animals.

That the derivation of each enamel organ is from a part of
that of its predecessor is very obvious ; the dentine organs
are formed in relation with the enamel germs, but apparently
independently of one another.

As the tooth sacs attain considerable dimensions, a curious

THE DEVELOPMENT OF THE TEETH.

127.

alteration in position takes place ; instead of preserving a
vertical position, they become recumbent, so that the form-
ing tooth lies more or less parallel with the long axis of the
jaw. The utility of such an arrangement is obvious : were

....I

the tooth to remain erect after it has attained to some little
length, its point would probably be forced through the
mucous membrane when the mouth was put upon the
stretch ; but while it lies nearly parallel with the jaw no-
such accident can occur.

The tooth does not resume the upright position until it
finally moves into its place upon the summit of the bone.

(J) Developing teeth of a Snake. /. Oral epithelium, e. Neck of the
enamel organs, b. Dentine pulp. c. Enamel cells, d. Dentine. 1, 2.
Very young germs. 3, 4. Older germs.

J28 A MANUAL OF DENTAL ANATOMY.

As has already been mentioned, there is a well-developed
enamel organ with large enamel cells : from these a thin
layer of enamel is formed and thus the thin exterior layer
upon the teeth of snakes is true enamel, and not, as has
been usually supposed, cementum.

Many points in the development of the teeth of reptiles I
have passed over very briefly for the want of space ; a more
full account of my observations will be found in the Philo-
sophical Transactions for 1875.

In Mammalia the earliest changes which will ultimately
result in the formation of a tooth are traceable at a very

FIG. 63

early period ; before the commencement of ossification, the
lower jaw consisting solely of Meckel's cartilage imbedded in
embryonic tissue, and the lateral processes which become
the upper maxillary bones having but just reached as far
as the median process which constitutes the intermaxillary
bone. That is to say, about the fortieth or forty-fifth day
(in the human subject), in the situation corresponding to
the future alveolar border, there appears a slight rounded
depression, extending the whole length of the jaw, it and

(*) Embryo at end of fifth week after Carpenter. 1, 2. First two
visceral arches, a. Superior maxillary process, t. Tongue. b. Eye.
c. Lateral nasofrontal process, nf. Nasofrontal process.

THE DEVELOPMENT OF THE TEETH. 129

its elevated borders being formed by an increase in the
thickness of the layer of epithelial cells ; while in trans-
verse sections the proliferation of epithelial cells is found to
have been even more energetic in a direction downwards
into the substance of the jaw than it is upwards, so that a
cul-de-sac of epithelium dips into the embryonic sub-mucous
tissue.1

In a certain sense, then, there is a dental groove, but it
is not the same thing as that described as such in the text-
books, and it therefore better to abstain from applying that
or any other name to the shallow furrow of which we are
now speaking, which is almost filled up with spherical or
flattened cells, the deepest layer being, however, columnar
cells. From the bottom, or the side near the bottom of the
depression, an inflection of epithelial cells takes its origin,
which does not dip downwards vertically, but inclines in-
wards. This secondary narrow inflection of epithelium,
which in section closely resembles a tubular gland, consti-
tutes the rudiment of the future enamel organ ; a prolife-
ration of the cells at its deepest extremity speedily takes
place, so that it expands, attaining somewhat the form of a
Florence flask (Fig. 64). It should, however, be noted, that
while the inflection of epithelium takes place around the
entire circumference of the jaw, so that that which appears
in sections like a tubular gland is really a continuous sheet
or lamina of epithelium, the dilatations of its extremity,
which I have compared to a Florence flask, occur only at the
several points where teeth will ultimately be developed.

The cells upon the periphery are columnar, polygonal
cells occupying the central area of the enlargement. Very

(!) The epithelium having been removed by maceration or by keeping a
specimen in dilute spirit, a groove would result, and this is probably what
was seen and described by Goodsir as the "primitive dental groove":
but, as the student will gather from the text, there is at no time any such
thing as a deep open groove like that described by him, unless it results,
from maceration and consequent partial destruction of the specimen.

K

130

A MANUAL OF DENTAL ANATOMY.

soon the terminal enlargement, as it grows more deeply
into the jaw, alters in form; its base becomes flattened.

FIG. 64 (l).

1 2

tr

and the borders of the base grow down more rapidly than
the centre, so that its deepest portion presents a concavity
looking downwards ; it might be compared to a bell, sus-

(l) Three stages in the development of a mammalian tooth germ (from
Frey). a. Oral epithelium heaped up over germ. &. Younger epithelial
cells, c. Deep layer of cells, or rete Malpighi. d. Inflection of epithelium
for enamel germ. e. Stellate reticulum. /. Dentine germ. g. Inner
portion of future tooth sac. h. Outer portion of future tooth sac.
i. Vessels cut across, k. Bone of jaw.

THE DEVELOPMENT OF THE TEETH. 131

pended from above by the thin cord of epithelium which
still connects it with the epithelium of the surface, or it
might in section be described as crescentic, the horns of the
crescent being long, and looking downwards. Coincident
with the assumption of this form by the enamel germ, is
the appearance of the dentine germ ; but it will facilitate
the description of the process to pursue a little farther
the development of the enamel organ.

The cells on its periphery remain prismatic or columnar,
but those in its centre become transformed into a stellate
network, in which conspicuous nuclei occupy the centre of
ramified cells, the processes from which anastomose freely
with those of neighbouring cells (see Fig. 65). This conver-
sion of the cells into a stellate reticulum is most marked
quite in the centre of the enamel organ ; near to its surfaces
the processes of the cells are short and inconspicuous.

The transformation of the cells occupying the centre and
constituting the bulk of the enamel organ into a stellate
reticulum goes on progressing from the centre outwards, but
it stops short of reaching the layer of columnar cells which
constitute the surface of the enamel organ, next to the den-
tine papilla ; a narrow layer of unaltered cells which remain
between the stellate cells and the columnar enamel cells
constituting the " stratum intermedium."

Thus far the cells constituting the periphery of the
enamel organ are alike : they are columnar or prismatic,
but from the time of the appearance of the dentine papilla
those which come into relation with it become much more
elongated and greatly enlarged, while those round the outer
or convex surface of the enamel organ do not enlarge ;
indeed, according to some authors, they even commence to
atrophy even at this early period. The cells which lie like
a cap over the dentine germ or "papilla" as they elongate
and their nuclei recede to their extremities, take on the
character to be presently described as belonging to the

K 2

132 A MANUAL OF DENTAL ANATOMY.

" enamel cells," (enamel epithelium, internal epithelium of
the enamel organ).

The enamel organ, then, consists (proceeding from with-
out inwards) of an " external epithelium," a " stellate reti-
culum," a " stratum intermedium," and an " internal epithe-
lium," the external and internal epithelia being continuous
at the edges or base of the enamel organ, while at its summit
the external epithelium remains still, through the medium
of the " neck of the enamel organ," in continuity with the
cells of the " stratum Malpighi."

Thus the enamel organ is entirely derived from the oral
epithelium, with which, by means of its "neck," it long
retains a connection, so that it, and whatever products it
may afterwards give rise to, are obviously to be regarded as
" epithelial structures." But it is the enamel organ alone
which is directly derived from the epithelium ; the origin of
the dentine germ is quite distinct.

In the embryonic tissue of the jaws, some little distance
beneath the surface, and at a point corresponding to that
ingrowth of cells and subsequent enlargement of the same
which goes to form the enamel organ, appears the first trace
of a dentine germ.Q This appears as a mere increase in
the opacity of the part, without at first any visible structural
change, and it occupies the concavity of the enamel organ.
Thus the dentine germ appears early, indeed almost simul-
taneously with the formation of a definite enamel organ, but
the enamel organ is far in advance of it in point of structural
differentiation, and the earliest changes which result in the
formation of the enamel organ are strikingly visible before a
dentine germ can be discovered. According to Dursy the

(T) The term "dental papilla," although eminently convenient, is asso-
ciated with an erroneous feature of the older views upon tooth develop-
ment ; where it is employed in the following pages, the student must
guard against the misconception that free papillae at any time exist in any
animal.

THE DEVELOPMENT OF THE TEETH. 133

dark halo which becomes the dentine bulb is, like the inflec-
tion of epithelium which forms the enamel germ, continuous
all round the jaw, while eventually it develops into promi-
nences at the points corresponding to the enamel germs of
future teeth, and atrophies in their interspaces.

From the base of the dentine bulb prolongations pass out-
ward and slightly upwards, so that they in a measure embrace
the free edge of the enamel organ, and at a somewhat later
period they grow upwards till they fairly embrace the whole
enamel organ.

These prolongations of the base of the dentine bulb are
the rudiments of the dental sac. In their origin, therefore,
the dental sac and the dentine organ are identical, and
spring from the submucous tissue : they contrast with the
enamel organ, which, as before said, is derived from the oral
epithelium.

To recapitulate briefly the facts which are now established
beyond all question, the early mammalian tooth germ con-
sists of three parts, one of which, the enamel organ, is
derived from the epithelium of the surface ; the other two,
the dentine organ and the dental sac, originate in the midst
of solid embryonic tissue at a distance from the surface.

The enamel organ is formed by a rapid increase of cells at
the bottom of a process which dips in from the stratum
Malpighi of the oral epithelium ; the dentine germ and the
dental sac are formed in close contiguity to this enamel
organ from the submucous tissue.

If there was a " basement membrane " demonstrable at
this early period (which there is not) the enamel organ and
the dentine organ would lie upon the opposite sides of it.

The description of the appearance of the several parts of
the tooth germ has brought us to the period at which cal-
cification first commences, but before proceeding further,
it will be well to examine more minutely the structure of
the several organs in which calcification takes place.

134 A MANUAL OF DENTAL ANATOMY.

ENAMEL ORGAN.

The enamel organ, as has already been stated, forms a
cap-like investment to the dentine bulb, and it is itself
thickest over the apex of the latter, thinning down some-
what as it approaches the base.

It is entirely surrounded by an epithelial layer, which
upon the inner surface applied to the dentine bulb consists
of much elongated columnar cells, and takes the name of
internal epithelium of the enamel organ, and upon its outer
surface the name of external epitlielium of the enamel organ.
The greater bulk of the enamel organ consists of a stellate
tissue, which passes gradually through the medium of a
layer of rounded cells, the stratum intermedium, into the
enamel cells, or internal epithelium. The essential portion
of the enamel organ is this layer of " enamel cells," which
by their calcification give rise to the enamel, and in lower
animals, such as most if not all reptiles, the whole enamel
organ is represented by little else than this layer of " enamel
cells."

The cells of the internal epithelium (enamel cells) form
an exceedingly regular and perfect columnar epithelium,
the individual cells becoming by result of their mutual
apposition very symmetrical hexagons.

They are four or five times as long as they are broad, and
the nucleus, which is large and oval, occupies that end
which is farthest from the dentine. It is said by Waldeyer
that the sides of the cells only are invested by membrane,
the protoplasm being without investment at its two ends.

Towards the base of the dentine germ, where the internal
epithelium merges into the external epithelium, the cells are
not so much .elongated, and they then pass gradually into
the cubical form of these latter cells. At their attached
extremities the enamel cells are prolonged into processes
which are continuous with the cells of the stratum inter-

Rt

THE DEVELOPMENT OF THE TEETH. : i

medium, so that it may fairly be concluded that the enamel
cells, as they are used up in the formation of enamel, are
recruited from the cells of this layer.

The " stratum intermedium" consists of cells intermediate
in character between those of the bordering epithelium and
the stellate reticulum ; they are branched, but less conspicu-
ously so than the stellate cells, with which on the one hand
they are continuous, on the other with the enamel cells.

The stellate cells proper are characterised by the great
^ength of their communicating processes, and the interspace
of the meshes is occupied by a fluid rich in albumen, so that
the consistence of the whole is little more than that of jelly ;
as the structure in question constitutes the major part in
bulk of mammalian enamel organs, these have been called
the enamel jellies, or enamel pulps.

Fro. 65

The function and destination of this portion of the enamel
organ is not very clear: enamel can be very well formed
without it, as is seen amongst reptiles and fish, and even
in mammalia it disappears prior to the completion of the
enamel, so that the external and internal epithelia come into
contact. It has been supposed to have no more important
function than to fill up the space subsequently taken up by
the growing tooth. (See page 156).

(l) Cells of the stellate reticulum of the enamel organ. From Frey's
Histology.

136 A MANUAL OF DENTAL ANATOMY.

The external epithelium of the enamel organ is composed
of cells cubical or rounded in form, and is of little interest
save in that it is a matter of controversy what becomes of
it. Waldeyer holds to his opinion that, after the disap-
pearance of the enamel pulp and the stratum intermedium,
it becomes applied to the enamel cells, and on the comple-
tion of the enamel becomes cornified and converted into
Nasmyth's membrane. Kolliker and Legros and Magitot
dissent from this opinion, the latter stating that the atrophy
of these cells commences early, and that they actually dis-
appear prior to the complete atrophy of the organ. For
reasons which I have given elsewhere, I do not agree with
Waldeyer in this matter, but rather with Magitot. The
external epithelium was seen by Nasmyth, Huxley, and
Guillot, but it was not very fully described until investigated
by Robin and Magitot.

So simple a matter as the vascularity or non-vascularity
of the enamel organ is not yet settled ; Wedl asserts that it
contains no vessels, Magitot and Legros sharing this opinion ;
Dr. Lionel Beale, on the other hand, stating that a vascular
network lies in the stratum intermedium.

The inner surface of the enamel organ, where it is applied
to the dentine bulb, presents a perfectly smooth outline, but
its outer surface is indented by numerous papillary projec-
tions, into which enter blood vessels of the dental sacculus.
These papillae are homologous with, and continuous with
those of the gum ; they may sometimes be traced along the
neck of the enamel germ, and it is believed that they
exercise an important influence on the formation of the
enamel, to which I shall again recur.

The narrow attenuated line of cells by which the enamel
organ retains its connection with the stratum Malpighi,
whence it was derived, varies much in length and direction
in different animals; in man it is short and straight, in
the calf it is larger, and undulates in its course. It does

THE DEVELOPMENT OF THE TEETH.

137

not remain quite that simple line of cells of which it con-
sisted when first formed, but varicosities, made up of poly-
hedral cells, bud out from it.

The origin of the dental germs of the permanent teeth
remains to be described : the twenty teeth which have
deciduous predecessors being derived from parts of the
germs of these, the twelve true molars having a distinct
origin. About the sixteenth week of intra-uterine life, from
the neck of cells which connects the enamel organ of the
temporary enamel germ with the stratum Malpighi, there
buds out a secondary inflection of epithelium, similar in
appearance to the first rudiment of the enamel germ of

FIG. 66 J.

the milk tooth ; it passes down to the inner side of the
temporary tooth sac, and by undergoing a series of changes
in all respects analogous with those resulting in the forma-
tion of the temporary tooth germ, gives rise to the perma-
nent tooth germ.

(!) Dental germ of temporary tooth of an Armadillo, showing its enamel
organ, and the enamel germ of the successional permanent tooth to the
left of it.

138

A MANUAL OF DENTAL ANATOMY.

The first permanent molar germ, however, is developed
about the sixteenth week by a similar budding out of epi-
thelium, from that same primary epithelial lamina, whence
the temporary enamel germs originated : whilst the second
permanent molar originates from the neck of the enamel
organ of the first molar after a long interval, i.e., about the
third month after birth.

The enamel germ of the wisdom tooth is similarly derived
from the neck of that of the second permanent molar, again
after a long interval ; (about the third year. Magitot).

The accompanying figure represents the enamel germ

FIG. 67 (').

for a permanent tooth budding off from the neck of the
enamel organ of the temporary tooth. Many differences of
detail, such as the point at which they arise, the depth to

(a) From the upper jaw of a kitten about the time of birth, a. Oral
epithelium, b. Bone of jaw. c. Neck of enamel organ, d. Dentine
papilla, e. Enamel cells. /. Stellate reticulum. h. Tooth germ of the
permanent tooth, the enamel organ of which is derived from the neck of
that of its predecessor.

THE DEVELOPMENT OF THE TEETH.

139

which they penetrate into the surrounding parts, and other
such characteristics exist not only between the germs of teeth
of different animals, but even between those of teeth situated
in different parts of the mouth of the same animal, so that
but little importance is to be attached to them.

DENTINE ORGAN.

The dentine germ, or dentine bulb, of which the origin
has been already described, at first was nothing more than

FIG. 63 (').

a part of the subuiucous tissue of the jaw which had become
more rich in vessels and cells than other neighbouring parts,

(l) Tooth sac of a calf. a. Tootli sac. a1 a2. Its outer and middle
portions, b. Stellate cells of enamel organ, c. External epithelium of
enamel organ, d. Internal epithelium of enamel organ, e. Odontoblasts.
/. Dentine bulb in papilla, g. Vessels in dentine bulb. i. Points where
the sac becomes fused with the base of the dentine papilla.

140 A MANUAL OF DENTAL ANATOMY.

but did not present any structures essentially different from
those found around it. It very speedily assumes the form
of the apex of the future tooth, becoming, if it be a canine,
simply conical, if a tooth with two cusps, bicuspid; and
coincidently with these changes the layer of cells forming
its surface, which is in close relation with the enamel cells,
becomes differentiated from the parts beneath it.

These cells, which become dentine by their calcification,
form a very distinct layer, which, after the commencement
of calcification, adheres more strongly to the formed cap of
dentine than to the rest of the pulp, and so is often pulled
away with the former when the two are separated ; hence
this layer of cells has obtained the name of " membrana
eboris," or membrane of the ivory ; but the student must
beware of falling into the mistake of supposing that it really
is a " membrane " properly so called.

The individual cells, which collectively constitute the
membrana eboris, are called odontoblasts ; they are variable
in shape, differing even in the same animal according as
the formation of dentine is going on actively or not, but
are always much longer than they are broad, and are fur-
nished with oval nuclei near to their deeper ends. The
odontoblast cells are furnished with several processes, named
according to their direction ; thus the process which pene-
trates the formed dentine is called the " dentinal process,"
that which, at its opposite pole, passes into the deeper
part of the pulp where it communicates with other cells,
the "pulp process," and those which communicate with
neighbouring odontoblasts, "lateral processes." Before
entering upon a detailed description of the transformation
which the various cells undergo in their conversion into
enamel, dentine, or cementum, it will not be out of place
to say a few words relative to the process of calcification
generally.

But before doing so it may perhaps assist the student, who may

THE DEVELOPMENT OF THE TEETH. 141

be perplexed in endeavouring to reconcile the statements of various
authors, to give a succinct history of the views from time to time
set forth. Q

Before the time of Goodsir (1838). the development of the teeth
was described by Kaschkow somewhat vaguely as proceeding under-
neath the mucous membrane, he did not, however, trace out in
what precise manner the several parts of the tooth germ originated.
The papers of Goodsir giving, in the place of somewhat vague and
general notions, a very definite and intelligible description of ob-
servations, was accepted without question by most anatomists, if
not by all. Accordingly we find in all the text-books at and after
that period, and in some even at the present day, the description
given by Goodsir reproduced almost without alteration, so that it
will be worth while to briefly relate what his views were.

He believed that at an early period in foetal life there appeared
a continuous open groove, running round the whole circumference
of the jaws ; that from the bottom of this groove there arose iso-
lated and uncovered papillas, corresponding in number to the milk
teeth ; that these papilla? became covered in by the deepening of
the groove and the meeting of its two edges over their tops, whilst
at the same time transverse septa were formed, so that the several
papillae became enclosed in their own separate follicles. With the
details of the process as described by him we are not concerned ; it
will suffice to remember that he distinguished the four stages ; a
primitive dental groove, a papillary stage, a follicular stage, and an
eruptive stage (the latter of course at a long subsequent period).

Not only were these views accepted quite without question, but
they were even extended to explain the development of the teeth
in Reptiles and Fishes, and thus in the Odontographies of Professor
Owen and Professor Giebel may be found accounts of the develop-
ment of the teeth in reptiles and fish which are perfectly in accord
with Goodsir's theory, but which in fact are far more inaccurate
than the same theories were as applied to mammalian teeth.

Even so careful a writer as Professor Huxley, who was the first
to point out that these stages really did not exist either in the frog,
the mackerel, or certain other fish, accepted them without question
as true of mammalia. Marcusen (2) (1849) gave upon the whole a
correct account of the process, referring the enamel to the oral epi-
thelium, and Professor Huxley (1854), whilst demonstrating that
the stage of free papillae was not to be found in certain fish and

(!) After the present summary had been partly prepared, the author
met with the very excellent r€sum6 given by Messrs. Legros and Magitot,
from which he has received much assistance.

(2) In the resume given by Messrs. Legros and Magitot, before referred
to, due credit is not given to the papers of Marcusen and Huxley (1849,
1854) (although they are alluded to), and it appears to me that too much
is given to that of Natalis Guillot (1858).

142 A MANUAL OF DENTAL ANATOMY.

reptiles (a fact also made out in the newt by Dr. Beale), clearly and
strongly expressed the same view as to the origin of the enamel
organ, and hence of the enamel. And whilst regretting that their
hold upon the minds of anatomists has been so strong as to en-
courage deductions therefrom going wider and wider of the mark, I
would not be understood to set small value upon the observations
of Arnold and Goodsir. They were a step in advance, and were
probably as accurate as the methods of investigation then in use
would allow of : moreover, the error in observation is very easy to
account for, as, the epithelium having peeled off as a result of de-
composition, or the use of weak spirit, the state of things left does
not widely differ from that described by Goodsir.

The subject rested for many years without further advances, but
in 1863 Professor Kolliker demonstrated, beyond all cavil, the real
origin of the enamel organ and its relations to the oral epithelium,
the dentine organ, and the dental sac.

His views, substantially correct, have been elaborated by Wal-
deyer, Kollmann, Hertz, Legros and Magitot, Wedl, and others, but
only in minor particulars have they been modified.

The development of the teeth of reptiles was found by a pupil of
M. Kolliker's, M. Santi Sirena, to have several features in accord
with that of mammalian teeth ; my own researches on the teeth of
Batrachia and Fish and Reptiles, elsewhere detailed, have proved a
striking general similarity in the process throughout the vertebrate
kingdom, though they are not in accord with the views of Pro-
fessors Owen and Giebel.

Dental Follicle. — In the foregoing account little mention
has been made of the tooth follicle or tissue forming a
capsule-like investment around the dentine germ and enamel
organ. At an early period of development the tissue form-
ing the dentine papilla of a mammalian tooth is seen to be
prolonged outwards and upwards from its base (see h in
Fig. 64) • these processes appear to grow rapidly upwards,
so as to embrace the enamel organ; but whether this is
really so, or whether it is merely that the ill-defined tissue
in which the dentine forming organ has itself originated is
in this region also becoming more pronounced, it is hard to
say. This up-growth from the base of the dentine papilla
is the first appearance of a special dental sacculus, which is
thus derived from a source identical with that of the forma-
tive organ of the dentine.

Fio. 69

12,

II

(!) TRANSVERSE SECTION OF THE LOWER JAW AND DEVELOPING BACK TOOTH OK A LAMB,
copied from Waldeyer (Henle's Zeitschrift f. Rat. Med. 1865). In its outlines the figure is
faithful to nature ; it is so far diagrammatic that more of structure than could be seen with the
magnifying power employed is introduced.

1. Dentine germ, with its border of pdontoblasts. 2. Formed dentine. 3. Formed enamel.
4. Points where the inner epithelium and the outer epithelium of the enamel become
continuous. 5. Enamel cells or internal epithelium, (i. External epithelium of enamel
organ. 7. Stellate reticulum of enamel organ. 8. Papillary projections into the enamel
organ. 9. Connective tissue around the sac, becoming continuous above with that of the
gum (9ft); this constitutes what is called the looth sac. 10. Vessels and nerves of the
jaw. 11. Bone of lower jaw. 12. Periosteum of the jaw. 13. Heap of epithelium over
the young tooth. 14, External skin with its epidermis. 15. Muscular bundles from floor
of mouth,

[To face p, 143.

THE DEVELOPMENT OF THE TEETH. 143

While these changes are going on, the tooth sac is becoming
lodged in a widely open gutter of bone, which is being rapidly
formed at its sides and under its base. If at this stage
(see Fig. 69) the gum be stripped off the jaws, the
developing tooth capsules are torn off with it, from
which they are inseparable except by actual cutting, thus
leaving the gutter of bone quite bare and empty. In fact
the capsule or sac which encloses the tooth germ consists of
almost the whole of the connective tissue which intervenes
between the special dentine and enamel germs and the bone.

In the first instance the follicle wall is only distinguished
from the connective tissue external to it by being somewhat
richer in cells, vessels, and fibrillar elements ; being, in fact,
more condensed or more compact. The sacs, when at their
fullest development, are divisible into two layers, an outer
thin firm wall, and an inner looser tissue, not very dense. At
the base of the tooth sac, the follicle wall is not separable
nor distinguishable from the base of the dentine papilla
with which it blends. The follicle wall is richly vascular ;
and over the surface of the enamel organ it is prolonged
inwards in the form of villous or papilliform eminences (8
in Fig. 69), projecting into the external epithelium of the
enamel organ ; to these prominences, which are analogous to
the papillae on the free surface of the gum, some authors
attach much importance, as having an influence upon the
direction of the enamel prisms, and so regulating the
pattern formed ; but this view is by no means universally
accepted. The internal or softer and looser portion of the
follicle wall, which has a consistency but little firmer than
that of the stellate reticulum of the enamel organ, is much
developed in Ruminants, where there is to be a deposition
of coronal cement. This differentiation of a portion of the
dental sac is thought by Messrs. Legros, Robin, and Magitot
to be sufficiently pronounced to justify its designation as a
distinct " cement organ."

144

A MANUAL OF DENTAL ANATOMY.

THE CEMENT ORGAN.

Cementum is, according to these authors, developed, just
as bone is, in two distinct methods.

Where it is not to be very thick, and is to clothe roots,
the ossification takes place in membrane (the alveolo
dentar periosteum), but where it is to form a thick layer

over the crown, as in Ruminants, a cartilaginous cement
organ is formed, and we have a calcification analogous to
formation of bone in cartilage.

Thus the cement organ is found in those animals only
which have coronal cement, such as the Herbivora. In a
calf embryo about the time that dentine calcification is
commenced, there may be distinguished beneath the follicle
wall and above the enamel organ a greyish layer of tissue,
thick enough to be distinguishable with the naked eye, and
of firmer consistence than the enamel organ, from which it
also differs in being richly vascular.

(!) Cement organ of a calf (after Magitot). a. Fibroid matrix.
6. Cartilage cells and capsules.

THE DEVELOPMENT OF THE TEETH. 145

But though it exists at this early period, it is not till
later, when, after the completion of the dentine and enamel
immediately beneath it, its own function is about to come
into play, that it attains to its characteristic structure.
This M. Magitot designates as fibro-cartilaginous, as there
appear in it characteristic cartilage cells or chondroplasts,
containing one, two, or rarely three cells, which have
spherical or ovoid nuclei.

In those creatures which have cementum upon the roots of
the teeth only, no special cement organ exists, but osteoblasts
which calcify into cementum are furnished by the tooth sac.

It is said that the inner layer of the tooth sac is concerned
in the formation of the cement ; that the outer layer, con-
jointly with the surrounding connective tissue, is converted
into the alveolo-dentar periosteum, but I cannot myself
recognise 'the justice of this distinction in practice. In
human teeth the parts of the follicle wall or sac cease to be
distinctly distinguishable at a comparatively early period,
and their importance is not such as to call for very detailed
description.

Another structure, once thought important, and now
known to be a mere bundle of dense fibrous tissue, is the
" gubernaculum." The permanent tooth sacs, during their
growth, have become invested by a bony shell, which is
complete, save at a point near their apices, where there is a
foramen. Through this foramen passes a thin fibrous cord,
very conspicuous when the surrounding bone is broken
away, which is called the " grubernaculum," from the notions
entertained by the older anatomists that it was concerned
in directing or effecting the eruption of the tooth. The
gubeniacula of the front permanent tooth sacs perforate
the alveolus and blend with the gum behind the necks of
the corresponding milk teeth; those of the bicuspids unit-
ing with the periosteum of the alveoli of their deciduous
predecessors.

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148 A MANUAL OF DENTAL ANATOMY.

CALCIFICATION.

A tissue is said to be " calcified " when the organic struc-
tures of which it is composed are hardened and stiffened by
impregnation with salts of lime. The impregnation with
lime salt may go on so far that the residual organic matrix
is reduced to a very small proportion, as is exemplified in
the case of adult enamel, in which the organic constituents
make up only from one to three per cent, of the whole, so
that practically the enamel wholly disappears under the in-
fluence of an acid j or the organic matrix may persist in
sufficient quantity to retain its structural characteristics
after the removal by solution in an acid of its salts, as is
the case with dentine, bone, and cementum. There are
two ways in which a calcified structure may be built up :
the one by the deposition of the salts in the very substance
of a formative organ, which thus become actually converted
into the calcified structure ; the other by a formative organ
shedding out from its surface both the organic and inorganic
constituents, and thus, so to speak, excreting the resultant
tissue.

An example of the latter method is to be found in the
shells of many mollusks, in which the mantle secretes the
shell, and is able to repair fractures in it, without itself
undergoing any apparent alteration j while the formation of
dentine, bone, and enamel (') are examples of calcification
by conversion.

The insoluble salts of lime are altered in their behaviour
by association with organic compounds, a fact which was
iirst pointed out by Rainie, and has been more recently
worked out by Professor Harting and Dr. Ord.

If a solution of a soluble salt of lime be slowly mixed
with another solution capable of precipitating the lime, the

(a) All observers are not, however, agreed as to the formation of the
enamel. (Cf. page 157.)

THE DEVELOPMENT OF THE TEETH. 149

resultant lime salt will go down as an amorphous powder,
or, under some circumstances, in minute crystals. But in
the presence of gelatine, albumen, and many other organic
compounds, the form and physical character of the lime
salts are materially altered, and in the place of an amor-
phous powder there are found various curious but definite
forms, quite unlike the character of crystals produced with-
out the intervention of the organic substance.

Mr. Rainie found that if calcium carbonate be slowly
formed in a thick solution of mucilage or albumen the re-
sultant salt is in the form of globules, laminated in structure,
so that the globules may be likened to tiny onions ; these
globules, when in contact, becoming agglomerated into a
single laminated mass, it appearing as if the laminse in
immediate apposition blended with one another. Globular
masses, at one time of mulberry-like form, lose the in-
dividuality of their constituent smaller globules, and become
smoothed down into a single mass ; and Mr. Rainie suggests
as an explanation of the laminated structure that the smaller
masses have accumulated in concentric layers which have
subsequently coalesced ; and in the substitution of the
globular for the amorphous or crystalline form in the salt of
lime when in contact with various organic substances,
Mr. Rainie claimed to find the clue for the explanation of
the development of shells, teeth, and bone. At this point
Professor Harting took up the investigation, and found that
other salts of lime would behave in a similar manner, and
that by modifying the condition of the experiment very
various forms (l) might be produced. But the most im-
portant addition to our knowledge made by Professor
Harting lay in the very peculiar constitution of the " calco-
spherites," by which name he designated the globular forms
seen and described by Rainie. That these are built up of

(*) Thus he was successful in artificially producing "dumb-bell'*
crystals.

150 A MANUAL OF DENTAL ANATOMY.

concentric laminae like an onion has already been mentioned,
and Mr. Rainie was aware that albumen actually entered
into the composition of the globule, since it retained its
form even after the application of acid.

But Professor Harting has shown that the albumen left
behind after the treatment of a calcospherite with acid is
no longer ordinary albumen ; it is profoundly modified, and
has become exceedingly resistant to the action of acids,
alkalies, and boiling water, and in fact resembles chitine,
the substance of which the hard skins of insects consist,
rather than any other body.

For this modified albumen he proposes the name of
" calcoglobulin," as it appears that the lime is held in some
sort of chemical combination, for the last traces of lime are
retained very obstinately when calcoglobulin is submitted
to the action of acids.

The " calcospherite," then, has a true matrix of calco-
globulin, which is capable of retaining its form and structure
after the removal of the great bulk of the lime.

Now it is a very suggestive fact that in the investigation
of calcification we constantly meet with structures remark-
able for their indestructibility : for example, if we destroy
the dentine by the action of very strong acids, or by
variously contrived processes of decalcification, putrefaction,
&c., there remains behind a tangled mass of tubes, the
" dentinal sheaths " of Neumann, which are really the
immediate walls of the dentinal tubes.

Or if bone be disintegrated by certain methods there
remain behind large tubes, found to be the linings of the
haversian canals (Kolliker), and small rounded bodies,
recognisable as isolated lacunse ; and in the cuticula dentis
we have another excellent example of this peculiarly inde-
structible tissue.

In point of fact, as will be better seen after the develop-
ment of the dental tissue has been more fully described, on

THE DEVELOPMENT OF THE TEETH. 151

the borderland of calcification, between the completed fully
calcified tissue and the formative matrix as yet unimpreg-
nated with lime, there very constantly exists a stratum of
tissue which in its physical and chemical properties very
much resembles " calcoglobulin."

It should also be noted that globular, spherical forms are
very constantly to be seen at the edges of the thin cap of
forming dentine, and may be also traced in and around the
interglobular spaces (see Fig. 34) ; moreover, isolated
spherules of lime salt have been described by Messrs. Kobin
and Magitot as occurring abundantly in the young pulps of
human teeth, as well as those in the herbivora, where their
presence was noted by Henle.

CALCIFICATION OF THE ENAMEL.

Although the calcification of the dentine commences
before that of the enamel, it will be convenient to describe
that of the enamel first, as being a somewhat simpler and
more easily intelligible process.

As has already been mentioned, I am distinctly of opinion
that the enamel is formed by the actual conversion of the
cells of the enamel organ into enamel, but as this view is
not held by all who have written upon the subject, I will
first mention the alternative theory, namely, that the enamel
is in some sense secreted or shed out by these cells. In
support of this latter theory the names of no less authorities
than Professor Huxley, Kolliker, Wenzel, and Magitot, may
be adduced, but the grounds on which their decisions are
based are appearances susceptible of a different interpreta-
tion. Kolliker considers that the cells do not undergo any
direct conversion, but that the enamel is shed out from the
ends of the enamel cells, the enamel fibres therefore corre-

152 A MANUAL OF DENTAL ANATOMY.

spending in size and being continuous with the enamel cells
whence they were shed out.

Professor Huxley's reason for doubting the direct con-
version of the enamel cells into enamel was that a membrane
could be raised from the surface of growing enamel, at any
period of its development, by the use of acid reagents, this
membrane necessarily intervening between the formed
enamel and the enamel cells ; hence he denied that the
enamel organ contributed in any way directly, though it
might indirectly, to the development of the enamel.

To the nature of this " membrane " I shall have again to
refer, so that for the present it will suffice to say that the
structure in question cannot be demonstrated, and in fact
has probably no existence, prior to the use of the reagent.

The cells of the internal epithelium of the enamel organ
or enamel cells have been already in some measure de-
scribed : they are elongated cells, forming a very regular
columnar epithelium, and are hence rendered hexagonal by
mutual apposition ; they vary in their length and diameter
in different animals.

To secure uniformity of nomenclature, the name adaman-
toblast has recently been proposed for them, as being better
comparable with the term odontoblast and osteoblast.

Although they are connected with the cells of the stratum
intermedium by a process at their base, they often adhere
more strongly to the enamel, when once this has begun to
be formed, than to the rest of the enamel organ, so that
when a dental sac is opened the enamel cells are most easily
obtained by scraping the surface of the enamel. The cells
thus torn away often have tapering processes at the ends
directed towards the enamel, which were first described by
my father, and go by the name of " Tomes' processes." The
cells are also slightly enlarged at these extremities, especially
if they have been immersed in glycerine or any such fluid
which causes their shrinkage, for this end of the cell having

THE DEVELOPMENT OF THE TEETH.

153

received a partial impregnation with lime salt at its peri-
phery, and so being rigid, is unable to contract with the rest
of the cell. These enlarged, everted ends, often show a very

FIG. 71

sharp contour, their trumpet-like mouths tending to confirm
the statement of Waldeyer that the protoplasm of the cell
is not covered in by membrane at its ends. The impregna-
tion with calcareous salts commences at the free end of the

FIG. 72 (2).

enamel cell, and at the periphery before the central portion,
and it is to this fact that the existence of " Tomes' pro-
cesses " is due, for when the enamel cell is dragged away
from the formed enamel prism, it separates across the line
of calcification ; and thus the axial part of the cell, when
torn away, projects out further than its periphery, in con-

(1) Enamel cells with Tomes' processes.

(2) Enamel cells ; the two on the right have been shrunk by immersion
in glycerine, and present the open trumpet-shaped ends described in the
text.

154 A MANUAL OF DENTAL ANATOMY.

sequence of calcification having extended less far at this
central portion of the cell.

In other words, if the forming enamel were freed from
the adherent enamel cells, its surface would be pitted,
each little pit marking the centre of an enamel prism \ and
if a thin section of this immediate surface could be taken
off, it would be pierced with holes at regular intervals.

The enamel cell with its process is like an odontoblast with
a very short dentinal fibril, which has been pulled out of the
formed dentine, and the nature of the " Tomes' processes " is
well illustrated in the enamel organs of marsupials. It
will be remembered that their enamel is permeated by a
large number of canals, which become continuous at the
junction of the dentine and enamel, with the dentinal tubes.
Accordingly the enamel cell of a marsupial, engaged in the
formation of a permanently tubular enamel, is just like an
odontoblast in that it has a long, fine process, pulled out of
the already formed enamel.

As the youngest part of the enamel has by no means
attained to its full hardness, it is quite possible to obtain, in
small pieces, sections parallel to its surface ; the nearer they
are to the surface, the larger will be the perforations, show-
ing what has already been stated respecting calcification
commencing at the periphery of each cell to be true. And
it is possible, by the use of an acid, to obtain such sections
upon a larger scale, for under the influence of such a reagent,
this youngest layer of the enamel peels off in a sheet,
bringing with it in places enamel cells, in places enamel
prisms, adhering to its opposite sides. When destitute of
adherent enamel cells or prisms, this so-called membrane is
foraminated ; and the processes of the ends of the enamel
cells are fitted into and passed through these perforations.

The real nature of the membrane which could be raised
from the surface of growing enamel was first demonstrated
by my father, and his explanation has been accepted by

THE DEVELOPMENT OF THE TEETH. 155

Waldeyer and other authorities ; it will be seen that this
sheet, produced solely by the destructive action of reagents,
corresponds with the membrana preformativa of some writers
(see page 171), and with the membrane described by Pro-
fessor Huxley as intervening between the enamel cells and
the enamel. Hence it will be seen that the fact of acids
raising a membrane from the surface of the enamel does not
really militate against the theory that the enamel is due to
the direct conversion of the enamel organ into enamel.

The ends of the enamel cells near to the formed enamel
are granular, this granularity being due to the deposition of
particles of lime salts, as is indicated by its clearing up
when treated with acid.

The cells on the one side of the membrane readily separate
from one another, adhering, however, slightly by their
dilated ends (vide supra), and the fact that we are able to
isolate the youngest layer of enamel as a thin sheet is
probably to be explained by its chemical nature. It ap-
pears to belong to that class of peculiarly resistant sub-
stances which are to be found on the borders of calcification,
and behaves very much like Professor Harting's " calco-
globulin" (see page 150); at all events it may safely be
said to have undergone some chemical change preparatory
to the reception of its full amount of lime salts.

The calcification of the enamel should be so complete
that its fibrous structure is but slightly apparent in longi-
tudinal sections, and the individual fibres should appear
structureless, with the exception of the feebly marked
striation (see page 50). In enamel of imperfect structural
character the centre of the fibre is not completely calcified,
the arrest of development having taken place short of its
full conversion.

The stellate tissue of the enamel organ disappears some
time before the whole thickness of the enamel is formed,
and changes go on in the latter up to the time of the erup-

156

A MANUAL OF DENTAL ANATOMY.

tion of the tooth ; the enamel of a tooth prior to its erup-
tion having a chalky, opaque surface.

The enamel of the teeth of reptiles is developed from an
enamel organ which at no time possesses any stellate tissue ;
this is also the case in all fish which I have hitherto ex-
amined. In the poison fangs of snakes the enamel cells,

over the interior of the poison tube, appeared to be trans-
formed into a stellate reticulum, which change in this case
would appear to be a retrograde metamorphosis.

The nuclei of the enamel cells, which lie at the extre-
mities furthest from the enamel, appear to recede as calcifi-
cation goes on ; they do not exercise any special influence
on the process as far as can be seen.

(*) Transverse section of the tooth sac of a poison fang (Viper). The
crescentic pulp (a) is surrounded by a layer of dentine (d) ; external to
this is a layer of columnar enamel cells, which, upon the exterior of the
tooth, upon which a thin layer of enamel is to be formed, are large con-
spicuous cells. Where they pass in between the horns of the crescent,
into that part which will ultimately be the poison canal, their character
is lost, and their place taken by stellate cells (/ ). No enamel is formed
in this latter position.

THE DEVELOPMENT OF THE TEETH. 157

As has been already mentioned, Kolliker dissents from the above
account of the calcification of. the enamel, partly on the ground
that enamel cells may be seen of the same size and form at all
stages of the formation of enamel.

The process he regards as one of secretion, the enamel being shed
out, so to speak, from the free end of each enamel cell ; hence the
prisms of the enamel will correspond in size and number with the
cells of the enamel epithelium ; the processes of the enamel cells
he regards as being fragments of this hardened secretion which are
Btill clinging to the parent cell.

M. Magitot (Journal de 1'anatomie de M. Ch. Robin, 1879) has
revived this view, describing each cell as terminated, towards the
forming enamel, by a little plate of dense material through which
by some process of exosmosis the constituents of enamel travel out.
He notes that these plates often cohere so as to form a sheet (cf .
page 154), but says nothing of their being perforated. No one,
however, who had seen the enamel cell of a marsupial with the
tapering process five or six times as long as itself which had been
pulled out of the young enamel would be satisfied with the excre-
tion theory.

The reasons for adopting the opposite view will have been ga-
thered from the text ; they are, in brief, the occurrence of the
11 Tomes' processes," especially in marsupials ; the rigidity of the
open mouths of the enamel cells ; the pitted surface of the youngest
layer of enamel, the foraminated membrane which can be raised
from it, and the relation of these facts to the occurrence of the
processes of the enamel cells.

Schwann believed that the enamel cell was constantly increasing
at its free end («.«?., that next to the enamel), and that the new
growth, or youngest part of the cell, is calcified as fast as it is
formed ; this view differs little from that of Kolliker, who prefers
to express it by saying that this end of the cell is constantly shed-
ding off or secreting a material which becomes external to itself.
My father, Waldeyer, Hertz, and many others, believe that the cell
growth takes place not at this free end, but at the attached nucle-
ated end, and that it is the oldest portion of the cell itself which
receives an impregnation with salts and forms the enamel.

Professor Huxley's opinion (page 152) is, I take it, based on the
fact that a membrane could be raised from the surface of young
enamel, which must have intervened between the enamel cells and
the enamel prisms ; if my father's explanation of the nature of this
membrane be accepted, the difficulty vanishes.

My own researches upon the development of the teeth of fishes
also furnish evidence ten ding in the same direction ; as has been al-
ready mentioned, the enamel cells in some parts of the enamel organs
of certain fish, such as the eel and perch, and certain Batrachia,
e.g., the newt, have dimensions very greatly exceeding those of the
cells in the remainder of the organ. These highly developed cells,

158 A MANUAL OF DENTAL ANATOMY.

three times as long1 as the corresponding cells lower down upon the
dentine papilla, are in the position of the terminal cap of enamel
which characterises these teeth. Moreover in the tooth sac of the
poison fang of a viper, the distribution of the large cells coincides
with that of the enamel on the finished tooth.

Calcification of the Dentine. — The dentine is formed
upon the surface of the dentine bulb, or papilla, from with-
out inwards, so that no portion of dentine once calcined can
receive any increase in external dimensions ; all additions
must take place upon the interior of the dentine cap. The
nature of the dentine bulb has already been to some extent
described ; it remains to consider somewhat more minutely
the nature of its surface. The cells constituting the mem-
brana eboris, to which Waldeyer has given the convenient
name of " odontoblasts," form an exceedingly sharply denned
layer upon the surface of the dentine wall, being arranged
in a single row ; the cells immediately beneath them differ
strongly from them, so that there is not so marked an
appearance of transitional structure as may be seen in the
stratum intermedium of the enamel organ. Nothing what-
ever like the linear succession of formative cells, which, by
coalescence at their ends went to form the dentinal tubes,
as described by the older writers, is to be seen.

The odontoblast cells vary in form according as the den-
tine formation is actively going on or not, but at the period
of their greatest activity they are broad at the end directed
towards the dentine cap, so as to look almost abruptly
truncated. The several processes of the cells have already
been described; there are, however, sometimes several
" dentinal processes " proceeding from a single cell, and
Boll has counted no less than six.

The cells are finely granular, and are, according to
Waldeyer and Boll, destitute of all membrane ; the nucleus
is oval, lies in that extremity of the cell which is farthest
from the dentine, and is sometimes prolonged towards the
dentinal process so as to be ovoid or almost pointed.

THE DEVELOPMENT OF THE TEETH. 159

The dentinal process passes into the tubes of the dentine,
and it frequently happens that when the membrana eboris
is only slightly separated from the dentine these processes,

FIG. 74 1.

which constitute the dentinal fibrils, may be seen stretching
across the interval in great numbers.

The odontoblasts, as may be seen from figures 30 and 31,
are fitted closely together, and there is no room for any other
tissue between them, so long as the formation of dentine is
actively going on. Prior to its commencement, however,
the cells are not so square at their ends, and the appear-
ance of the thin edge of such a pulp suggests the idea that
they are bedded in a transparent and structureless jelly,
which projects a little beyond them. To render my mean-
ing more clear by a homely illustration, the surface of the
pulp at this stage reminds one of the clear jellies put upon
the table with strawberries or the like buried in them, near
to, but beneath, the surface. But no such substance can
be seen when once calcification has actively set in.

When the pulp has completed, for the time being at all
events, the formation of the dentine, the odontoblast cells
become more elongated and more rounded in their outline
and taper off towards and into the dentinal process, instead
of having truncated ends.

The cells figured by Lent as the formative cells of dentine
I regard as odontoblasts taken from an adult tooth, the

0) Isolated odontoblast cell.

160 A MANUAL OF DENTAL ANATOMY.

period of formative activity being past, and I am inclined to
think that his views on the subject of development are open
to criticism, as being based upon the appearances presented
by such old cells.

FIG. 75 (').

The dentine is, I believe, formed by the direct conversion
of the odontoblast cells, just as is the enamel by that of
the enamel cells, and is derived from them, and from them
alone.

According to this view, which is supported by Waldeyer,
Frey, Boll, Dr. Lionel Beale, and many other writers, the
dentinal fibrils, the dentinal sheaths, and the matrix between
these latter, are alike derived from the metamorphosis of
the odontoblast cell. In other words, the three structures
in question may be taken as being three stages in the con-
version of one and the same substance : thus we have the
dentinal fibril in its soft condition, little more than the
unaltered protoplasm of the cell, then the dentinal sheath,
one of those peculiarly resistant substances which lie on the
borders of calcification; and lastly, the matrix, the com-
pleted, wholly calcified tissue.

That some such relation exists seems to be indicated by
the fact that dentinal tubes once formed are capable of

(') Odontoblasts in situ. After Waldeyer.

THE DEVELOPMENT OF THE TEETH. 161

further calcification, by which their calibre becomes sensibly
diminished. Thus, my father states (speaking of the incisor
teeth of rodents), " the tubes which proceed from the pulp
cavity near the base of the tooth, are, in most cases, per-
ceptibly larger than those that are situated higher up ;
hence it follows that, as the latter were once near the base
of the tooth, the dentinal tubea undergo a diminution of
calibre after their original formation. In the teeth of the
Sciuridse I have found a difference of size amounting to a
third or half between the tubes near the base and those
near the surface in wear."

And Dr. Lionel Beale calls attention to the fact that the
hollows of the canals are largest nearest to the pulp, and
smallest at the periphery of the tooth, in other words, at
the oldest pail ; also that calcification is still slowly going
on even in advanced life, so as often to lead to the oblite-
ration of the peripheral tubes. There is, too, the statement
of Robin and Magitot, that the teeth become more rich in
calcareous salts as age advances, so that analyses of human
teeth show great discrepancies.

It is difficult to see how a dentinal tube once formed can
become contracted to a third or half of its diameter unless
we believe that that which was at first the soft tissue (den-
tinal fibril) occupying its canal may become at its periphery
metamorphosed into " dentinal sheath," while that which
was originally this latter has passed into the condition of
matrix. Other illustrations of this fact, observed by inde-
pendent writers, suggest themselves to me; the converti-
bility of the dentinal fibril into dentinal sheath and of the
latter into matrix, seems to be of necessity implied by the
narrowing of the calibre of a tube once formed, for the tubes
thus narrowed present no special character ; their walls do
not appear any thicker, nor do they in any way become dif-
ferent save in the one matter of diameter. The phenomena
of dental caries also appear to lend some support to this

M

162 A MANUAL OF DENTAL ANATOMY.

view, that dentinal fibril, dentinal sheath, and matrix, are
but three ages of the same tissue.

For under the influence of caries the walls of the tubes,
invisible, or almost so, in perfectly healthy dentine, become
apparent.

As I have elsewhere expressed it, the most external
portions of the odontoblasts undergo a metamorphosis into
a gelatigenous matrix, which is the seat of calcification,
while their most central portions remain soft and unaltered
as the fibrils. Intermediate between the central perma-
nently soft fibril and the general calcified matrix, is that
portion which immediately surrounds the fibril, namely, the
dentinal sheath ; as expressed by Dr. Lionel Beale they are
protoplasm, formed material, and calcified formed material.

That the whole of the dentine is derived from a conversion of the
odontoblast cells is not agreed to by all writers. Thus Kolliker and
Lent believe that while the canals and their contents are continua-
tions of the odontoblasts, the matrix is a secretion either from these
cells or from the rest of the pulp, and so is an " intercellular " sub-
stance. Their view is therefore intermediate between the excretion
and conversion theories; and Kolliker goes on to say, "since the
dentinal cells are immediately drawn out at their outer ends into
the dentinal fibres, and do not, as was formerly thought, grow out
in such a manner that the dentinal fibre is to be regarded only as
the inner part of the cell, so it is not possible to derive the dentine
Immediately from the cells." But is not Professor Kolliker think-
ing and writing of those aged, spent cells which his pupil Lent
figured ? No one could speak of a young, active odontoblast as
•" drawn out into the dentinal fibril." A good section of young
developing dentine shows that the cells are square and abrupt
towards the dentine ; they do not taper into the dentinal process in
the smallest degree, and there is no room for any intercellular sub-
stance whatever.

Hertz coincides with Kolliker in regarding the matrix as a " se-
cretion from all the dentinal cells in common which stands in no
definite histological relation to the individual cells," but his figure
also I believe to be representative of an adult inactive surface of
pulp, in which dentine formation had almost ceased.

Kolliker and Lent are of opinion that a single cell is sufficient to
form the whole length of a dentinal fibril, not having seen evidence
of active cell growth in the subjacent layer of the pulp, from which
they would infer that the membrana eboris was supplemented by

THE DEVELOPMENT OF THE TEETH. 163

new cells from below. In the latest edition, however, Kolliker
speaks with much more hesitation on this point.

Magitot now (1881) holds that the whole of the dentine is " a
product elaborated by the odontoblasts," but neither secreted by nor
formed by the conversion of the odontoblasts, and he denies the
existence of the sheaths of Neumann in toto.

Klein believes that the odontoblast forms the matrix only,
whilst the dentinal fibrils are processes continued up between the
odontoblasts from a subjacent layer of stellate cells.

Robin and Magitot formerly held that the dentine matrix was
formed by the transformation of the odontoblast cells, but that the
tubes were interspaces between these latter, not corresponding with
the axes of the cells.

The thinnest layer of dentine, such as may be found at
the edges of the dentine cap, is soft and elastic, and so
transparent as to appear structureless. Where it has at-
tained a somewhat greater thickness, globules begin to
appear in it, which are small in the thinner, and larger in
the thicker portion of the dentine cap. As they are actually
in the substance of the cap, their growth and coalescence
obviously go on without any very immediate relation to
the cells of the pulp; in point of fact, a process strictly
analogous to that demonstrated by Mr. Kainie and Professor
Harting (see page 148), is going on. Thus, in the formation
of the first skin of dentine, a stage of metamorphosis pre-
paratory to impregnation with calcareous salts distinctly
precedes that full impregnation, which is marked by the
occurrence of globules and their subsequent coalescence.
The occurrence of these globular forms and consequent large
interglobular spaces, in the deeper parts of adult dentine, is
therefore an evidence of arrest of development rather than
of any otherwise abnormal condition.

When the formation of the dentine and enamel has gone
on to the extent of the crown of the tooth having attained
its full length, the reproduction of new formative pulp (in
teeth of limited growth) takes place only over a contracted
area, so that a neck, and finally one or more roots are the
result of its conversion into tooth substance. In teeth of
constant growth, however, no such narrowing of the forma-

M 2

164 A MANUAL OF DENTAL ANATOMY.

live pulp takes place, but the additions to the base of the
tooth are of constant, or ever-increasing dimensions, as is
the case in some tusks, which are of conical form.

It is said that the number of roots which would have been
developed at the base of a particular dentine organ may be
inferred from the vessels, i. e., that in a single rooted tooth
the vessels would, even at an early period, form a single
fasciculus, in a double rooted one similarly they would be
arranged in two bundles, so that the ultimate formative
activity will be exercised around one, two, or three centres
of nutrition. I am not however able, from my own ob-
servations, to thrown any light upon this matter.

THE CALCIFICATION OF VASO-DENTINE.

During the conversion of the membrana eboris into
ordinary hard unvascular dentine the vessels of the formative
pulp recede, so that, whilst at all stages a capillary plexus
is to be found just below the odontoblast layer, no vessels
are to be found amongst the cells which constitute it.

Nevertheless a moment's reflection will show that (except
in the earliest stages, before any dentine is formed) the
plexus must at a prior time have occupied the place now
taken possession of by the inward marching odontoblasts and
dentine.

But in the calcification of a formative pulp into vaso-
dentine this recession of its vessels before the advancing
border of calcification does not take place ; the whole vascular
network of the papilla remains and continues to carry blood
circulating through it, even after calcification has crept up
to and around it.

So that if we imagine a vascular papilla to have its
stroma suddenly petrified whilst its circulation went on all
the same, we should have something like a vaso-dentine.

Just as in hard dentine, the odontoblast layer is distinctly

TEE DEVELOPMENT OF THE TEETH. 165

marked oft' from the rest of the dentine organ, and the
dentine is wholly derived from its conversion into calcined
material, so that the difference between vaso-dentine and
hard dentine is not one of a very fundamental character.

Indeed, as we have seen (p. 85), the same formative pulp,
the same odontoblast layer, is able at one time to form hard
tubular dentine, at another vaso-dentine. All, therefore,
that has been before said of the calcification of odontoblasts
will apply equally to those of a vaso-dentine pulp, save only
that in a typical tissue of this latter kind each cell calcifies
solidly, and does not leave the axial portion soft, to remain
as a dentinal fibril.

Of the development of Plicidentine nothing more need
be said, as it presents no peculiarities which are not the
obvious result of the folding of the surface of its formative

THE CALCIFICATION OF OSTEO-DENTINE.

With the exception of the thin external layers (see fig. 47),
which are developed from a superficial layer of not very
highly specialised cells, osteo-dentine is built up in a
manner fundamentally different from that in which hard
dentine, plicidentine, and vaso-dentine, are constructed.

For it is not, like these, a surface formation ; it is not
laid down in a regular manner upon the exterior of a pulp,
and it has no relation to an odontoblast layer, if we except,
perhaps, its thin exterior shell.

So soon as this has been formed its inner surface becomes
roughened by trabeculse shooting inwards into the substance
of the pulp, which speedily becomes traversed completely
by them, as well as by the connective tissue bundles which
are continuous with them. Thus the pulp being pierced
through in every direction by these ingrowths cannot be
withdrawn, like the pulp of a hard or of a vaso-dentine tooth,

166 A MANUAL OF DENTAL ANATOMY.

from the interior of the dentine cap. Osteoblasts clothe,
like an epithelium, the trabeculae and the connective tissue
fibres attached to them, and by the calcification of these the
osteodentine is formed.

The process is exactly like the calcification of any mem-
brane bone, and the connective tissue bundles remind one
of those which are believed to be the occasion of the
formation of Sharpey's fibres in bone. In the case of teeth
which are going to be anchylosed to the subjacent bone,
these fibres run continuously from the interior of the dentine
cap down to the bone, and calcification in and around them
binds the two inseparably together.

It is interesting to note, especially in connection with
the fact that some observers believe Sharpy's fibres to be
elastic, that the hinged teeth of the pike (see fig. 88) owe
their power of resilience entirely to the elasticity of these
connective tissue bundles, which do not become completely
calcified, although at an early stage it would be quite im-
possible to say whether the tooth under observation was
going to be anchylosed, or to be a hinged tooth tied down by
elastic strings.

The Calcification of Cementum. — Just as is the case
with bones elsewhere in the body, cementum may be formed
in two distinct ways, by membranous ossification, and by
ossification in a fibro-cartilage, the former method obtaining
upon the roots of teeth, and the latter upon those crowns
where the cement organ described by Magitot exists.

At the time when the crown of a tooth appears through
the gum, it alone is complete, and the root has yet to be
calcified ; as each portion of dentine of the root is completed
it is coated with a closely adherent vascular membrane which
is in fact the follicle wall, and which is to become, when the
cement is formed, the alveolo-dentar periosteum.

The inner or dentinal face of this membrane presents a
layer of large cells, the osteoblasts of Gegenbaur, and it is

THE DEVELOPMENT OF THE TEETH. 167

by their calcification that bone or cementum is directly
formed. These osteoblasts are themselves a special de-
velopment where bone is about to be manufactured, as is
clearly explained in the following extract from a paper by
my father and the late Mr. De Morgan, who termed them
osteal cells : —

" Here (towards the bone) in the place of cells with
elongated processes, or cells arranged in fibre-like lines, we
find cells aggregated into a mass, and so closely packed as to
leave little room for intermediate tissue. The cells appear
to have increased in size at the cost of the processes which
existed at an earlier stage, and formed a bond of union be-
tween them. Everywhere about groiving bone a careful ex-
amination ivill reveal cells attached to its surface, while the
surface of the bone itself ivill present a series of similar bodies
ossified. To these we propose to give the name of osteal
cells, as distinguished from lacunal and other cells."

Externally to the osteoblast layer, but still very near to
the perfect cementum, lies a reticulum or network made up
by the inosculating branches of cells. The cells have largish
round nuclei, and are each furnished with three or four homo-
geneous processes, so that the tissue, save in very thin sec-
tions, looks hopelessly confused from the interlacing of the
cell processes. Many of these processes pass into, and are lost
in the clear, structureless matrix of the already formed
cementum ; the functions which they perform in its deve-
lopment are not very apparent, as they do not correspond
to anything which can be traced in the completed tissue.

Externally to the fine-meshed net-work which has been
well figured and described by Dr. Lionel Beale, the soft
tissue surrounding the root partakes more of the character
of ordinary fibrous tissue, and may be teased out into
fibrils. The fibrous bands run mainly in a direction from
the alveolus towards the tooth. Many of them pass through
the whole thickness of the soft structures, extending from the

168 A MANUAL OF DENTAL ANATOMY.

bone of the alveolus to the cementum of the tooth, becoming
lost at each extremity in the one tissue or other.

The osteoblasts form both matrix and bone corpuscles :
in Professor Klein's words " each osteoblast by the peripheral
portion of its cell substance gives origin to the osseous ground-
substance, while the central protoplasm round the nucleus
persists with the latter as the nucleated bone-cell. The bone-
cell and the space in which it lies become branched. For
a row of osteoblasts we then find a row of oblong or round
territories, each composed of matrix, and in it a nucleated
branched cell. The outlines of individual territories are
gradually lost, and we have then a continuous osseous
lamina, with its bone-cells. The ground substance is from
the outset a network of fibrils ; it is at first soft, but soon
becomes impregnated with inorganic salts, the process com-
mencing at the ' point of ossification.' The bone cells, with
their processes, are situated in corresponding lacunae and
canaliculi, just as in the adult osseous substance."

Thus just as calcification in an enamel cell or in an odon-
toblast commences upon its surface, and proceeds inwards
till it has more or less completely pervaded it, so in the case
of the osteoblast the deposition of calcareous salts proceeds
from without inwards. To use a rough comparison we
might imagine a calcifying osteoblast as like an egg-shell,
the central cavity of which was being gradually obliterated
by the addition of successive layers on its interior (it is not
to be understood that any such lamination is to be detected
in an individual osteoblast). In a certain number of osteo-
blasts this process of calcification does not proceed with
such regularity as to obliterate their centres, and at the
same time to fuse together their exteriors, but as it pro-
gresses with some degree of irregularity towards the centre,
tracks of uncalcified matrix are left, and finally it stops
short of obliterating the central portion of the cell. Al-
though for the purpose of description I have spoken of the

THE DEVELOPMENT OF THE TEETH. 169

centre of the osteoblast cell as a ' space,' of course it is not
hollow, but consists of uncalcified matrix, and in this situa-
tion lies the nucleus of the cell.

In carmine-stained preparations from the teeth of calves
a round nucleus may sometimes be seen lying in the stellate
" lacuna ; " the nucleus soon disappears, and plays no active
part in determining the form of the lacuna. The nucleus
may also be seen in the developing bones of human foetuses
and, though this is difficult to understand, the traces of the
nucleus seem to be beautifully preserved in the lacunse of
a supposed Pterodactyle bone from the Wealden, a section
from which was figured by my father in the paper referred
to. Exactly as calcification, advancing with irregularity in

FIG. 76 (M.

the interior of an individual cell, fails to render it homo
geneous by pervading its whole substance, so it may fail so
completely to unite contiguous cells as to obliterate their
contours. A lacuna, surrounded by such a contour line,
mapping the limits of the original cell, or cluster of cells, is
what is termed an " encapsuled lacuna."

That which determines the formation of a lacuna, or an
encapsuled lacuna, at any particular spot, is unknown : all
that can certainly be said upon the subject is embodied in

(') Encapsuled lacunae

170 A MANUAL OF DENTAL ANATOMY.

the following extract from the paper by my father and Mr.
De Morgan, above alluded to :— " We see the boundary of
the original lacunal cells only in those cases where the
lacunae have but few, or are entirely devoid of canaliculi.
It would appear to be a law, to which there are few, if any,
exceptions, that when anastomosis is established between
adjoining lacunse, the lacunal cells blend with the con-
tiguous parts, and are no longer recognisable as distinct
bodies."

According to Kolliker, the cementum first is deposited in
isolated scales, which coalesce with one another, rather than
in a continuous sheet. In the teeth of the Primates, the
Carnivora, Insectivora, &c., the cementum, at least in any
appreciable thickness, is confined to the roots of the teeth.
Various reasons, however, exist, for regarding Nasmyth's
membrane as an exceedingly thin layer of cement, which
have been entered into in the section relating to that
structure, and need not be recapitulated here. It will
suffice to say, that it appears to be one of those structures
midway betwixt full calcification and full vitality, and
shares with such substances the power of resistance to
chemical reagents which characterises them.

M. Magitot states that the calcification of the cartilaginous
cement organ of Herbivora differs in no respect from that of
other cartilages, but in his description he merely states that
patches of calcification appear here and there in the deepest
portion of the organ, coalesce, and come to invade its entire
thickness ; and further that the cement at the period of
eruption is constituted of " osteoplasts " regularly grouped
round vascular canals, and included in a ground substance
finely striated. (Journal del'anatomie, 1881, p. 32.) Where
mtra-cartilaginous ossifications occur elsewhere in the body
a temporary bone is formed by the calcification of the
cartilage matrix, which is subsequently absorbed and swept
away, as marrow-containing channels appear in it, and bore

THE DEVELOPMENT OF THE TEETH. 171

their way through it, substituting for the calcined cartilage
a bone developed from osteoblasts, and ultimately all remains
of the calcined cartilage or temporary bone disappear. Thus
all bone whether developed in cartilage or in membrane
is formed alike, the calcined cartilage merely forming a
temporary framework or scaffolding, in and amongst which
the bone is formed from osteoblasts. But M. Magitot does
not describe in much detail this calcification of cartilage and
subsequent removal to give place to an osteoblast-derived
bone, though he speaks of the cartilaginous cement organ
as a transitory or temporary structure.

Membrana Preformativa. — To the student of dental
development few things are more perplexing than the con-
flicting statements which he reads in various works as to
the nature and position of the Membrana preformativa, of
which I have hitherto studiously avoided all description;
while it is not encouraging, after having mastered with
difficulty some one description of its character, to find that
many of the most recent authors altogether deny its exist-
ence. I will endeavour, therefore, so far as I am able,
although not myself believing in its presence, to put the
matter in a clearer light, and to point out wherein lie the
discrepancies of statement.

According to the older theories of tooth development,
under the thrall of which most authors have written, the
tooth germ was in the first instance a free, uncovered papilla
of the mucous membrane, which subsequently sank in and
became encapsulated, &c., &c., (see page 129). Moreover, it
was taught by the older histologists that fine homogeneous
" basement membranes " were to be found in a great variety
of situations, amongst others beneath the epithelium of the
mucous membrane, and that these were of (physiologically)
much importance, inasmuch as they formed defining limits,
through which structures did not pass. As a necessary con-
sequence of these views, it was assumed as a matter of

172 A MANUAL OF DENTAL ANATOMY.

course that the " dentine papilla " was covered over by a
" basement membrane," or membrana preformativa.

Thus this membrane necessarily intervened between the
enamel organ and the dentine papilla, and hence gave rise
to difficulties in the understanding of the calcifying process.
Henle considered that evidences of its presence speedily
became lost, but that ossification proceeded in opposite
directions upon the two sides of this membrane : from
within outwards in the case of the enamel, from without
inwards in the case of the dentine.

Prof. Huxley, starting on the same hypothesis as to its
position, namely, that it was between the enamel organ and
the dentine papilla, came to a different conclusion as to its
after fate ; relying upon the fact that a continuous sheet of
tissue or membrane can be raised from the surface of the
developing enamel (see page 152), he concluded that this
was the original membrana preformativa, that it afterwards
became the Nasmyth's membrane, and that enamel was
developed without the direct participation of the enamel
organ, seeing that a membrane separated the two. My
reason for doubting the correctness of these conclusions has
been there given ; the membrane so demonstrable is, I
believe, artificial, and does not represent any naturally
existing structure.

Kblliker strongly affirms the existence of the membrana
preformativa, and in the older edition of his Histology, held
that it became converted into Nasmyth's membrane ; al-
though he now gives a different explanation of the origin
of Nasmyth's membrane, I have not found a definite state-
ment as to his recent views of the ultimate fate of the
membrana preformativa.

We have thus three destinations assigned to the mem-
brane^ covering the dentine papilla, or membrana prefor-
mativa.

THE DEVELOPMENT OF THE TEETH. 173

(i.) Between the dentine and the enamel (Henle).
(ii.) Between the enamel and the enamel organ, or out-
side the enamel (Huxley).

(iii.) Between the dentine and the pulp (several writers of
less authority).

We come next to those writers who deny its existence
altogether, explaining on other grounds the appearances
observed.

Markusen believed that it was nothing more than the part
of the papilla first ossified ; and Dr. Lionel Beale definitely
denies the existence of a membrane in any one of the three
situations above detailed, as do also Hertz, Wenzel, and
Waldeyer.

Messrs. Robin and Magitot have offered a plausible ex-
planation of the appearance of a limiting membrane over
the pulp, which is briefly this : the formative pulp is rich
in a clear substance of gelatinous consistency (which in
fact forms its chief bulk), and which reminds the observer
of the tissue contained in an umbilical cord. This is some-
what more dense towards the surface, where it forms a
matrix for the odontoblasts and projects beyond them, so
as to look, in section or at a thin edge, like a sort of
varnish to the papilla. From its greater density near the
surface, it may become corrugated, and so look like a
folded or torn membrane. I am quite inclined to agree
with the foregoing explanation.

I am inclined to think, that but for the erroneous theories
that the dentine germ originated as a free papilla on the
surface, which would according to the prevalent view have
been necessarily invested by a basement membrane, we
should never have heard of a membrana preformativa. At
all events it is difficult to imagine that such a membrane
exists upon papilla formed at such a great distance from the
surface as those of the snake or the lizard (Figs. 61 and 62) ;

174 A MANUAL OF DENTAL ANATOMY.

and if there be such a membrane, it must be a secondary
development upon the surface of the mass of cells which
primarily constitute the rudiment of the dentine papilla, and
in that case is not a part of the general basement membrane
of the oral mucous membrane ; or else it must have been
carried above as a sort of cul de sac in front of the inward
growing process of epithelium, to which in that case it would
belong rather than to the dentine germ. Neither of these
suppositions commend themselves as probable ; and a still
greater obstacle to the acceptance of a membrane in this
position is afforded by the structure of Marsupial teeth (see
fig. 23), in which the membrane would be everywhere per-
forated by the soft contents of the dentine and enamel
tubes.

ROBIN ET MAGITOT. Journal de I'anatomie. 1866.

LEGEOS ET MAGITOT. Follicule Dentaire. Journal de I'anatomie

de M. Ch. Robin, 1873.
Morphol. du follicule dentaire. 1879.
Formation de 1'organe dentaire. 1881.
KLEIN. Atlas of Histology. 1880.
WALDEYEE. Strieker's Histology. 1870.
HUXLEY. Quart. Jour. Micros. Science. 1853.
KOLLIKEE. Gewebelehre.
TOMES, J. Quart. Jour. Micros. Science. 1853.

Dental Surgery. 1859.
TOMES, CHAELES S. Develop, of Vascular Dentine. Philos. Trans.

1878.

Develop, of Teeth of Batrachia, Ophidia, Se-
lachia, and Teleostei. Phil. Trans. 1875 —
1876.
On Nasmyth's Membrane. Q. J. Microsc

Science, 1872.
OWEN. Odontography. 1845.

Anatomy of Vertebrates. 1870.
NASMYTH. Med. Chirurg. Transact. 1839.

Observations on the Teeth. 1835.

MAECUSEN. Bulletin de 1'Acad. de S. Petersburg. 1849.
GOODSIE. Edinburgh Med. and Surg. Journal. 1838.
BEALE, DE. LIONEL. Structure of the Simple Tissues. Archives

of Dentistry, vol. i.
DUESY, EMIL. Entwickelungsgeschichte des Kopfes. 1869.

THE DEVELOPMENT OF THE JAW. 175

HERTWIG. Entwickelung der Placoidscliuppen und Zahne. Jen-

aisclie Zeitschrift. 1874.
Zahnsystem der Amphibien. Archiv. f. Mik. Anat.

1874.

RASCHKOW. Meletemata circa Dentium Evolutionem. 1835.
HEINCKE. Zeitschrift f. Wiss. Zool. Bd. xxiii. 1873.
WEDL. Pathologie der Zahne. 1870.
TOMES AND DE MORGAN. On Development of Bone. Phil. Trans.

1852.
GEGENBAUR. Manual of Comparative Anatomy. Translated by

Jeffrey Bell, 1878.

ROLLET. Connective Tissues, in Strieker's Histology.
HARTING. Quart. Journal Micros. Science. 1872.
RAINIE. Brit, and Foreign Med.-Chirurg. Review. 1857.
DEAN, M. S. Annotated Translation of Robin and Magitot on the
Origin of the Dental Follicle. Chicago, 1880.

CHAPTER V.

THE DEVELOPMENT OF THE JAWS — THE ERUPTION AND THE
ATTACHMENT OF THE TEETH.

AT an early period in the development of the embryo
there is a single primitive buccal cavity, which is subse-
quently divided into a nasal and an oral cavity by the
palatine plates growing horizontally across it ; the pharynx
behind the hinder end of the primitive buccal cavity
remains undivided. Both upper and lower jaws make their
appearance about the twentieth day as little buds from the
first visceral arch, and grow inwards towards the middle
line : those which form the lower jaw reach to the middle
and there coalesce, those for the upper jaw stop short, and
the gap left between them is filled by a double downward
sprouting process from the forehead, which afterwards forms
the intermaxillary bone. A failure in the coalescence of the
maxillary processes with this intermaxillary process, on
one or both sides, results in a single or double hare-lip.

In the lower jaw or mandibular processes there appears,
about the end of the first month, a dense cord of cartilaginous
consistence, Meckel's cartilage, which seems to serve as a
scaffolding, giving form and consistency to the lower jaw
prior to the occurrence of calcification. Meckel's cartilage,
formed as two distinct halves, soon unites in the middle, and
then forms a continuous curved bar, the hinder ends of
which reach up to the tympanum.

About the fortieth day a centre of ossification appears in
the mandibular process, which, spreading rapidly, soon forms

THE DEVELOPMENT OF THE JAW. 177

a slight osseous jaw outside Meckel's cartilage, which is not
however in any way implicated in it, and very soon begins
to waste away, so that by the end of the sixth month it has
disappeared : that end of it alone which extended up to the
tympanum does not so waste away, but becomes ossified into
the malleus. There are, however, observers who hold that
in some animals, at all events, Meckel's cartilage plays a
more active part in ossification of the jaw.

In the upper jaw the suture separating the intermaxillary
from the maxillary bones becomes obliterated very early on
the exterior surface, but it long remains distinguishable on
the palatine aspect of the bones.

The later changes which are undergone by the jaws during
the development, eruption, and loss of the teeth, have long
engaged the attention of anatomists, and amongst others of
Hunter, who was the first to arrive at a tolerably correct
appreciation of the process. In the first edition of my
father's " Dental Surgery," the results of a very extensive
series of observations carried out upon maxillae collected by
himself, were detailed, confirming in the main Hunter's
conclusions, but adding many new points to our knowledge ;
and from this work I have borrowed largely in the present
chapter. Professor Humphrey, who had overlooked these
descriptions, which were never published in any other form
than as an introduction to the " Dental Surgery," instituted
a series of experiments upon growing animals, which tended
towards the same conclusions.

As a means of giving the student a guide in his reading
of the following pages, and a clue to the results towards
which he is being led, a preliminary statement, which does
not pretend to scientific accuracy, may perhaps be useful ;
while the description given will relate for the most part to
the lower jaw, because its isolated position, bringing it into
relation with fewer other bones, renders it more easy to
study; not that any difference of principle underlies the

178 A MANUAL OF DENTAL ANATOMY.

growth of the upper jaw. The different parts of the lower
jaw answer for different purposes ; one division of its body
having a very close and intimate relation with the teeth ,
the other serving a distinct purpose, and being only secon-
darily connected with the teeth.

The alveolar portion of the jaw, that which lies above the
level of the inferior dental canal, is developed around the milk
teeth : when they are lost, it disappears, to be re-formed
again for the second set of teeth, and is finally wholly
removed after the loss of the teeth in old age.

The portion of jaw below this line, which is essential to
deglutition and respiration, is late in acquiring any con-
siderable development. Once formed it is never removed,
save that when in advanced old age the muscles of mastica-
tion are no longer in full use, it becomes, to a slight extent
only, wasted.

In order to understand the drift of the following descrip-
tion, it is essential to keep in view the different life histories
of those two parts of the jaw just alluded to.

In an early foetus, long before the necessity for respiratory
movement or deglutition has become imminent, a thin lamina
of bone has begun to be formed beneath the tooth germs,
forming, as it were, a semicircular gutter running round the
jaw, in which the developing tooth sacs are lodged. The
thin gutter of bone thus formed is above and outside
Meckel's cartilage, and intervenes between the rudimentary
inferior maxillary vessels and nerves, and the teeth. The
sides of the bony furrow rise as high as the top of the tooth
germs, but they do not arch over and cover them in, in such
manner as the permanent tooth germs are arched in, for the
long furrow is widely open at the top.

Passing on to the condition of the mandibles at the time
of birth, the two halves are as yet not anchylosed, but are
united only by fibre-cart ilage. " The alveolar margins are
deeply indented with large open crypts, more or less per-

THE DEVELOPMENT OF THE JAW.

179

fectly formed. The depth of these bony cells is only suffi-
cient to contain the developing teeth and tooth pulps, the

FIG. 77 (!).

0) Upper and lower jaws of a nine months foetus, the teeth having
been removed from the jaws on one side to show the extent to which they
are calcified at this period. (Two-thirds life size.) a. Alveoli of lateral
incisors, b. Alveoli of canines, c. Alveoli of second temporary and first
permanent molars. A bristle has been passed through the inferior dental
canal.

N 2

180 A MANUAL OF DENTAL ANATOMY.

former rising to the level of the alveolar margins of the
jaws. At this period the crypts or alveoli are not arranged
in a perfectly uniform line, nor are they all equally complete.
The septa, which divide into a series of cells that which at
an earlier age was but a continuous groove, are less perfect
at the back than at the front part of the mouth. The
alveoli of the central incisors of the upper and the lower
jaws are a little larger within than at the orifice, and this
diiference is made still greater by a depression upon the
lingual wall of each for the reception of the pulp of the
corresponding permanent tooth. They are divided from
the crypts of the lateral incisors by a septum which runs
obliquely backwards and inwards towards the median line.
The sockets for the lateral incisors occupy a position slightly
posterior to those for the central teeth, and are divided
from the canine alveoli by a septum which proceeds obliquely
backwards, and in the lower jaw, as regards the median line
of the mouth, outwards. By the arrangement of these
divisions the alveoli of the central incisors are rendered
broader in front than behind, and the relative dimensions
of the sockets of the lateral teeth are reversed, as shown in
Fig. 77. The crypts of the canine teeth are placed a little
anteriorly to those of the laterals, and nearly in a line with
those of the central incisors, giving to the jaws a somewhat
flattened anterior aspect."

While the main bulk of the lower jaw is made up by the
alveoli of the teeth, in the upper jaw the alveoli descend
but little below the level of the palatal plates, though the
sockets are tolerably deep. The antrum as a special distinct
cavity cannot be said to exist, being merely represented by
a depression upon the wall of the nasal cavity, the alveolar
cavities therefore being separated only by n thin plate of
bone from the orbits.

The figure represents also the extent to which calcification
has advanced in the various teeth.

THE DEVELOPMENT OF THE JAW. 181

A full half of the length of the crowns of the central
incisors, about half that of the laterals, and the tips only of
the canines are calcified ; the first temporary molars are
complete as to their masticating surfaces ; the second tem-
porary molars have their cusps more or less irregularly
united, in many specimens the four cusps being united into
a ring of dentine, the dentine in the central depression of
the crown not being yet formed. During the formation of
the permanent teeth, very similar relations exist between
the amount of calcification in the incisors and canines ; thus
when, as sometimes happens, the development of the teeth
proceeds very imperfectly up to a certain date, and then
changes for the better, it may be that the lower half of the
crown of the central incisor, somewhat less of the lateral,
and the extreme tip of the canine will be honeycombed,
while the remainder of the tooth will be perfect, thus per-
petuating an evidence of the stages to which each of these
teeth had at that particular period attained.

Having noted in some detail the characters of the jaws of
a nine months foetus, we may pass on to the consideration
of those changes which precede the cutting of the deciduous
teeth. A general increase in size takes place, new bone
being developed at all those points where the maxillse are
connected by soft tissue with other bones, as well as from
their own periosteum. But the increase in dimensions does
not take place in all directions equally, so that material
changes of form result.

In correspondence with the elongation of the tooth sacs,
the alveoli become increased in depth, and their edges circle
inwards over the tooth sacs; active development of bone
takes place in the sutures uniting the two halves of the jaws
to one another, which is compensated by the inclination
inwards of the alveoli of the central incisors. In the lower
jaw the articular process, at first hardly raised above the
level of the alveolar border, rises rapidly up, the direction

182 A MANUAL OF DENTAL ANATOMY.

of the ramus at first remaining oblique, though the angle of
the jaw becomes developed as a stout process for the attach-
ment of muscles. At the age of six months the syniphysis
is still well marked, and the mental prominence first becomes
noticeable.

FIG. 78

An additional bony crypt for the first permanent molar
has also appeared, though its separation from that of the
second temporary molar, from which it was at first in no
way distinct, is yet incomplete, especially in the lower jaw.
In the upper jaw the first permanent molar crypt has no
posterior wall ; bony cells for the permanent central incisors
are well marked, but those for the laterals are mere deep
pits in the palatine wall of the crypts of the temporary teeth.

At the age of eight months, or thereabouts, the process
of the eruption of the teeth, or "teething," has fairly set in ;
anchylosis has taken place at the symphysis of the lower
jaw, the mental prominence is well marked, and in the upper
jaw the antrum has become a deep depression, extending
under the inner two-thirds of the orbit.

Postponing for the moment the consideration of the erup-
tion of the teeth, in order to follow up the growth of the
jaws, it becomes necessary to take some fixed points as
standards from which to measure the relative alteration of
other portions of the bone. In most bones, processes for
the attachment of muscles would be very unsuitable for the
purpose, because they would alter with the general altera-

(*) Lowci* jaw of a niue months foetus.

THE DEVELOPMENT OF THE JAW. 183

tion in the dimensions of the bone : thus a process situated
at a point one-third distant from the articular extremity of
a large bone, will still be found one-third distant from the
end, though the bone has doubled in length. The four
little tubercles which give attachment to the genio-hyo-
glossus and genio-hyoid muscles are not, however, open to
these objections, as they are already, so to speak, at the end
of the bone, or, at least, of each half of it ; and their general
correspondence in level with the inferior dental canal, which
can hardly be imagined to undergo much alteration, in-
dicates that their position is tolerably constant.

The points selected as landmarks are then, the spinse
mentales, the inferior dental canal and its orifice, and the
mental foramen. The mental foramen itself does undergo
slight change in position, but this change can easily be
estimated, and may as well at once be mentioned. As the
jaw undergoes increase in size, large additions are made to
its surface by deposition of bone from the periosteum, neces-
sarily lengthening the canal. The additions to the canal
do not, however, take place quite in the line of its original
course, but in this added portion it is bent a little outwards
and upwards. If we rasp off the bone of an adult jaw down
to the level of this bend, a process which nature in great
part performs for us in an aged jaw, or if instead we make
due allowance for the alteration, the mental foramen becomes
an available fixed point for measurement.

The mental foramen, which undergoes most of its total
change of position within a few months after birth, comes
to correspond with the centre of the socket of the first tem-
porary molar ; later on it corresponds with the root of the
first bicuspid, which is thus shown to succeed, in exact
vertical position, the first temporary molar.

On the inner surface of the jaw the tubercles for the
attachment of the genio-hyo-glossus and genio-hyoid muscles
are in the foetus, opposite to, and very little below the base

184 A MANUAL OF DENTAL ANATOMY.

of the alveoli of the central incisors, a position which they
afterwards hold with regard to the permanent incisors. The
upper of the two pairs of processes are about at the same
general level as the mental foramen.

The general result arrived at by measurements taken
from these fixed points is that the alveolar arch occupied
by the teeth which have had deciduous predecessors, namely
the incisors, canines, and bicuspids, corresponds very closely
with the whole alveolar arch of the child in whom the tem-
porary dentition is complete ; and that the differences which
do exist are referable, not to any fundamental alteration in
form or interstitial growth, but to mere addition to its
exterior surface. Or more briefly, that the front twenty of
the permanent succeed vertically to the places of the tem-
porary teeth, the increase in the size of the jaw in an adult
being due to additions at the back, in the situation of the
true molars, and to other points on the surface.

If measurements be taken across between the inner platen
of the alveoli on either side at the points where they are
joined by the septa between the first and second temporary
molars, and at about the level of the genio-hyo-glossu&
tubercles, it will be found that the increase is slight, if any,
notwithstanding that in other dimensions there is a very
great difference between the jaws of a nine months foetus
and of a nine months child.

Again, if an imaginary line be stretched across between
these two points, and from its centre a line be drawn for-
wards to the spina mentalis in the same two jaws, this will
be found to differ but little in length in the two specimens.

But, if instead of measuring to the spina mentalis, the
line had been carried to the anterior alveolar plate, a great
difference would have been observable ; in point of fact, con-
temporaneously with the development of the crypts of the
permanent teeth inside them, the temporary teeth and their
outer alveolar plates are slowly pushed outwards, a process,.

THE DEVELOPMENT OF THE JAW.

185

the results of which we see in the separation which comes
about between each one of the temporary teeth, prior to
their being shed, where the process of dentition is being
carried on in a perfectly normal manner.

Measurements taken for the sake of comparing adult jaws

with those of an eight months child, give closely similar
results, which I have endeavoured to roughly embody in
the accompanying figures.

In these it is shown that the increase in the dimensions
of the jaw has taken place in two directions; by prolongation
backwards of its cornua concomitantly with the addition at
the back of the series of teeth of the true molars, which follow

(*) Diagram representing a jaw of a nine months fetus, superimposed
upon an adult jaw, to show in what directions increase has taken place.

186 A MANUAL OF DENTAL ANATOMY.

one another at considerable intervals ; and by additions to
its exterior surface by which it is thickened and strengthened.
The study of the growth of the jaw in vertical depth is also
very instructive. We find that, as has already been men-
tioned, the history of that part of the jaw which lies below
the inferior dental canal is very different from that which
lies above. From the time of birth to that at which the
temporary teeth begin to be cut, the jaw below that line
has been making steady but slow progress in vertical depth ;
the alveoli, above that line, have been far more active but
far more intermittent in their development.

Again, passing from the nine months foetus to the seven
years old child, in whom the temporary dentition is complete,
the framework of the jaw below our imaginary line has

f years

attained to a depth almost equal to that which it is seen to
have in an adult ; in the adult again it corresponds pretty
well with that in an aged jaw. The alveolar portion, how-
ever, is far deeper in the adult than in the child (this
difference is not sufficiently well marked in the figure), and

(!) Lower jaw. The horizontal line marks the level of the inferior
dental canal.

THE DEVELOPMENT OF THE JAW.

187

in fact constitutes almost the whole increase in vertical
dimensions down the passage from the child's to the adult's
form of the jaw.

FIG. 81 (').

FIG. 82 (2).

In the lower jaw we may take.it as proven that the
basal portion has little relation to the development of
the teeth, but that the alveolar, or upper portion is in
entire and absolute dependence upon them, a point to

(J) Lower jaw of an adult.

(2) Lower jaw of an aged person, the dotted lines indicating the outline
of the parts removed by absorption, as the jaw assumes the form charac-
teristic of advanced age.

188 A MANUAL OF DENTAL ANATOMY.

which I shall again return in speaking of the eruption of
the teeth.

It remains to speak in some further detail of the
precise means by which the enlargement of the jaw is
effected.

To a slight extent there is formation of bone going on at
the symphysis, prior to the complete anchylosis taking
place : the share taken by this in increasing the size of the
jaw would, however, appear to be but small, after the ter-
mination of the intra-uterine period. Additions to the
surface, at the edges of the alveoli and at the base of the
jaw are continually going on, and bring about that addition
to the exterior already noticed.

But the main increase in the size of the jaw has been in
the direction of backward elongation; in this, as Kolliker
first pointed out, the thick articular cartilage plays an im-
portant part. The manner in which the jaw is formed
might almost be described as wasteful ; a very large amount
of bone is formed which is subsequently, at no distant date,
removed again by absorption; or we might compare it to a
modelling process, in which thick, comparatively shapeless
masses, are dabbed on to be trimmed and pared down into
form.

To bring it more clearly home to the student's mind, if
all the bone ever formed were to remain, the coronoid pro-
cess would extend from the condyle to the region of the first
bicuspid, and all the teeth behind that would be buried in
its base: there would-be no "neck " beneath the condyle,
but the internal oblique line would be a thick bar, corre-
sponding in width with the condyle. It is necessary to
fully realise that the articular surface with its cartilage
has successively occupied every spot along this line ; and as
it progresses backwards by the deposition of fresh bone in
its cartilage, it has been followed up by the process of
absorption removing all that was redundant.

THE DEVELOPMENT OF THE JAW. 189

On the outer surface of the jaw we can frequently discern
a slight ridge, extending a short distance from the head of
the bone ; but if the prominence were preserved on the
inner surface, the inferior dental artery and nerve would be
turned out of their course. We have thus a speedy removal
of the newly-formed bone, so that a concavity lies imme-
diately on the inner side of the condyle ; and microscopic
examination of the bone at this point shows that the lacunae
of Howship, those characteristic evidences of absorption,
abundantly cover its surface, showing that here at least
absorption is most actively going on.

In the same way the coronoid process, beneath the base
of which the first, second, and third molars have successively
been formed, has moved backwards by absorption acting on
its anterior, and deposition on its posterior surfaces.

The periosteum covering the back of the jaw is also active
in forming the angle and the parts thereabouts.

It is worth while to add that the direction of growth in
young jaws is marked by a series of minute ridges ; in like
manner the characteristic marks of absorption are to be
found about the neck of the condyle, and the front of the
coronoid process, and those of active addition about the pos-
terior border, so that the above statements rest upon a basis
of observation, and are not merely theoretical. Two cases of
arrested development of the jaw ("Dental Surgery," p. 108)
lend a species of experimental proof to the theory of the
formation and growth of the jaw above given.

There are authors, however, who maintain that the growth
of 'the jaws is not merely a backward elongation of the
cornua, together with additions to the external surface, but
that an " interstitial growth" takes place.

AVedl inclines to this latter view, and the question cannot,
I think, be held to be absolutely settled. Although it is
difficult to form any definite conception of interstitial growth
in a tissue so dense and unyielding as bone, so that the doc-

190 A MANUAL OF DENTAL ANATOMY.

trines promulgated in the foregoing pages have the support
of ct, priori probability, there are some rather paradoxical
facts to be met with in comparative odontology. Never-
theless, there can be no doubt, that backward elongation as
teeth are successively added, &c., is sufficiently near the
truth in the case of human and most mammalian jaws for
practical purposes.

It remains to notice the changes in form which the
ascending ramus and the angle of the jaw undergo. In
the foetus the ramus is but little out of the line of the body
of the jaw, and the condyle little raised above the alveolar
border.

Gradually the line of development, as is indicated even in
the adult jaw by the course of the inferior dental canal, takes
a more upward direction ; copious additions of bone are
made on the posterior border and about the angle, so that in
an adult the ramus ascends nearly at right angles to the body
of the jaw.

In old age, concomitantly with the diminution of muscular
energy, the bone about the angle wastes, so that once more
the ramus appears to meet the body at an obtuse angle.
But all the changes which mark an aged jaw are the simple
results of a superficial and not an interstitial absorption, cor-
responding with a wasting of the muscles, of the pterygoid
plates of the sphenoid bone, &c.

ERUPTION OF THE TEETH.

THE mechanism by which teeth, at the date of eruption,
are pushed upwards into place, is far from being perfectly
understood. The simplest theory would appear to be that
they rise up, in consequence of the addition of dentine to
their base ; in fact, that their eruption is due to the elonga-
tion of their fangs.

ERUPTION OF THE TEETH. 191

Various very strong objections have been brought forward,
clearly proving that this cause is quite inadequate to explain
all that may be observed. In the first place, teeth with very
stunted roots — which may be practically said to have no
root — are often erupted. Again, a tooth may have the
whole length of its roots completed, and yet remain buried
in the jaw through half a person's life, and then, late in life,
be erupted. Moreover, when a healthy normal tooth is being
erupted, the distance travelled by its crown materially ex-
ceeds the amount of addition to the length of its roots which
has gone on during the same time.

To turn to comparative anatomy, the tooth of a crocodile
moves upwards, tooth pulp and all, obviously impelled by
something different from mere elongation; and my own
researches upon the development and succession of rep-
tilian teeth clearly show that a force quite independent of
increase in their length shifts the position of, and " erupts"
successive teeth. But what the exact nature of the impulse
may be, is an unsolved riddle : the explanations which I have
read, being, to my mind, less satisfying than the admission
that we do not know.

V-^Towards the eighth month of childhood the bony crypts
/ which contain the temporary teeth in the front of the mouth
begin to be renewed. The process of absorption goes on
with greater activity over the fronts of the crowns than over
their apices, so that almost the whole outer wall of the
alveoli is removed. At the back of the mouth the crypts
still retain their inverted edges ; indeed, development of the
crypts is still going on in this part of the mouth.

When a tooth is about to be cut, very active absorption of
its bony surroundings goes on, particularly on the anterior
surface, the bone behind it being still required as forming
part of the crypt of the developing successional tooth. But
no sooner has the crown passed up through the very wide
and free orifice so formed, than absorption gives place to

192 A MANUAL OF DENTAL ANATOMY.

deposition, and the bone rapidly developes so as to loosely
embrace the neck of the tooth.

Additions to the margin of the alveoli keep pace with

FIG. 83

the gradual elongation of the roots of the teeth ; as this is
a moderately rapid process, the alveolar portion of the jaw
increases in depth almost abruptly.

But it does not do so uniformly all over the mouth ; if it
did, the teeth could only be closed at the back of the mouth,
unless the rami elongated by an equally sudden accession of
new bone.

The front teeth are erupted first, and the jaw deepens
first in front : later on the back teeth come up and the jaw
is deepened posteriorly ; meanwhile the elongation of the

(j) Jaws of a male nine months old, in which the eruption of the teeth
is just commencing.

/£•- VrV\

/ o^ n i r \

i UNIVERSITY 7)
ERUPTION OF THE TEETH. 193

rami has been going on slowly, but without interruption.
Thus is brought about a condition of parts allowing of
the whole series of teeth coming into their proper mutual
antagonism.

It was pointed out by Trousseau that the eruption of the
teeth is not a continuous process, which, once commenced, is
carried on without intermission to its completion, but that it
is interrupted by periods of repose. The teeth are, according
to his statement, cut in groups ; the eruption of the teeth of
each group being rapid, and being succeeded by a complete
cessation of the process. Individual variations are nume-
rous ; the following may be taken as an approximation to
the truth : —

The lower centrals are erupted at an age ranging from six
to nine months ; their eruption is rapid, and is completed in
ten days or thereabouts ; then follows a rest of two or three
months.

Next come the four upper incisors ; a rest of a few
months ; the lower laterals and the four first molars \ then
a rest of four or five months.

The canines are peculiar in being the only teeth of the
temporary set which come down between teeth already in
place. To this, as well as to the greater length of their
root (though it is not quite clear what this has to do with
it), Trousseau ascribes the great length of time which their
eruption occupies, it taking two or three months for its com-
pletion. According to him, children suffer more severely
from constitutional disturbance during the cutting of these
teeth than that of any other, but Dr. West thinks that the
eruption of the first molars causes the most suffering. It
may also be noted that the canines during their develop-
ment lie farther from the alveolar border than do the other
teeth, so that they travel a greater distance ; obviously, not
merely from the elongation of the root, which is wholly
inadequate to effect such a change in position.

194

A MANUAL OF DENTAL ANATOMY.

The dates of the eruption of the milk teeth vary much,
no two authors giving them alike ; but the whole of the
deciduous teeth are usually cut by the completion of the
second year. Cases in which incisors have been erupted
before birth are not very uncommon. At a time when the
crowns of all the deciduous teeth have been fully erupted,
their roots are still incomplete, and are widely open at their
bases, so that it is not till between the fourth and sixth
years that the temporary set of teeth can be called abso-
lutely complete. \/

At the sixth year, preparatory to the appearance of any

of the permanent teeth, the temporary teeth may be observed
to be slightly separated from each other ; they have come to
occupy a more anterior position, pushed forward, it may be,
by the great increase in size of the crypt of the permanent
teeth behind them. The general relation of these to the

(J) Normal well-formed jaws, from which the alveolar plate has been in
great part removed, so as to expose the developing permanent teeth in
their crypts in the jaws.

ERUPTION OF THE TEETH. 195

temporary teeth may be gathered from the accompanying
figure, in which it will be noticed that the canines lie far
above and altogether out of the line of the other teeth, and
that a slight degree of overlapping of the edges of the per-
manent central and lateral incisors exists.

The bicuspids lie in bony cells which are embraced pretty
closely by the roots of the temporary molars, and it hence
happens that extraction of the latter sometimes brings them
away in their entirety.

The first permanent molars are erupted in a manner closely
similar to that described as occurring with the temporary
teeth ; that is to say, their bony crypts become widely
opened out by absorption, the crown passes out, and new
bone is rapidly formed which embraces the neck, prior to
any considerable length of root being formed.

Last, then, follows the absorption of the root of the tem-
porary teeth, a matter first accurately investigated by my
father. The root at or near to its end, becomes excavated
by shallow cup-shaped depressions ; these deepen, coalesce,
and thus gradually the whole is eaten away. Although ab-
sorption usually commences on that side of the root which
is nearest to the successional tooth, it by no means invari-
ably does so ; it may be, and often is, attacked on the oppo-
site side, and in many places at once.

The cementum is usually attacked first, but eventually
dentine, and even enamel come to be scooped out and
removed by an extension of the process. That part of the
dentine, however, which immediately surrounds the pulp
appears to have more power of resistance than any other
part of the tooth, and thus often persists for a time as a sort
of hollow column. The absorption of the temporary teeth
is absolutely independent of pressure ; the varying position
of the excavation has already been noticed, and it may be
added that in many lower animals, for example, the frog or
the crocodile, the growing tooth sac passes bodily into the

o 2

196

A MANUAL OF DENTAL ANATOMY.

excavation made before it in the base of the tooth which has
preceded it, while if pressure had had any share in the matter
the cells of its enamel organ, &c., must have inevitably been
crushed and destroyed.

Again, when the absorption and shedding of the first teeth

FIG. 85

have taken place early, before their successors are ready to
appear, perfect little sockets are formed behind the lost

(!) Jaws of a six year old child. In the upper jaw complete sockets are
seen where the temporary incisors have been shed.

ERUPTION OF THE TEETH. 197

temporary teeth, cutting them off from the permanent teeth
destined to follow them. Absorption, too, may attack the
roots of permanent teeth, which is another reason for regard-
ing the process as not necessarily dependent upon the approach
of a displacing tooth. Closely applied to the excavation
produced by absorption is a mass of very vascular soft tissue,
the so-called absorbent organ. The surface of this is com-
posed of very large peculiar-looking cells, bearing some little
resemblance to those known as " myeloid cells," or the
" giant cells " of recent authors. Microscopic examination
of the excavated surface shows it to be covered with small
hemispherical indentations, the "lacunse of Howship," into
each of which one of the giant cells fitted, and in which they
may sometimes be seen in situ.

In what manner these giant cells, or " osteoclasts," effect
their work is not known, but their presence where absorption
of hard tissues is going on is universal. Some suppose that
they put forth amoebiform processes, others that they secrete
an acid fluid, but nothing very definite is known ; a curious
parallel is afforded by the manner in which a fungus can
drill and tunnel through and through the dentine, as may be
very constantly observed in teeth long buried.

The process of absorption once commenced does not neces-
sarily proceed without intermission, but may give place for
a time to actual deposition of osseous tissue on the very sur-
face eroded ; probably by the agency of the absorbent cells
themselves, which are capable of being calcified in the exca-
vations they have individually made.

These alternations of absorption and deposition, so com-
mon a result of inflammations of the pulp, or of the alveolo-
dentar periosteum, as to be diagnostic of the former occurrence
of these maladies, often occur during the normal process of
the removal of the deciduous teeth, and result in the deposi-
tion of a tissue not unlike cementum in excavations made
in the dentine, or even in the enamel.

198 A MANUAL OF DENTAL ANATOMY.

The eruption of the permanent teeth is a process closely
analogous to that of the temporary set. Rapid absorption
of the bone, especially on the exterior surface of the crypts,
takes place, and an orifice very much larger than the crown
of the tooth is quickly opened out.

Hence it is that the slightest force will suffice to determine
the direction assumed by the rising crown : a fragment of a
root of a temporary tooth, the action of the lips and tongue,
&c., are all potent agencies in modifying the arrangement of
the teeth.

The temporary teeth stood vertically, the permanent
teeth in front of the mouth stand obliquely, thus opening
a space between the lateral incisors and the first bicuspid
for the canine, which during development was out of the
line altogether. And, inasmuch as the crowns of the teeth
are on the whole much larger than their necks, it would
be manifestly impossible for them all to come down simul-
taneously.

^ The permanent teeth usually make their appearance in
the following order : — First permanent molars, about the
seventh year ; a little later, the lower central incisors, upper
centrals and laterals, the first bicuspids, the canines, the
second bicuspids, the second permanent molars, the third
permanent molars.

The period of eruption is variable. From a comparison
of several tables, I find the principal discrepancies to relate
to the date of the appearance of the canines and the second
bicuspids. The canine would certainly appear to belong to
the eleventh and twelfth years ; but some authors consider
that the second bicuspid is usually cut earlier, others later
than this date.

We may now revert to the phenomena observed in the
alveolar processes. They were first built up as crypts with
overhanging edges enclosing the temporary teeth : then they
were swept away, in great part, to allow of the eruption of

ERUPTION OF THE TEETH. 199

the temporary teeth : and next they were rebuilt about the
necks, to form the sockets, of the deciduous teeth.

Once more, at the fall of the deciduous teeth, the alveoli
are swept away, the crypts of the permanent teeth are
widely opened, and the permanent teeth come down through
the gaping orifices.

When they have done so, the bone is reformed so as to
closely embrace their necks, and this at a period when but
little of the root has been completed.

Take for example the first upper or lower molars : their
short and widely open roots occupy the whole depth of the
sockets, and reach respectively to the floor of the antrum
and the inferior dental canal. No growth, therefore, can
possibly take place in these directions ; the utmost available
depth has already been reached, and as the roots lengthen
the sockets must be deepened by additions to their free
edges.

It is impossible to insist too strongly upon this fact, that
the sockets grow up with and are moulded around the teeth
as the latter elongate. Teeth do not come down and take
possession of sockets more or less ready made and pre-
existent, but the socket is subservient to the position of the
tooth ; wherever the tooth may chance to get to, there its
socket will be built up round it.

Upon the proper appreciation of this fact depends our
whole understanding of the mechanism of teething; the
position of the teeth determines that of the sockets, and the
form of the pre-existent alveolar bone has little or nothing
to do with the disposition of the teeth.

During the period of eruption of the permanent teeth the
level of the alveolar margin is seen, in a dried skull, to be
extremely irregular, the edges of the sockets corresponding to
the necks of the teeth, whether they have attained to their
ultimate level, or have been but just cut.

And when temporary teeth have been retained for a longer

200

A MANUAL OF DENTAL ANATOMY.

period than is natural, they sometimes become elevated to
the general level of the permanent teeth (which is consider-
ably higher than that of the temporary teeth), so that they
take their share of work in mastication. When this is the
case the alveoli are developed round them, and co/ne to
occupy with the tooth a higher level than before.

FIG. 86 (»).

j*

Enough has perhaps been said to illustrate the entire
dependence of the alveoli upon the teeth, a relation of which
dentists every day avail themselves in the treatment of
i regulation cases : it remains to say a few words as to the
^forces which do determine the position of the teeth.

Inasmuch as when a tooth leaves its bony crypt, the
bone does not at first closely embrace it, but its socket is

(!) From a child aged fourteen. The specimen well exemplifies the fact
that the height of the alveolar edge corresponds exactly to the position of
the neck of each tooth, on which it is wholly dependent. A temporary
tooth (the first right lower temporary molar) has been elevated, so that it
has attained to the level of the surrounding permanent teeth, and the edge
of the socket follows the level of the neck of the tooth.

ERUPTION OF THE TEETH. 201

much too large for it, a very small force is sufficient to
deflect it. And, indeed, a very slight force, constantly
operating, is sufficient to materially alter the position of a
tooth, even when it has attained to its full length.

Along the outside of the alveolar arch the muscular lips
are exercising a very symmetrical and even pressure upon
the crowns of the teeth ; so also the tongue is, with equal
symmetry, pushing them outwards : between the two forces,
the lips and the tongue, the teeth naturally become moulded
into a symmetrical arch. That the lips and tongue are tho
agencies which mainly model the arch is very well illustrated
by that which happens in persons who have from childhood
suffered from enlargement of the tonsils, and are conse-
quently obliged to breathe through the mouth, which is thus
pretty constantly open. This causes a slight increase in the
tension of the lips at the corners of the mouth, and is im-
pressed upon the alveolar arch as an inward bending of the
bicuspids at that point ; thus persons with enlarged tonsils
will be found, almost invariably, to present one of the forms
of mouth known as V-shaped.

But Dr. Norman Kingsley attaches far more importance
to disturbed innervation than to any mechanical causes, and
refers most dental irregularities to unhealthy conditions of
the child's nervous system.

f\ When the crowns of the teeth have attained such a level
as to come in contact with their opposing teeth, they very
speedily, from readily intelligible mechanical causes, are
forced into a position of perfect correspondence and an-
tagonism ; and even at a somewhat later period than that
of eruption, if this antagonism be interfered with, the teeth
will often rise up so as to readjust themselves in position. \/

202 A MANUAL OF DENTAL ANATOMY.

THE ATTACHMENT OF TEETH.

Although the various methods by which teeth are fixed in
their position upon the bones which carry them pass by
gradational forms into one another, so that a simple and at
the same time absolutely correct classification is impossible,
yet for the purposes of description four principal methods
may be enumerated, namely, attachment by means of fibrous
membrane, by a hinge, by anchylosis, and by implantation
in bony sockets.

Attachment by means of Fibrous Membrane. — An"
excellent illustration of this manner of implantation is
afforded by the Sharks and Rays, in which the teeth have
no direct connection with the cartilaginous, more or less
calcified, jaws, but are imbedded solely in the tough fibrous
mucous membrane which covers them. This, carrying with
it the teeth, makes a sort of sliding progress over the curved
surface of the jaw, so that the teeth once situated at the
inner and lower border of the jaw, where fresh ones are
constantly being developed, rotate over it, and come to
occupy the topmost position (cf. description of the denti-
tion of the sharks). That the whole fibrous gum, with the
attached teeth, does really so slide over the surface of the
jaw, was accidentally demonstrated by the result of an injury,
which had been inflicted upon the jaws of a shark.

The fibrous bands by which each individual tooth of the
shark is bound down are merely portions of that same sheet
of mucous membrane which furnished the dentine papillae ;
and the gradual assumption of the fibrillated structure by
that portion of the mucous membrane which is contiguous
to the base of the dentine papilla may be traced, no such
fibrous tissue being found at the base of young papillae, and
very dense bands being attached to the bases of the com-
pleted calcified teeth.

Attachment by an Elastic Hinge. — The possession

THE ATTACHMENT OF TEETH. 203

of moveable teeth, able to yield to pressure and subsequently
to resume the upright position, was formerly supposed to be
confined to the Lophius (Angler) and its immediate allies.
I have however found hinges in the common Pike (Esox),
and in the Gadidce (Cod tribe) ; so that, as they occur in these
fish so widely removed from one another in other respects, it is
probable that further investigation will bring to light many
other examples of this very peculiar method of attachment,
eminently suited to, and hitherto only discovered in, fish of
predatory habits.

In the Angler, which obtains its food by lying in ambush
on the bottom, to which it is closely assimilated in colour,
many of the largest teeth are so hinged that they readily
allow an object to pass into the mouth, but rebounding
again, oppose its egress. These teeth are held in position
by dense Iioro^s ligaments radiating from the posterior side
of their bases on to the subjacent bone, while the fronts of the
bases of the teeth are free, and when the teeth are pressed
towards the throat, rise from the bone. The elasticity of
the ligament is such that when it has been compressed by
the tooth being over towards it, it returns it instantly into
position with a snap. Many of the teeth of the Angler are,
like most fishes' teeth, anchylosed firmly.

The Hake (Merlucius, one of the Gadidce) possesses two
rows of teeth, the inner and shorter of which are anchylosed,
whilst the outer and longer are hinged.

In some respects these hinged teeth are more highly
specialised than those of the Angler, which they resemble
in being attached by an elastic hinge fixed to their inner
sides, the elasticity of which is brought into play by its being
compressed, or at all events bent over, upon itself.

The pulp is highly vascular, and its vessels are so arranged
that, by entering the pulp through a hole in the ligament,
which is about at the axis of motion, they escape being stretched
or torn during the movements of the tooth. But the base

204

A MANUAL OF DENTAL ANATOMY.

of the tooth itself is modified so as to be particularly fitted
for resisting the jars to which a moveable tooth must at times
be exposed, and so is the bone upon which it is set.

As is seen in the figure, the base of the tooth, or the side

FIG. 87 (J).

opposite to the hinge, is thickened and rounded, the advan-
tages which such a form must possess over a thin edge when
bumping upon the bone being sufficiently obvious. This

(*) Hinged tooth of Hake. a. Vaso dentine. 6. Pulp. c. Elastic
hinge, d. Buttress of bone to receive /, formed out of bone of attach-
ment, e. Bone of jaw. f. Thickened base of tooth, g. Enamel tip.

THE ATTACHMENT OF TEETH. 205

thickened edge is received upon a little buttress of bone,
and it occupies a much higher level than the opposite thin
edge to which the hinge is attached, so that the tooth cannot
possibly be bent outwards without actual rupture of the
ligament.

And what is not a little remarkable is, that whilst the
Hake, the most predatory of all the Gadidce, is possessed of
these very perfectly hinged teeth, other members of the
family have teeth moveable in a less degree, whilst others
again have teeth rigidly fixed. So that within the limits of
a single family we have several steps in a gradual progres-
sion towards a very highly specialised organ.

In the hinged teeth already alluded to the purpose served
by their mobility seems to be the catching of active fish, and
the elasticity resides solely in the hinges ; but the common
Pike possesses many hinged teeth which seem to be con-
cerned in the swallowing of the prey after it has been caught,
and there is no elasticity in the hinges, the resilience of the
teeth being provided for in another way.

The teeth which surround the margins of the jaws are
anchylosed, and they are more or less solidly filled up in
their interior with a development of osteodentine, which, by
becoming continuous with the subjacent bone, cements them
upon it. The manner of development of this is by rods of
calcifying material shooting down through the central pulp
(see page 165) j in the hinged teeth also these trabeculse shoot
down, and become continuous with the subjacent bone, only
instead of rigidly ossifying they remain soft and elastic, so
that the tooth is like an extinguisher fastened down by a
large number of elastic strings attached to different points
on its interior, and hinged at one side.

The elasticity is very perfect, so that the teeth depressed
and suddenly released return with an audible snap, but it
resides solely in these strings, for if these be divided by
carefully slipping a cataract needle under the tooth without

206

A MANUAL OF DENTAL ANATOMY.

injuring the hinge, the tooth will stay in any position into

which it is put.

FIG. 88 1.

The points most noteworthy are, (i.) that hinged teeth have
arisen independently in three families of fish widely removed
from one another, and (ii.) that, whilst the general object of
mobility and elastic resiliency is attained in all, it is by a
different mechanism, and by the least modification possible
of the existing fixed teeth of the family.

(1) Hinged tooth of Pike. a. Dentine. 5. Elastic rods, formed of un-
calcified trabeculae which might have become bone. c. Hinge, not itself
elastic, d. Bone of attachment, e. Bone of body of jaw.

THE ATTACHMENT OF TEETH. 207

Attachment by Anchylosis. — In both the socketed and
the membranous manners of attachment an organised, more
or less vascular membrane, intervenes between the tooth and
the jaw-bone ; in the method now under consideration there
is no such intervening membrane, but the calcined tooth
substance and the bone are in actual continuity, so that it
is often difficult to discern with the naked eye the line of
junction.

The teeth may be only slightly held, so that they break
off under the application of only a moderate degree of force,
or they may be so intimately bound to the bone that a
portion of the latter will usually be torn away with the
tooth.

A very perfect example of attachment by anchylosis is
afforded by the fixed teeth of the Pike, of which the central
cone is composed of osteodentine. The method by which the
entire fusion of this tissue with the bone beneath it takes
place has already been alluded to, the similarity of its
method of calcification with that of bone rendering the
fusion easy and complete.

And in certain extinct fish, whose nearest ally is the now
anomalous Australian shark, the Cestracion philippi, the
lower part of the tooth is composed of osteodentine, which
so closely resembles bone itself that it is impossible to say
at which point the bone may be said to commence and the
tooth to end ; but even where this intimate resemblance in
histological character does not exist, there is often to be
found more or less blending of the basal dentine with the
bone beneath it, so that there is even here a sort of transi-
tional region.

From the accounts which pass current in most text books
it would be supposed that the process of attachment by
anchylosis is a very simple matter, the base of the dentine
papilla, or the dental capsule, by its calcification cementing
the tooth on to a surface of the jaw-bone already formed.

208

A MANUAL OF DENTAL ANATOMY.

In the few animals which I have examined (*), however, I
have found that this conception does not at all adequately
represent what really takes place \ it seldom, perhaps never,
happens that a tooth is attached directly to a plane surface
of the jaw which has been formed previously ; but the union
takes place through the medium of a portion of bone (which
may be large or small in amount) which is specially developed

FIG. 89 (2).

to give attachment to that one particular tooth, and after
the fall of that tooth is itself removed.

For this bone I have proposed the name of "bone of
attachment," and it is strictly analogous to the sockets of
those teeth which have sockets. It is well exemplified in
the Ophidia, a description of the fixation of the teeth of

(a) Transactions of the Odontological Society, Dec. 1874. — "Studies on
the Attachment of Teeth."

(2) Section of tooth and a portion of the jaw of a Python, showing the
marked difference in character between the bone of attachment and the
rest of the bone.

THE ATTACHMENT OF TEETH. 209

which will serve to convey a good idea of its general cha-
racter. If the base of one of the teeth, with the subjacent
jaw-bone, be submitted to microscopic examination we shall
find that the layer of bone which closely embraces the tooth
contrasts markedly with the rest of the bone. The latter is
fine in texture, its lacunae, with their very numerous fine-
canaliculi, very regular, and the lamination obviously refer-
able to the general surface of the bone. But the "bone
of attachment" is very coarse in texture, full of irregular
spaces, very different from the regular lacunee, and its lamin-
ation is roughly parallel with the base of the tooth. The
dentine of the base of the tooth also bends inwards (Fig. 89),
and its tubes are lost in the osseous tissue, a blending so-
intimate resulting, that in grinding down sections the tooth
and the bone of attachment often come away together,, the-
tooth and this bone being more intimately united than this,
special bone is with that of the rest of the jaw.

A study of its development also proves that it has an in-
timate relation with the tooth with which it is continuous,
for it is wholly removed with the fall of the tooth, and is.
specially developed again for the next tooth which comes,
into position. The periosteum of the rest of the jaw-bone^
appears to take an important share in the formation of this,
special bone substance, and the tooth capsule, by its ossifi-
cation, apparently contributes little.

In the frog the teeth are commonly described as being
attached by their bases and outer surface to a continuous
groove, of which the external wall is the highest. Such is,
however, an inadequate description of the process, the-
tooth, as seen in section, being attached on its outer side
by a new development of special bone, which extends for a.
short distance up over its external surface ; and for the
support of its inner wall there springs up from the sub-
jacent bone a pillar of bone, which is entirely removed when
that tooth falls, a new pillar being developed for the next tooth.

210

A MANUAL OF DENTAL ANATOMY.

When the teeth are, as in many fish, implanted upon
what to the naked eye appears nothing more than a plane
surface of bone, a microscopic examination generally, in fact
in all specimens which I have examined, reveals that the
individual teeth are implanted in depressions much larger
than themselves, the excess of space being occupied by new
and specially formed bone, or else that the teeth surmount
pedicles, which are closely set together, the interspaces being
occupied with a less regular calcified structure.

A good example of the latter method is afforded by the Eel

Fro. 90 (').

(Fig. 90), in which each tooth surmounts a short hollow cylin-
der of bone, the lamination, &c., of which differs strongly
from that of the body of the jaw-bone. When the tooth which
it carries is shed, the bone of attachment, in this case a hollow
cylinder, is removed right down to the level of the main
bone of the jaw, as is well seen in the figure to the left of
the teeth in position. Under a higher magnifying power

('*) From lower jaw of ail Eel. a. Bone of jaw. 6. Bone of attach-
ment, d. Dentine. /. Enamel, g. Space vacated by a shed tooth.

THE ATTACHMENT OF TEETH. 211

the bone at this point would be found to be excavated by
"Howship's lacunae." As an anchylosis, the implantation
of the teeth is less perfect than that of those of the snake,
for the dentinal tubes at the base of the tooth are not de-

FlG. 91

fleeted, and do not in any sense blend with the bone beneath
them. Accordingly, the teeth are far less firmly attached,
and break off quite readily.

A transition towards the socketed type of implantation is
furnished by some of the cod tribe. In the haddock, for
example, the teeth surmount hollow cylinders of " bone of
attachment," resembling in many particulars those of the
eel ; the teeth do not, however, simply surmount the boiiv
cylinders, but are continued for a short distance within
them, definite shoulders being formed which rest on the
rims of the cylinder. The base of the tooth does not, how-
ever, contract or taper any more, and is widely open, so that
it cannot be considered that any close approximation to a
root is made. The pulp cavity of the tooth becomes con-

(*) From lower jaw of a Haddock, a. Bone of jaw. 1>. Bone of attach-
ment. </. Dentine of tooth.

p -J

212 A MANUAL OF DENTAL ANATOMY.

tinuous with the cavity of the osseous cylinder, into which
it is for a short distance continued.

The bony supports of the teeth originate in many osseous
trabeculse which spring up simultaneously from the bone of
the jaw beneath the new tooth ; these coalesce to form a
net-like skeleton, which rapidly becomes filled in by the
progress of ossification. So far as my own researches enable
me to say, there is this much in common in all forms of
attachment by anchylosis, no matter how different the
naked eye results of the process may be ; the tooth, as it
comes into position, is secured by an exceedingly rapid
development of bone, which is more or less directly an
outgrowth from the jaw-bone itself, which is in some un-
seen manner stimulated into activity by the proximity of the
tooth. In amount this specially formed bone varies greatly,
but in all instances it is not the tooth capsule, but tissues-
altogether external to this, which serve to secure the tooth
in its place by their ossification.

The teeth of the mackerel present an interesting variety of
attachment by anchylosis. The margins of the jaws are very
thin, and by no means fleshy, and in this thin margin there
is a deep groove between the outer and inner plate of the
bone. In this groove are the teeth, their sharp points pro-
jecting beyond the edges of the bone, and they are held in
their place by a network or scaffolding of bone of attach-
ment which is developed between their sides and the inner
surface of the bone. They are, so to speak, hung up in their
place, and their open bases rest on nothing, or at least on
nothing hard.

Attachment by implantation in a socket. — In this,
as in anchylosis, there is a special development of bone,
which is modelled to the base of the tooth, but instead of
its being in actual close continuity with the dental tissues,
there intervenes a vascular organised membrane. The
manner in which the sockets are, so to speak, plastered

THE ATTACHMENT OF TEETH. 213

around the roots of the teeth, and are perfectly subservient
to and dependent 011 them, has already been described ; little,
therefore, need be added here, save that the soft tissue
intervening between the bone and the tooth is not sepa-
rable, either anatomically or from the point of view of
development, into any two layers, but is a single mem-
brane, termed the "alveolo-dentar-periosteum." That it is
single, is a matter of absolute certainty \ there is no difficulty
in demonstrating it in situ, with vessels and bundles of
fibres traversing its whole thickness from the tooth to the
bone, or vice versd.

The nature and development of the sockets in those
few reptiles and fishes which have socketed teeth require
further examination. I am not, from what I have seen in
sections of the jaws of a young crocodile, inclined to regard
them as in all respects similar to the alveoli of mammalian
teeth. At all events they are not developed in that same
subserviency to each individual tooth; on the contrary,
successive teeth come up and occupy a socket which is
already in existence.

Although there are animals in which implantation in a
spurious socket is supplemented by anchylosis to the wall
or to the bottom of the socket, no example of anchylosis
occurring between the tooth and the bone of the socket has
ever been met with in man, or indeed in any mammal ex-
emplifying a typical socketed implantation of the teeth.

HUNTER. On the Anatomy of the Human Teeth.

TOMES, J. Dental Surgery. 1859.

HTJMPHERY. Transact. Camb. Philos. Soc. 1863.

WEDL, Pathology of the Teeth.

HEUDNER. Beitrage zur Lehre von der Knochenentwickelung,
&c.

TOMES, CHARLES S. On Vascular Dentine and Hinged Teeth.
Philos. Transac., 1878, and Quart. Journal
Micros. Science, vol. xvii. new series.
Transac. Odontolog. Soc. 1874 — 1876.

CHAPTER VI.

THE TEETH OF FISHES.

IN the following pages nothing more than a brief account
of a few typical forms can be attempted ; the limits of space
forbid the mention of many creatures, or the insertion of
detailed descriptions of the dentition even of the few which
are included in its pages. In the class of fish the task of
selection of the forms for description is no easy one ; for
the almost infinite diversity of dentition which exists in
it makes it a matter of peculiar difficulty to frame any
general account, or to do more than present before the
reader a description of a few individual forms from which he
may gather, as. best he can, a general idea of piscine
dentition.

Fish may be grouped into —

I. PHARYNGOBRANCHII. IV. GANOIDEI.

II. MAESIPOBEANCHII. V. TELEOSTEI.

III. ELASMOBEANCHII. VI. DIPNOI.

I. PJiaryngobranchii comprise only the Amphioseus,
II. Marsipobra/neMi comprise the Lampreys and the parasitic
Myxine.

III. Elasmobra/nohii comprise the Sharks and Rays (Plagiostom-i)

and the Chimera and its allies (Holocepliali). Their skele-
tons are cartilaginous, with an ossified crust.

IV. Ganoldei. A large number of extinct fish — of existing- fish the

Lepidost&us, or Bony Pike, is the most familiar.
V. Tcleostei comprise the ordinary Fish in our seas and rivers.
VI. Dipnoi. The Lepidosireng, or Mud Fishes, capable of living for
a long time in moist mud.

The Marsipobranchii need not detain us long ; they are

THE TEETH OF FISHES. 215

destitute of true calcified or dentinal teeth, the armature of
the mouth consisting of horny cones or serrated plates.

,The parasitic Myxine, which is found in the interior of
other larger fish, is furnished with a median curved conical
tooth, of horny consistency, which is believed to act as a
holdfast, while the serrated edges of the horny plates upon
the tongue are brought into play in boring a way into the
interior of its host.

The concave circular disc which surrounds the mouth of
the Lamprey is covered with concentrically disposed horny
teeth, of simple conical form ; in addition to these there
are lingual and palatal horny plates.

The teeth of Elasmobranchii present rather more of sim-
plicity and uniformity of plan than do those of most fish,
and it will hence be convenient to describe their teeth first,
although in most respects they stand at the head of the
class of fishes, and present many indications of affinity with
the Batrachia.

In the Plagiostomi the mouth is a transverse, more or less
curved fissure, opening upon the under surface of the head
at some little distance behind the end of the snout. Hence
it is that a shark in seizing its prey turns over upon its back
or at all events upon its side.

The jaws, which are made up of the representatives of the
palato-quadrate arch, and of Meckel's cartilage, neither true
maxillse nor premaxillae being present, are cartilaginous in
the main (although covered with a more or less ossified crust),
and therefore shrink and become much distorted in drying.
The shape of the jaws differs in the various groups of
Playiostomi, in some each of the two jaws being a tolerably
perfect semicircle, while in others they are nearly straight
and parallel to one another (see fig. 92 and fig. 96); but in all
the rounded working surface of the jaw is clothed or encased
by teeth, which are arranged in many parallel concentric
rows.

216 A MANUAL OF DENTAL ANATOMY.

The teeth, which are situated upon the edge or exposed
"border of the jaw, are usually erect, whilst the rows which
lie behind them, farther within the mouth, point backwards,
and are more or less recumbent, not having yet come into
full use.

In this respect, however, marked difference exists among
various genera of sharks ; for instance in the great tropical

FIG. 92 (l).

IN..S.

white shark the teeth which lie on the border of the jaw
are erect, and all the successive rows are quite recumbent,
whereas in many of the dog-fishes the inner surface of the
jaws forms an even rounded surface along which the rows of
teeth are disposed in every intermediate position between
those fully recumbent at the innermost part of the jaw,
and those fully erected upon its exposed borders. Only a
few of the most forward rows of teeth are exposed, a fold
or flap of mucous membrane covering in those teeth which

(1) Lower jaw of Lamna. a. Edge of flap of mucous membrane which
covers in the teeth not yet completed.

THE TEETH OP FISHES.

217

are not as yet fully calcified and firmly attached to the
gum.

In Lamna, which may be taken as fairly illustrative, the
teeth are arranged round the jaws in concentric rows with
great regularity, the teeth of the successive rows correspond-
ing in position to the teeth of older rows, and not, as is the
case in some other sharks, to their interspaces. They are

FIG. 93 f1).

attached by being embedded in a densely fibrous gum, which
closely embraces their bifurcated bases ; and this dense gum,
carrying with it the teeth, slides bodily upwards over the
inner face of the jaw, and outwards over its border, beyond

0) Transverse section of lower jaw of a Dog-fish, a. Oral epithelium.
b. Oral epithelium passing on to flap. c. Protecting flap of mucous mem-
brane (thecal fold), d. Youngest dentine pulp. e. Youngest enamel
organ. /. Tooth about to be shed. g. Calcined crust of jaw.

218 A MANUAL OF DENTAL ANATOMY.

which it, to borrow a phrase from geological science, has an
" outcrop."

In Lamna the second and third rows of teeth are only
partially erect, the rows behind these lying recumbent, and
being in the fresh state covered in by the fold of mucous
membrane, which, being dried and shrunk in the specimen
figured, falls short of its original level.

Thus rows of teeth originally developed at the base of
the jaw are carried upwards, come to occupy the foremost
position on the border of the jaw, and are cast off when
they pass the point/ in the figure. It is thus easy to under-
stand why sharks' teeth are so abundantly found in a fossil
condition, although other indications of the existence of the
fish are rare enough ; for every shark in the course of its
life casts off great numbers of teeth, which fall to the
bottom of the sea and become bedded in the deposit there
forming.

The teeth are never anchylosed to the jaw, nor have they
any direct connection with it, but, as before mentioned, are
retained by being bedded in a very tough fibrous mem-
brane ; the nature of their fixation has been more exactly
described at another page (page 202).

The sheet of fibrous gum slides bodily over the curved
surface of the jaw, continually bringing up from below
fresh rows of teeth, as was proved by Andre's specimen,
and it may be worth while to condense from Professor
Owen the description of the manner in which it was thus
proved that an actual sliding or rotation of the membrane
does really take place, and that the whole bony jaw itself
does not become slowly everted. The spine of a sting ray
had been driven through the lower jaw of a shark (Galeus),
passing between two (vertical) rows of teeth which had
not yet been brought into use ; when the specimen came
under observation the spine had remained in this situation,
transfixing the jaw, for a long time, as was evidenced by all

THE TEETH OF FISHES. 219

the teeth of these two rows, both above and below it, being
stunted and smaller than their neighbours.

Hence the development of these teeth, which ultimately
came to be at some little distance from the spine, had been
profoundly modified by its presence, and it is difficult to
understand in what manner this could have affected them
had they not, at an earlier period of their growth, lain in,
more immediate proximity to it. But if the membrane,
with the teeth attached, does move slowly along the surface
of the jaw, this difficulty at once disappears.

The forms of the teeth in various sharks are different
and characteristic ; nevertheless they vary somewhat with
age in some species, and present differences in size and
form in the upper and lower jaws, or in different parts of the
mouth of the same individual. For instance, in Lamna, in
the upper jaw, the third teeth of each horizontal row, count-
ing from the middle line, are very small, while in both jaws-
there is a gradual diminution in the size of the teeth towards
the back of the mouth.

Thus, although it is often possible to refer a particular
tooth to its right genus or even species, much care is re-
quisite in so doing.

The teeth of the bloodthirsty white shark (Carcharias)
are triangular flattened plates, rounded on their posterior
aspect, with trenchant slightly serrated edges ; it is pointed
out by Professor Owen that if the relation between the size
of the teeth and that of the body were the same in extinct
as in recent sharks, the dimensions of the teeth of the
tertiary Carcharodon would indicate the existence of sharks.
as large as whales.

The intimate relationship between the teeth and the
dermal spines, which from the standpoint of development,
has been illustrated at page 2 and page 119, is apparent also
in their histological structure. There are many dermal
spines to be met with in the sharks, which seen alone could

220 A MANUAL OF DENTAL ANATOMY.

not possibly be distinguished from teeth, the resemblance
both in outer form, in minute structure, and manner of
development being most complete. The tooth figured on
page 90 is a fair example of a structure very common among
the sharks, viz., a central body of osteo-dentine, the outer
portion of which has dentinal tubes so fine, regular, and
•closely packed as to merit the name of hard unvascular
dentine, and over this again a thin varnish of enamel. (?)

And yet no observer from its structure alone could feel
.sure whether it was a large dermal spine, or a tooth. Dental
tissues occur in other parts of the mouths of Selachia than
upon the jaws, not only in the embryonic stages, but in the
.adult. Thus Professor Turner has described (Proc. Roy.
Society, Edinburgh, 1880), very numerous comb-like ap-
pendages 5 inches long upon the branchial arches of the
Basking Shark (Selache maxima), which apparently perform
the same function as whalebone in straining the water.
These combs are formed of a variety of dentine (? osteo-
dentine), and closely resemble in structure the true teeth,
which are however very small in this shark.

In the seas of Australia there exists a Shark, the
Cestracion Philippi, with a very aberrant dentition, to which
great interest attaches, inasmuch as it is the sole surviving
representative of forms once spread all over the world. In
the front of the mouth the teeth are small and very
numerous ; they are flat plates fitted by their edges to one
another, while from their centres spring up sharp points,
soon worn off when the tooth reaches such a position upon
the jaw that it comes into use.

Proceeding backwards, the teeth cease to be pointed,
increase in size, and become fewer in each row ; a reference
to the figure will convey a better idea of their general form
than any description. Those which have come into use
are, towards the back of the mouth, always much worn ;
their shedding and renewal takes places, as in other sharks,

THE TEETH OF FISHES. 221

by a rotation of the mucous membrane over the surface
of the jaw, so that, as might have been expected, large
numbers of the isolated fossil teeth of Cestracionts are to be
met with.

The teeth of the Cestracion are fitted for the trituration
of hard substances, and for such they are used, its food

FIG. 94 !.

consisting of shell-fish, &c. The teeth consist of vaso- and
osteodentine, protected by what is apparently a structure-
less layer of enamel.

The extinct Cestracionts extended far back in time, being
met with in palaeozoic strata, and they were equally widely
distributed in space ; the size of many of the teeth also
indicates the existence of forms much larger than the recent
timid and inoffensive Cestracion Philippi. Many of the
extinct forms are known only by isolated teeth ; of others

(*) Lower jaw of Cestracion Philippi. a. Young teeth not yet in use.
b. Large grinding back teeth, c. Small pointed front teeth.

The new teeth are developed at the bottom of the series on the inner
side, and, just as in other sharks, are covered in by a flap of mucous
membrane.

222

A MANUAL OF DENTAL ANATOMY.

FIG. 95 (»).

portions of the jaw with teeth in situ have been discovered ;
thus fragments of the jaw of Acrodus,
the igoiated fossil teeth of which have
been compared to fossil leeches, with
seven teeth arranged in series, have been
met with.

The Pristis, or Saw fish, so far as the
mouth is concerned, is in no way re-
markable, its teeth being small and
blunt, like those of many rays. The
snout is, however, prolonged to an
enormous length, and is shaped like a
gigantic spatula, its thin edges being
beset by dermal spines of large size,
arranged at regular intervals, and im-
planted in distinct sockets. These
dermal spines, or rostral teeth, as they
are sometimes termed, are not shed
and replaced, but grow from persistent
pulps ; in structure they closely re-
semble the teeth of Myliobates (see
page 82), being made up of parallel
denticles, in the centre of each of
which is a pulp cavity or medullary
canal.

What use the Saw fish makes of its
armed snout is not very certainly
known, but its rostral teeth are of
interest to the odontologist for several
reasons — the one that they are dermal
spines, having a structure all but iden-

(J) Rostrum and under side of the head of a small Pristis. a. Mouth.
6, Rostrum, c. One of the rostral teeth.

The teeth, with which the margins of the jaws are covered, are so small
that they cannot be represented in this figure.

THE TEETH OF FISHES. 223

tical with that of the actual teeth of another ray, the
Myliobates ; the other that they are socketed, a manner of
implantation not at all common amongst the teeth of fishes ;
and yet another, that they grow from persistent pulps, also
unusual in fishes.

Broadly speaking, the teeth of the Kays (skates) differ
from those of typical sharks by being individually blunter,
and being more closely set so that they form something ap-
proaching to a continuous pavement over the jaws, with but
little interspace left between the teeth.

FIG. 96 (

The dentigerous surface of the jaw is very much rounded,
and in some is completely encased under a pavement of teeth.

C1) Upper and lower jaw of Myliobates. At a, the mosaic pavement
formed by the broad flattened plates which constitute its teeth is seen,
these being the oldest teeth which are about to be shed off in consequence
of the rotation of the whole sheet of mucous membrane over the surface
of the jaws. The letter 6 indicates the under surface of one of the plates,
which is seen to be finely fluted on its edge.

224 A MANUAL OF DENTAL ANATOMY.

Thus, in Myliobates, the powerful jaws are straight from side
to side, while their working surfaces from back to front are
segments of a circle. The teeth form a thick and strong
pavement over the jaws, in the manner of their formation
and renewal conforming with the teeth of other Plagiostomi ;
the severe use to which they are put being indicated by the
extent to which the grinding surfaces of those teeth which
have come into use are worn down.

Several genera have the jaws thus covered, the number
of the teeth differing ; thus Myliobates has a central series
of very broad, oblong teeth, to the outer sides of which are
three rows of small hexagonal teeth ; in (Etobatis the large
oblong central plates constitute the whole armature of the
jaw.

The structure of the teeth of Myliobates has already been
described and figured (see page 82).

The Teleostei, or osseous fish, form, the group which com-
prises all the fish most familiarly known to us, and within
its limits the variation in dentitions is so great that few, if
any, general statements can be made about them. It is not
uncommon to find teeth crowded upon every one of the
bones which form a part of the bony framework of the
mouth and pharynx, and the teeth are sometimes in count
less numbers. And so great is the variability that even
within the limits of single families differences in the teeth
are to be found.

In the common pike the mouth is crowded with sharply-
pointed teeth, having a general inclination backwards, and
being in some parts of the mouth of larger size than in
others. The margin of the lower jaw is armed with teeth of
formidable size and sharpness, the smallest teeth being at
the front, where they are arranged in several rows, and the
largest being about the middle of the side of the jaw. A
pike, as is well known to anglers, when it has seized a fish
often holds it across its mouth, piercing and retaining it by

THE TEETH OF FISHES. 225

means of these largest teeth ; then, after holding it thus for
a time, and so maimed it and lessened its power of escape, it
swallows it, generally head foremost. The tenacity of the
pike's hold is often illustrated when it takes a bait, and
retains it so firmly that when the angler " strikes " the
hooks do not get driven into the fish's mouth; but after
tugging at the bait for a time the pike releases it, and the
angler finds that it has never been hooked at all.

The margin of the upper jaw is not bordered by teeth,
save at the front, where the intermaxillary bones carry a
few teeth of insignificant dimensions ; indeed, it is rather
exceptional for the true maxillary bones to carry teeth in
osseous fish. The roof of the mouth presents three wide
parallel bands of teeth, those in the median band (on the
vomer) being directed backwards, those upon the lateral
bands (on the palatine bones) backwards and inwards.
Some of the latter teeth are very large, but not quite so
large as those at the sides of the lower jaw.

The marginal teeth are firmly anchylosed, but the teeth
upon the palate are all hinged, and in such a manner that
they can only bend exactly in one direction. Those of the
vomerine band which lie in the middle line, will bend back-
wards only ; those upon the outer margins of this band
backwards, with an inclination outwards. Those of the
lateral or palatine bands bend obliquely backwards and
inwards, about at an angle of 45 with the median line of
the mouthj or somewhat more directly backwards. To a
body sliding over them in one direction they offer no
resistance, bending down as it passes, and springing up as
the pressure is removed from them, but to anything moving"
in any other direction they are rigidly fixed sharp curved
stakes impeding its further progress.

An elongated body of some size, such as a living fish,
can only be swallowed by the pike when it is arranged
lengthwise in the mouth; crosswise it cannot possibly enter

Q

226

A MANUAL OF DENTAL ANATOMY.

the throat. The hinged teeth on the palate seem ad-
mirably arranged for getting the fish into a longitudinal

FIG. 97 (x).

•S-3T.S.

position and keeping it there ; for, if we imagine the fish's

(*) Jaws of a Pike, viewed from the front, with the month opened more
widely than is natural, so as to bring the teeth into view. a. Group of
teeth situated on the palatine bone. 6. Group of teeth situated on the
vomer. c. Group of teeth situated on the lingual bone. d. Specially
large teeth, placed at intervals round the margin of the lower jaw.
e. Group of teeth on the intermaxillary bones.

The diagram beneath represents the direction in which the hinged teeth
of the vcmerine and palatine bands can bend.

THE TEETH OF FISHES. 227

body held up against these teeth, and consider the direction
in which the hinging of the teeth allows them to yield, it
will be seen that every motion tending to arrange the body
lengthwise, either in the median line of the mouth or in
either of the interspaces between the vomerine and palatine
bands of teeth, will meet with no obstruction, but in every
deviation from this position it will be caught on the points of
the teeth and resisted. Thus with the pike's mouth shut,
and the fish kept up against the palatine teeth, even its own
struggles will be utilised by every movement tending to
place it aright being allowed, and every other stopped by the
bands of hinged teeth entangling it.

The structure of these teeth, and the mechanism by which
they are rendered elastic, have been already described
(page 206).

The lingual bone, and the three median bones behind it,
carry small teeth arranged in oblong patches ; the internal
surfaces of the branchial bones (which support the gills)
are armed with similar small teeth ; while the last or fifth
branchial arch (which carries no gills, the bones forming-
it being called inferior pharyngeal bones,) carry larger
teeth. The superior pharyngeal bones (which are median
portions of the four anterior branchial arches) also carry
recurved teeth larger than those which line the rest of the
internal surfaces of each of the branchial arches.

The pike's mouth and pharynx thus fairly bristle with
teeth, all directed somewhat backwards ; and any one who
has been unfortunate enough to have allowed his fingers
to get entangled in the mouth of a living pike will realise
how small a chance its living prey has of escape, when once
it has been seized.

The teeth of the pike are composed of a central body of
osteo-dentine, on the outside of which is a layer in which
the dentinal tubes are directed towards the surface, as in
hard or unvascular dentine ; while the outermost portion of

Q 2

228 A MANUAL OF DENTAL ANATOMY.

all is a very dense and hard, and apparently structureless,,
enamel film. The teeth are anchylosed to the bone, and arc
very frequently renewed, their successors being developed at
one side of their bases.

Though the pike has rather more teeth than many other
fish, it may be taken as a fair example of most osseous fishes;
in this respect. Space will only allow of a few of the more
exceptional forms being here described.

The angler (Lophius piscatorius), another predatory fish,
with an enormous mouth and disproportionately small body
and tail, lies hidden in the mud, or crouched upon the
bottom, and makes a rush upon smaller fishes which ap-
proach sufficiently near to it ; it is remarkable for the
manner of attachment of the teeth, some of the largest of
which upon the edges of its jaws do not become anchylosed,
but are so attached, as has been described at p. 203, as to
allow of their bending in and towards the mouth, but not in
the opposite or any other direction. The teeth of the outer
row are firmly anchylosed to the margins of the jaw, and the
far larger hinged teeth form a sort of irregular second row.

The benefit of such an arrangement to a fish of its habit
is sufficiently shown ; its teeth allow the utmost freedom of
entry, but offer obstacles to anything getting out again.

This arrangement of teeth, long supposed to be unique, is
closely paralleled in a very different fish, the Hake (Mer-
lucius, one of the Gadidce). This fish, the most active and
predatory of the Cod family, follows shoals of pilchards and
of herrings, themselves active fish, and feeds upon them. The
margins of the jaws carry two distinct and regularly ar-
ranged rows of teeth, an outer smaller row which are
anchylosed, and an inner longer row which are hinged.
They are very sharp, being tipped with spear points of
enamel, and are recurved. In the fresh state they look
quite red, being composed of a richly vascular vaso-
dentine.

THE TEETH OF FISHES.

229

Another curious dentition is possessed by the Wolf-fish
(Anarrhicas lupus), also an inhabitant of British waters,
and sometimes to be seen in London fishmongers' shops

FIG. 98

under the name of the sea cat. The intermaxillary teeth
are conical, bluntly pointed, and set forwards and outwards;
these are antagonised by somewhat similar teeth in the
front of the lower jaw. The palatine bones carry short,
bluntly conical, or round topped crushing teeth in a double

(J) Bones of the mouth of the Wolf-fish (Anarrhicas lupus). The letter
a. indicates the divergent pointed teeth which occupy the intermaxillary
bone ; the letter d. indicates the similar teeth which are attached to the
front of the mandible, on the middle and back parts of which are round -
topped crushing teeth (e). Strong crushing teeth are found also upon the
palatine bones (i), and upon the vomer (c).

230 A MANUAL OF DENTAL ANATOMY.

row ; the vomer is also armed with a double row of very
much larger and shorter teeth ; the lower jaw, with the
exception of its anterior part, is occupied by teeth of similar
character.

All the teeth of the Wolf-fish are anchylosed slightly to
the bone, a definite process from which forms a sort of short
pedestal for each tooth. The jaws are worked by muscles
of great power, and it seldom happens that a specimen is
examined in which some of the teeth are not broken. It
feeds upon shell fish, the hard coverings of which are
crushed by the blunter teeth, while the pointed front teeth
apparently serve to tear the shell fish from the rocks to
which they are commonly attached.

In the group of fish known as " Gymnodonts " (naked
toothed), the teeth and the margins of the dentigerous bones
form a sort of beak, which is not covered by the lips. The
example here figured consists of the upper and lower jaws of
the Diodon, so called because it appears to casual observators
to have but two teeth. A kindred fish in which the division
of each jaw in the middle line is conspicuous, is similarly
called Tetrodon. The jaw consists of teeth and bone very
intimately fused together; the broad rounded mass (c. in
the figure), which lies just inside the margin of the jaws, is
made up of a number of horizontal plates of dentine, the
edges of which crop out upon its posterior surface ; and
these are united to one another by the calcification of the
last remains of the pulp of each plate into a sort of osteo-
dentine, the different hardness of the two tissues keeping
the surface constantly rough, as the plates become worn away.
The whole margin of the jaw is similarly built up of smaller
horizontally disposed denticles, or plates of dentine, which
are, as they wear down, replaced by the development of
fresh plates, which are added from beneath, where they are
developed in cavities situated low down in the substance of
the bone.

THE TEETH OF FISHES. 231

The new teeth or plates of dentine thus formed at the
base of the hemispherical masses within the jaws (at the
point a), or low down in the substance of the jaw, do not

FIG. 99 •(')•

corne into use by the ordinary process of displacing their
predecessors, and being in turn themselves replaced, but
fresh plates only come into use by the actual wearing away
of all that is above them, both dentine and bone, so that
they come to be the topmost portion of the jaw. The
margins of the jaw are, however, mainly built up of dental
tissues, there being but little bone in their interspaces.

Tetrodon has not the rounded triturating disk of the
Diodon, or has it but feebly represented ; and the margins
of the jaws are sharper.

In the Parrot-fishes (Scarus), which are not very nearly
allied to the Gymnodonts, somewhat similar beaks are found,

(]) Jaws of the Diodon. a. Base of the dental plates, where new lamella?
of dentine are being developed, b. Margin of jaw, formed mainly by the
sides of the denticles, c. Compound tooth, made up of the superimposed
lamella) of dentine anchylosed together.

232 A MANUAL OF DENTAL ANATOMY.

the individual teeth being more conspicuous. The whole
outer surface of the jaw near to its working edge is covered
by a sort of tesselated pavement, formed by the several
teeth which are pressed together into a mass, but they form
only the outer surface and the immediate edge, so that the
soft bone forms a part of the working surface, or would do
so but that, by its more speedy wear, it leaves the edge,
formed by dentine and enamel, always prominent and more
or less sharp.

The structure and succession of these teeth have been care-
fully described by J. von Boas (Zeits. f. Wissen. Zool. xxxii.),
and the differences between the several genera pointed out.
He describes cementum as binding the denticles together
and forming a part of the working edge, but that which he
describes as cementum appears to me to be that tissue
which I have termed " bone of attachment." See page 208.
In a section of a jaw in my possession, which I believed
to have belonged to a Gymnodont fish but which bears a
remarkably close resemblance to that figured by von Boas
as being a jaw of Pseudoscarus, a very beautiful arrange-
ment serves to preserve the sharpness of the edge of the jaw.
The denticles are conical, and form a series of hollow
superimposed cones with the points upwards ; they consist
of dentine and enamel, and the point of the subjacent cone
fits closely up into the hollow of that above it, so closely
that in von Boas' specimen the dentine of the older tooth is
in great part absorbed to make way for the point of its
successor, so that the working denticle comes to be little
more than a hollow cone of enamel. This is not the case in
my specimen in which there is a quantity of dentine left
in each denticle. This vertical series of superimposed
sharp cones lie in the midst of the somewhat thin jaw bone,
fused together by cementum (1 bone of attachment), and
enclosed between the inner and outer plates of the jaw.
The bone being much softer than the denticle, wears

THE TEETH OF FISHES. 233

down much faster, so that the edge is always formed by a
prominent sharp tooth, which, as the wearing down of the
bone progresses, falls off, and the next one beneath it comes
into play. The arrangement recalls the way in which a
scythe or a chisel is assisted in keeping its edge by being
made of a plate of steel welded between two plates of
softer iron.

The pharyngeal bones are also remarkable ; the two lower
are united into one, and the stout bone so formed is armed
with teeth ; it is antagonised by two upper pharyngeal bones
similarly armed. It carries teeth which are anchylosed to
it, and which are so disposed as to keep the surface con-
stantly rough. When they are freshly formed the teeth

c—

have flattened thin edges, something like human incisors.
The teeth are coated with enamel, and thus, when calcifi-
cation has proceeded so far as to obliterate their central
pulp cavities, after the tooth is worn to a certain point
(c in Fig. 99) it presents a ring of enamel, inside which
comes a ring of dentine, and inside this a core of secondary
dentine, as seen in the figure. Owing to the different hard-

(l) Lower pharyngeal bone of Pseudoscarus. a. Posterior border, at
which the teeth are unworn, c. Oval areas formed by teeth, the points
of which are worn off. b. Anterior edge of bone, at which the teeth are
almost completely worn away.

234 A MANUAL OF DENTAL ANATOMY.

ness of the three tissues a constant roughness of surface is
maintained. The upper pharyngeals are similarly armed ;
and as the teeth and the supporting bone wear away, fresh
teeth are developed at the front, so that the whole bone
undergoes a sort of gliding motion backwards, the armature
of the lower pharyngeal being renewed in a similar manner,
save that new teeth and bone are developed at its posterior
instead of its anterior extremity.

The teeth are developed in bony crypts,beyond the youngest
functional teeth, and perforations in the roofs of the crypts
give passage to the connecting band between the tooth sac
and the mucous membrane.

No more fitting place will occur for noticing the stout
pharyngeal teeth whicn are met with in so many fish.
Some fish, which are edentulous so far as the mouth is
concerned, have the pharyngeal bones armed with teeth ;
in the carp and its allies, edentulous so far as the mouth
proper is concerned, the two lower pharyngeal bones carry
long pointed teeth, which partly oppose one another, and
partly oppose a sort of horny tubercle, which is supported
on a process of the base of the occipital bone.

A few fish are quite without teeth ; the sturgeon, whose
mouth forms a protrusible sucker, is edentulous, as are also
the pipe fish, and the little sea horse (Hippocampus), now so
common in aquaria.

But as a rule fish are remarkable for the great number of
their teeth, which are being constantly shed off' and replaced
by successors an indefinite number of times.

In all the fish hitherto mentioned in these pages, it
happens that the teeth in different parts of the mouth differ
in size and in the function which they have to perform ;
but this is only so because a few striking forms have been
naturally selected for description. It is far commoner for
all the teeth of fish, particularly of those fish which have
countless numbers of teeth, to be very nearly alike in form

THE TEETH OF FISHES. 235

and size in all parts of the mouth. As a general rule, fish
do not comminute their food very fully, but make use of
their teeth simply for the apprehension of prey, not sub-
mitting the food to any mastication whatever ; their teeth
are hence often mere sharp cones, slightly recurved, or set
looking backwards. Thus, though the mouth of the common
pike is beset with an immense number of sharp teeth, its
food is swallowed whole, and very often is alive when it
reaches the stomach, the sole purpose served by the teeth
being the prevention of its escape when once it has been
seized.

Implantation of the teeth in sockets is not usual in the
class of fish, but it does occur : for example the Barracuda
pike (Sphyrasna) has its lancet-shaped teeth implanted in
distinct sockets, to the walls of which they are said to
become slightly anchylosed; the file-fish and others might
also be cited. And although the succession of teeth is
usually from the side, in some cases the successional teeth
are developed in alveolar cavities within the substance of
the bone, and displace their predecessors in a vertical direc-
tion, as happens in the pharyngeal teeth of the Wrasses, or
the curiously human-looking incisors of the Sheep's-head fish
(Sargus) ; the Lepidosteus also has its teeth affixed in incom-
plete sockets, to the walls of which they are anchylosed ;
this is not a very uncommon arrangement with the teeth of
fish when they are socketed at all.

The teeth of fish are of all degrees of size and of fineness ;
in some (Chsetodonts) the teeth are as fine as hairs, and are
so soft as to be flexible.

Teeth which are very fine and very closely set are termed
"dents en velours;" when they are a little stouter, " dents
en brosse," and when still stronger and sharper, "dents en
cardes." Teeth that are conical, wedge-shaped, spheroidal,
and lamelliform, are all to be met with ; in fact there is
infinite diversity in the form of fishes' teeth.

236 A MANUAL OF DENTAL ANATOMY.

And there are some fish, e.g., some of the large Siluroid
fishes, which have very strong, large teeth, an inch and a
half or more long, and very firmly anchylosed to the bone.

It is not common for sexual differences to be met with
between the teeth of the male and female, though a slight
difference exists between the sexes in some species of Skate.
And although not strictly speaking a dental character, it
may not be out of place to mention here the peculiar
armature of the jaw of the male Salmon at the breeding
season.

The end of the lower jaw becomes produced, and turned
upwards at its point; the stout cartilaginous hook thus
formed is of such dimensions that it has to be accommo-
dated in closure of the mouth in a deep cavity formed
for it between the intermaxillary bones. In some Canadian
salmon this process is supposed to be constant in the older
males, but in the British fish it disappears, and only exists
at the breeding season. A fish in which it is strongly
developed is a foul fish, and is called a Kelt. It is used
apparently as a battering ram, and such salmon are con-
stantly found killed, with their sides deeply gashed by the
charges of their opponents.

Not much can be said in general terms of the structure
of the teeth of fish. The bulk of the teeth of most fishes
is made up of one or other modification of vasodentine or
osteodentine ; this is often glazed over upon its exterior by
a thin film of enamel, so thin as often to appear structure-
less.

Unvascular dentine also forms the teeth of many fish,
and in some is remarkable for the fineness of its tubes ; in
fact, every form of dentine, from fine-tubed hard dentine
to tissue indistinguishable from coarse bone is to be found
in this class.

Dentine of very complex structure (labyrintho-dentine)
is met with in some fish ; and an example from the Lepi-

THE TEETH OF FISHES. 237

dosteus (American garpike, a ganoid fish) has been figured at
page 78.

Enamel is often present in a very thin layer, glazing the
exterior of the dentine (see Fig. 48) ; sometimes it forms a
mere tip, a sort of spear-point to the tooth as in the Eel and
the Hake (see Figs. 87 and 90), and sometimes it is very
thick, and itself permeated by systems of tubes (see Fig. 24).

Cementum is of comparatively rare occurrence in fish.

Professor Kolliker has shown that in a very large number
of fishes the skeleton more nearly resembles dentine than
true bone in its structure ; whilst the dermal scales and
protective spines of fish are often made up of a tissue much
resembling dentine (cf. Professor Williamson, Philos. Trans.
1849). We may say, then, that just as in the external skin,
bony or dentinal plates are developed for the purpose of
protecting it from destruction by attrition, so for a similar
purpose teeth are developed in that portion of the mucous
membrane which covers the jaws.

Near the borderland between fish and amphibia is the
Lepidosiren, or Mud-fish, which is a fish rather than an
amphibian. The armature of its mouth is peculiar, the
margins of the lower jaws being formed by dental plates
anchylosed to the bone. These plates have upon their
edges five deep angular notches, the prominence of the
upper plate corresponding to the notches of the lower ;
and the edge is kept somewhat sharp by the front surface
being formed of very dense hard dentine, while the bulk
of the tooth is permeated by large medullary canals, which
render it softer. The cutting plates of the upper jaw are
developed in the median line of the palate, and there are
in front of them conical piercing teeth upon that forward
prolongation of the cartilage which takes the place of a
distinct vomer; these have sometimes been described as
being upon the nasal bone.

It would seern that the two conical piercing teeth serve

238 A MANUAL OF DENTAL ANATOMY.

as holdfasts, while the cutting edges of the deeply-notched
plates are brought into play to slice up the food.

Both in structure and general disposition the dental plates
in Lepidosiren are paralleled by the teeth of Ceratodus, for
some time known only as a fossil, but of which recent
examples have been captured near Queensland; this re-
semblance was suspected some years ago by my friend,
Mr. Moseley, of the Challenger) and has been since worked
out by other observers.

CHAPTER VII.

THE TEETH OF BATRACHIA AND REPTILES.

Ix these classes the teeth are never so numerous nor so
widely distributed upon the bones of the mouth as in fish ;
a double row of teeth arranged in concentric lines in the
upper jaw, between which a single row of teeth upon the
lower jaw passes when the mouth is closed, is an arrange-
ment rather common amongst Batrachia. Almost all
Batrachians and Reptiles have an endless succession of
teeth ; but there are a few lizards (e.g., Hatteria), in which
the manner of succession, if there be any, has not been
definitely ascertained. The outer of the two rows of teeth
in the upper jaw is situated upon the premaxillary and
maxillary bones, and usually extends further back than
the vomerine or inner row.

From this type of dentition there are many deviations ;
thus the toads are edentulous, and the frog has no teeth in
the lower jaw.

The teeth of the frog form a single row upon the margin
of the upper jaw, their points projecting but little above
the surface of the mucous membrane, and the vomerine
teeth are few in number and cover only a small space.

The edentulous lower jaw passes altogether inside the
row of upper teeth, and, itself having rounded surfaces
and no lip, fits very closely against the inner sides of the
teeth. Thus it leaves very little room for the young
developing tooth sacs, which are accommodated with the

240 A MANUAL OF DENTAL ANATOMY.

space required for the attainment of their fall size, by the
absorption of the older solid bone and the tooth which
has preceded them, in the following manner. The teeth
are attached to the bone by anchylosis, each tooth being
perched upon a little pedestal of bone which is specially
formed for it; and the successional teeth, the germs of
which originally lay at the inner sides of the old teeth,
commonly undermine the side of the pedestals and the
bases of the latter, and move bodily beneath them, so that
the new tooth completes its development in what was once
the pulp cavity of its predecessor.

The teeth of the frog consist of a body of hard dentine,
coated with an exceedingly thin layer of enamel, the exist-
ence of which has been doubted by some writers ; but a
study of the tooth sac of the animal renders it probable that
the transparent layer which is undoubtedly there is really
enamel.

The teeth of the newt and its ally the salamander are
remarkable for having tips of enamel, somewhat like those
of the eel (see Fig. 90), save that they are bifurcated, the
one point being larger and longer than the other.

The tadpole has its jaws armed with tough horny plates
something like a turtle's bill, which are shed off, prior to
the development of any true teeth; at all events I have
myself been unsuccessful in discovering any tooth germs at
the period when its horny bills are still in use.

Some extinct batrachia were of large size ; the Laby-
rinthodon, the structure of whose teeth has already been
described (page 79), was furnished with a marginal row of
teeth in the upper jaw, of which some few were of larger
size and greater length than the others. In the lower jaw,
the teeth, which are similar to those of the upper, are
disposed in some sense in an incomplete double row, the
series of smaller teeth not being interrupted by the occur-
rence of the larger tusks, but passing in unbroken series

THE TEETH OF REPTILES. 241

outside them. The Labyrinthodon was possessed also of
palatine teeth.

The teeth were anchylosed to slight depressions or sockets,
and the successional teeth were probably developed, as in
the frog, at the inner side of the bases of the teeth already in
position, as there are no indications of crypts within the bone.

In many reptiles teeth are developed for the merely tem-
porary end of effecting an exit from the egg-shell. This
purpose is sufficiently answered by the hard snout of the
•crocodiles, and by a sort of snout developed in Ckelonia, but
snakes and lizards have sharp teeth, which afterwards are
lost, developed on the preinaxillary bones (Owen).

The CHELONIA, comprising the Tortoises and Turtles, have
no teeth, but the margins of the jaws are sheathed in horny
cases, which are variously shaped in accordance with the
habit of the animal, being sharp and thin edged in carnivor-
ous, and blunt and rugged in herbivorous species.

SAURIAN reptiles (lizards, &c.), have, as a rule, rather
•simple teeth, which are confined to the margin of the jaws,
the occurrence of palatal teeth being less usual. The
teeth are of various forms, being blunt and rounded in many
genera, whilst in others they are long and pointed. They
are generally made up of a central body of hard dentine,
more or less completely invested by a cap of enamel ; and
they are attached to the bone by anchylosis.

When the tooth is anchylosed by its outer side to an
external parapet of bone, the creature is said to be " pleu-
rodont," when by the end of its base it is attached to the
summit of a parapet it is " acrodont."

The succession of teeth in the Lizards is constant, new
teeth being developed at the inner side of the bases of the
old teeth, which become undermined by absorption and fall
off when the successional tooth has attained to a certain
stage in its development.

The accompanying figure of the lower jaw of a Monitor

B

242 A MANUAL OF DENTAL ANATOMY.

lizard will give an idea of a dentition common in the
group. The teeth are not very large nor very numerous,
there being about 30 in the jaw ; towards the front of the
mouth they are a little more pointed than at the back, but
the differences in this respect are not striking.

At the inner side of the bases of the teeth are seen

FIG. 101

Nat Siz«

foramina which lead into the spaces in which new teeth are
being developed.

Amongst the lizards considerable variety in the form of
the teeth themselves exists, some having thin serrated
edges, others being exceedingly blunt and rounded, but in
the general disposition of the teeth there is considerable
uniformity.

The teeth of some lizards consist at their apices of ordinary
hard dentine, with a simple central pulp cavity, but at
their bases of plicidentine with numerous subdivisions of
the pulp cavity, as is seen in the Monitor lizards (Varanus,
see p. 77). One Mexican lizard (Helodermus), has the re-
putation of being poisonous, and has teeth which are
grooved both back and front ; but it is doubtful whether its
harmful powers have not been exaggerated.

(*) Lower jaw of a Lizard (Varanus Gouldii). a. Foramina leading to
cavities of reserve.

THE TEETH OF REPTILES. 243

Vase-dentine also occurs in the teeth of some saurians, as
for example, in those of the great extinct Iguanodon. in
which it, roughly speaking, formed the inner half of the
crown, the outer moiety consisting of hard dentine. In
addition to this peculiarity, the teeth of Iguanodon were
remarkable for the partial distribution of the enamel, which
was strongly ridged, the ridges being serrated, and was con-
fined to the outer side of the crown. Thus at the outside
came the hardest tissue, the enamel; next the harder dentine
and on the inside, the softer vase-dentine. Hence, as the
tooth wore down, a sharp edge was long preserved.

There is a New Zealand lizard, to which the several names
of Hatteria, Sphenodon, and Rhyncocephalus have been given,
which has a very peculiar dental armature (Dr. Giinther,
Phil. Trans., 1867).

The inter-maxillary bones are armed with two teeth, so
large as to be co-extensive with the whole bone in width,
and of a form which recalls that of the gnawing incisors
of Rodents ; the other teeth are quite small, and " acrodont "
in their attachment.

But the great peculiarity of Hatteria is that the alveolar
margins of the jaws are sharp, and when the teeth are worn
down, which would happen in adult specimens, the actual
sharp margins of the bone come into play as masticatory
organs, near to the front of the mouth. It occurred to me
as probable that the surface thus exposed might be coated
with dentine, but a microscopic examination of one of the
specimens in the British Museum, which I was, by the
kindness of Dr. Giinther, enabled to make, proved that the
dense ivory-like surface which serves the purposes of masti-
cation is true bone, and has no relation to dental structure.

There are very few other instances of actual bone, un-
coated by dental tissues, being used for masticatory purposes.

The great extinct Dicynodon, an African fossil, also
had sharp trenchant margins to its jaws ; it is not

R 2

244 A MANUAL OF DENTAL ANATOMY.

known whether these were sheathed in horny cases like
those of the turtles, or whether the bones themselves came
into use, as in Hatteria. But the most striking peculiarity
of Dicynodon was the co-existence with such jaws of a pair
of very large -caniniform tusks, extending downwards and
forwards from the upper jaw, and growing from persistent
pulps, a thing altogether exceptional in the reptilian class.

The dentition of Ophidian reptiles (snakes) is very uni-
form; they maybe conveniently divided into two groups, the
poisonous and the non-venomous snakes.

Non-venomous snakes have one row of teeth in the lower
jaw, and two rows in the upper jaw; in the latter the
maxillary bones carry one row, while a parallel internal row
is supported upon the palatine and pterygoid bones.

The teeth are in both groups strongly recurved, and are
firmly anchylosed to the bone ; they consist of a central
body of unvascular dentine, coated by a very thin layer of
enamel (there is not, as is generally supposed, any layer of
cementum, the enamel having been erroneously supposed to
be such).

The two halves of the lower jaw are connected at the
symphysis by an exceedingly elastic ligament ; their articu-
lation with the base of the skull through the medium of an
elongated movable quadrate bone, is also such as to allow
of their being widely separated from the skull and from one
another, which allows of the dilatation rendered necessary
by the large size of the creatures which a snake swallows
whole.

The teeth of the snake are simply available for seizing-
prey and retaining it, as the snakes invariably swallow their
prey whole, and in no sense masticate it.

As the object to be swallowed is often so disproportionately
large as to make the process of deglutition appear an im-
possibility, the mouth and pharynx have to undergo great
dilation. The arrangements which combine to give to the

THE TEETH OF REPTILES.

245

lower jaw its mobility have just been alluded to ; the
successional tooth germs, which are very numerous, are
also arranged in the snake in an. unusual position, which by

bringing them very close to the surface of the bone, to
which they lie parallel, renders them less liable to displace-
ment and injury than they would have been had they been
placed vertically, as they are in all other creatures ; while
in addition to the advantage of protection by position, they
are wrapped round by a sort of adventitious capsule of con-
nective tissue.

(!) Developing teeth of a Snake. /. Oral epithelium, e. Neck of the
enamel organs. 6. Dentine pulp. c. Enamel cells, d. Dentine. 1, 2.
Very young germs. 3, 4. Older germs.

246

A MANUAL OF DENTAL ANATOMY.

As the teeth during their development are thus lying down
parallel with the length of the jaw-bone, when the period
for their replacing a predecessor arrives, they have not only
to move upwards, but also to become erected ; how this
is done remains a mystery, for I have been quite unable to
discern the means by which it is accomplished.

When a snake has seized its food, which it retains by
means of its many sharp recurved teeth, it slowly swallows

FIG. 103 (M.

it by advancing first its lower, then its upper jaw, till it
thus, so to speak, forces itself over the body of its prey.
When this latter is large, deglutition is a very lengthy

(J) One half of the skull of a Python (without the lower jaw) seen from
below, a. Intermaxillary bone. 6. Maxillary bone, carrying the outer
row of teeth, c. d. Palatine bone and pterygoicl bone, the teeth upon
which constitute the inner or second row of teeth.

THE TEETH OF REPTILES. 247

process, but an English snake can swallow a moderate-sized
frog with considerable rapidity.

There is an African snake (Rachiodon) which has none but
rudimentary teeth; its food consists of eggs, which thus
escape breakage until they reach the oesophagus, into which
spinous processes from the under surface of the vertebrae
project, and there serve to break the egg; snakes with
their dentitions similarly modified exist also in India (e.g.,
Elachistodon).

It has already been mentioned that the non-venomous

FIG. 104 !.

snakes have two complete rows of teeth in the upper jaw,
the outer row being situated on the maxillary bones, the
inner upon the palatine and pterygoid bones. The teeth of
such snakes as the Pythons are all simple recurved cones,
and^, are none of them either grooved or canaliculated.2

0) Head and jaws of Hydrophis. The maxillary bone (6), instead of

•carrying a complete series of teeth, is armed with a few teeth only near to

the front. The foremost tooth is canaliculated, and forms the poison fang.

(2) It has been proposed to divide the Ophidia into groups, distinguished

*by the presence or absence of grooved teeth, thus : —

i. Agfypkodontia. No grooved or canaliculated maxillary teeth,
ii. Opisthoglyphia. Some of the posterior maxillary teeth grooved,
iii. Protcroylypliia. Anterior maxillary teeth grooved.

Posterior maxillary teeth solid.

iv. Solenoglyphia. Maxillary teeth few, canaliculated — poisonous
snakes.

248 A MANUAL OF DENTAL ANATOMY.

Some of the harmless snakes, however, have particular
teeth which are developed to a greater length than the rest,
and others have the posterior teeth on the maxillary bones
grooved ; but the statement that this grooving serves to
convey an acrid saliva into the wound inflicted rests on in-
sufficient foundation. The poisonous snakes are charac-
terized by a shortening of the series of teeth carried upon
maxillary bone, and by the front tooth of the series being-
developed to much greater length than those which lie
behind it. Thus Hydrophis, a genus of poisonous sea-snakes,
has five or more teeth upon the maxillary bone, the fore-
most of which is much the largest, and this largest tooth is
so deeply grooved upon its anterior surface as to be converted
into a tube, the tube serving to convey the poison into the
wounds inflicted by it.

Poisonous snakes which have several teeth upon the
maxillary bone for the most part present some little external
resemblance to the harmless snakes, and are called "colu-
brine poisonous snakes " (coluber being the name of a genus
of harmless snakes) ; they present transitional characters
between these and the more specialised or "viperine" poison-
ous snakes. The Cobra is a familiar example of a colubrine
poisonous snake, and almost all the venomous snakes of
Australia belong to this group. Their poison fangs are not
very long, and they remain constantly erect, being anchy-
losed to the bone (the maxilla) which is long and not move-
able, and which also carries a varying number of small
insignificant teeth behind the poison fang.

In the viperine poisonous snakes (Puff-Adder, Rattle-
snake, Vipers, &c.,) the poison apparatus is yet more
specialised. The maxillary bone carries no teeth at all
behind the poison fang ; it is so reduced in length as to be
of squarish form, and is so articulated to the skull as to be
movable.

The poison fang is of great length, so that if constantly

THE TEETH OF REPTILES.

249

erect it would be much in the way ; when it is out of use,
however, it is laid flat along the roof of the mouth, and is
only erected for the purpose of striking j when in repose it
is altogether hidden by a fold of mucous membrane, which,
when it is erected, becomes tightly stretched over a part of
its anterior surface, and serves to direct the poison down
the poison canal by, to a great extent, preventing its escape
around the exterior of the tooth.

The mechanism by which the poison fang is erected is
thus described by Professor Huxley (Anatomy of Verte-

FIG. 105 (*).

brated Animals, p. 241) : — " When the mouth is shut the
axis of the quadrate bone is inclined downwards and back-
wards. The pterygoid, thrown back as far as it can go,
straightens the pterygo-palatine joint, and causes the axis
of the palatine and pterygoid bones to coincide. The trans-
verse, also carried back by the pterygoid, similarly pulls the
posterior part of the maxilla and causes its proper palatine
face, to which the great channeled poison fangs are attached,

(!) Side and front view of the skuli of Craspedocephalus melas. A
bristle is passed down the poison canal. MX. Maxillary bones. Mn.
Mandible. PL Palatine bones. Pt. Pterygoid bones. Q,u. Quadrate
bone. T. Transverse bone.

A. Side view. B. Front view.

250 A MANUAL OF DENTAL ANATOMY.

to look backwards. Hence these fangs lie along the roof of
the mouth, concealed between folds of the mucous mem-
brane. But when the animal opens its mouth for the
purpose of striking its prey, the digastric muscles, pulling
up the angle of the mandible, at the same time thrust the
distal end of the quadrate bone forwards. This necessitates
the pushing forward of the pterygoid, the result of which
is twofold : firstly, the bending of the pterygo-palatine joint;
secondly, the partial rotation of the maxillary upon its
lachrymal joint, the hidden edge of the maxillary being
thrust downwards and forwards.

"In virtue of this rotation of the maxillary through about
a quarter of a circle, the dentigerous face of the maxilla
looks downwards and the fangs are erected into a vertical
position. The snake ' strikes ' by the simultaneous contrac-
tion of the crotaphite muscle, part of which extends over
the poison gland, the poison is injected into the wound
through the canal of the fang, and this being withdrawn,
the mouth is shut, all the previous movements reversed,
and the parts return to their first position."

The poison fang is a long, pointed, slightly recurved tooth,
traversed by a canal which commences on its front surface,
near to the bone, and terminates also on its front surface, a
little distance short of its point ; in the figure a bristle has
been passed through it, and shows the points where it com-
mences and terminates. This tube conveys the poison into
the puncture, its upper orifice being in close relation with
the end of the duct of the poison gland.

It has been mentioned that some snakes which have not
definite poison fangs have a few of the large posterior teeth
grooved upon their front surfaces, the object of this grooving
being, as a matter of conjecture, to convey a more or less
poisonous saliva into the wounds inflicted by them.

By imagining such an anterior groove to be deepened,
and finally converted into a canal by its edges growing up

THE TEETH OF REPTILES. 251

and meeting over it, we shall have a fair conception of the
nature of the tube in a poison fang, which is thus really
outside the tooth ; which might thus, as least in its canali-
culated part, be regarded as a thin flattened tooth bent round
so as to form a tube. Just as there are gradations in the
armature of the maxillary bone, which link together the
extreme form of the harmless Python, and the venomous
Rattlesnake, so there are gradations in the form of the
poison tooth, in the degree in which the groove is converted
into a canal.

In colubrine poisonous snakes the canal is visible on the

exterior of the tooth, where an apparent fissure marks the
point where the two lips of the groove have met. Thus the
poison fang of Hydrophis, although in a part of its length

(') Transverse section of tooth-sac of poison fang of Viper, prior to the
complete closure of the poison tube by the meeting together of the two
cornua of the dentine.

252

A MANUAL OF DENTAL ANATOMY.

the canal is quite closed in, has a very marked line along
its front, and in section it looks much as would the dentine
in Fig. 106, if the two cornuahad their rounded extremities
brought together into actual contact, without, however,
their rounded outline being altered.

But in the poison fang of a viperine snake the lips of the
groove are flattened and fitted to one another, so that not a
vestige of the join can be seen upon the smooth exterior of
the tooth. In the accompanying figure the pulp cavity is
seen to be a thin flattened chamber partly surrounding the
tube formed for the conveyance of the poison.

The poison-fang is exceedingly sharp, its point being con-
tinued some little distance beyond the place where the
poison canal opens on the front of the tooth ; this disposi-
tion of parts has been copied in the points of syringes for
making subcutaneous injections.

(*) Transverse section of the poison fang of a Rattlesnake, a. Pulp-
cavity, d. Dentine.

THE TEETH OF REPTILES. 253

The dentine is continued down to a very fine point, and
it is cased by an exceedingly thin layer of enamel, not
much more than ^-^ of an inch in thickness in our common
English viper : thus the utmost sharpness is secured,
without loss of elasticity, which would have ensued had
its point been made up of brittle enamel only. Enamel
covers the whole exterior of the tooth but does not extend
into the poison canal in the viperine snakes ; in Hydrophis
I believe that it does. As the point is simple, the tooth
germ of a poison-fang only becomes distinguishable from
that of another ophidian tooth after the tip of the tooth has
been formed, when a groove appears in its side (see 8 and 9,
in Fig. 108).

It being the habit of poisonous snakes to make use of
these weapons to kill their prey, which they consequently
do not swallow alive, it would obviously subject them to
no little inconvenience to be without these weapons for
any considerable length of time, while from their habit of
striking living prey the long fangs must be very liable to
being broken off by the jumping away of the creature struck,
to say nothing of the great force with which the blow is
given.

In the most typical (viperine) poisonous snakes the
succession of teeth is conducted upon a plan which is
unique, and which is excellently adapted to save loss of
time in the replacement of a lost poison fang. Upon the
movable maxillary bones there is space enough for two
poison fangs, side by side ; only one, however, is fully
anchylosed to the bone at a time, and occupies a place to
the extreme right or extreme left of the bone, leaving vacant
space for another by its side.

When the tooth in use falls, it will be succeeded by a
tooth upon the vacant spot by its side, not upon the spot
upon which itself stood, so that the places on the right
and the left of the bone are occupied alternately by the tooth

254

A MANUAL OF DENTAL ANATOMY.

in use. Thus, in Fig/ 105, the poison [fang of the snake's
right side is seen occupying a position on the extreme out-
side of the maxillary bone, while its left poison fang is fixed
on the inside of the maxillary bone.

The upper boundary of Fig. 108 is formed by the flap
of mucous membrane which covers in the poison fang when
at rest. Nos. 1 and 2 lie in the pouch formed by it, the

FIG. 108

iD

section happening to be taken from a specimen in which the
tooth was about to be changed. In most specimens one
tooth only, the tooth actually in use, is seen in this position.
A flap hanging free across this space serves apparently to

(l) Transverse section of the reserve poison fangs of a Viper. 1. Tooth
at present in use, in its recumbent position ; were it erect, it would be
withdraAvn from view, or else seen in longitudinal section. 2. Tooth which
will next succeed to No. 1. 3, 4, 5, &c. Tooth-sacs numbered in the order
in which they will succeed.

THE TEETH OF REPTILES. 255

keep teeth of the one series from getting over to the other
side, and probably serves to hold in place the reserve tooth
when the older tooth is erected for biting.

The reserve poison fangs, as many as ten in number in
the Rattlesnake, are likewise arranged in two parallel series,
in which the teeth exist in pairs of almost equal age ; the
tooth in use is thus derived alternately from the one and
the other series, as is indicated by the consecutive numbers
in the figure, a septum of connective tissue keeping the two
series of teeth distinct from one another.

The teeth being arranged in pairs of almost equal age,
suggest that the succession is both rapid and regular. All
the reserve teeth lie recumbent in and behind the sheath
of mucous membrane which covers in the functional tooth.

This arrangement of the successional teeth in a paired
series does not exist in the Cobra, in which the successional
teeth form but a single series ; perhaps this may serve to
explain the preference of the snake charmers for the Cobra,
which would probably take longer to replace a removed
poison fang than a viperine snake would.

But in the colubrine venomous snakes the successional
poison fang sometimes makes its way to a spot a little to the
side of its predecessor, so that there may possibly be no loss
of time : and notwithstanding that they are in a measure
transitional forms between the harmless and the viperine
snakes, some of them are most virulently poisonous and
deadly in their bite Q.

This arrangement of tivo distinct chains of younger de-
veloping organs, all destined to keep the creature always
supplied with one organ in a state of efficiency, is, so far as
I know, without parallel.

Like other ophidian teeth the poison fangs become an-

(*) I have given a more detailed account of the succession of poison fangs
in the Philos. Trans., 1876, Part i.

256

A MANUAL OF DENTAL ANATOMY.

chylosed to the bone which carries them, their secure
fixation being aided by the base of the tooth being fluted, as
well as by a sort of buttress work of new bone being thrown
out to secure each new poison fang as it comes into place.

The poison is secreted by a salivary gland homologous
with the parotid ; by an especial arrangement of the muscles
and fascia about it the erection of the poison fang and the
infliction of the bite cause a copious stream of poison to be

FIG. 109 1.

ejected. The duct terminates in a sort of papilla, close to
the superior orifice of the tube in the fang : the passage of
a considerable portion of the poison down the tube is secured
by the close apposition of a shield of mucous membrane,
which is strained over the erected tooth.

In Crocodilia the teeth are confined to the margins of

(*) Jaws of the Crocodile. The first, fourth, and eleventh teeth in the
lower jaw, and the third and ninth in the upper, are seen to attain to a
larger size than the others.

THE TEETH OF REPTILES. 257

the jaws, where they are very formidable in size and sharp-
ness. The individual teeth are generally conical, sharply
pointed, and often a little compressed from side to side, so
as to possess sharp edges ; but they vary much in form in
different species.

The teeth are lodged in distinct tubular alveolar cavities,
to the walls of which they do not become anchylosed, and
they are tolerably constant in number in the same species.

In parts of the mouth certain teeth are developed to a
greater length than those nearest to them ; thus, in the
Crocodile proper, the first and fourth lower teeth are spe-

FlG. 110 (»).

cially large, while in the extinct African Galesaurus the
difference is so marked that both in the upper and lower

(J) Transverse section of the lower jaw of a young Alligator, a. Oral
epithelium. 6. Bone of socket, d. Dentine of old tooth. 2. Tooth next
in order of succession, which is causing absorption of one side of the base
of the older tooth. 3. Young tooth germ.

258 A MANUAL OF DENTAL ANATOMY.

jaws the teeth might be grouped as incisors and canines,
so far as size and probable function go in such a classification.

In structure the teeth of crocodiles consist of hard, fine
tubed dentine, with an investing cap of enamel, and in
addition a coating of cementum on their implanted por-
tions. As already mentioned, they are implanted in tubu-
lar sockets ; new successional teeth are being continually
developed at the inner side of their bases, and as these at-
tain to a certain size, absorption attacks the base of the
older tooth, and its successor moves into the space so gained?
so that it comes to be situated vertically beneath the older
tooth. In its further growth it causes yet more absorption
of the older tooth, which it ultimately pushes out in front
of it, sometimes carrying the remains of the old tooth like
a cap upon its own apex when it first emerges. Each new
tooth vertically succeeds its predecessor ; hence no additional
teeth are added, but the young newly hatched crocodile has
as many teeth as a full grown one.

In the extinct Ichthyosaurus the teeth, while forming an
armature not unlike that of some of the crocodiles, were
not implanted in distinct sockets, but were lodged in a
continuous shallow groove, with but slight indications of
transverse divisions.

The huge Dinosauria, some of which must have been
thirty feet in length, had teeth implanted in imperfect
sockets, the outer alveolar wall being considerably higher than
the inner, and the transverse septa not very complete. The
roots of the teeth were more or less perfectly cylindrical,
and the enamelled crowns compressed and expanded, with
trenchant edges. The tooth of the Iguanodon will serve as a
fair example of a Dinosaurian tooth : the crown is greatly ex-
panded, and presents anterior and posterior sharp notched
margins ; the enamel is laid over the outer surface of upper
teeth, and the inner of lower teeth. The enamelled surface
is ridged, so that as it wears down a notched edge is main-

THE TEETH OF REPTILES. 259

tained. Moreover the maintenance of a sharp edge is further
secured by the dentine on the enamelled side of the crown
being of the hard unvascular variety, that on the inner being
vaso-dentine and therefore softer. The remnant of the pulp
ossifies, and comes into use, as these teeth remained at work
until worn quite to a flat surface. The root portion was
smooth, round, and curved.

Professor Marsh (American Journal of Science, March,
1880) has described and figured a peculiar Dinosaurian den-
tition, in a reptile to which he gives the name of Stegosaurus ;
the teeth are slightly compressed transversely, and are
covered with a thin enamel ; the roots are long and slender,
implanted w^eakly in separate sockets. But at the inner side
of the roots of the teeth in use were no less than five suc-
cessional teeth, in graduated stages of development, ready
to ultimately take its place ; so large a number of successional
teeth has not hitherto been met with in a Dinosaur.

A very remarkable carnivorous reptile as large as a lion
has been described by Professor Owen (Quart. Journal
Geolog. Society, 1876,) under the name of Cynodraco major
for the reception of which he proposes a new reptilian order,
that of Theriodontia. Its dentition is not completely known,
but it possessed in the lower jaw eight incisors, of which the
first is the smallest, and a canine of moderate size. The
upper incisors are not known, but there were a pair of upper
canines of such size that they extended down along the
outside of flattened portion of the lower jaw, like the
canine teeth of Machairodus. The hinder margins of these
canines were trenchant, and finely serrated.

The Fterosauria, or flying reptiles, have, since the
discovery of toothed birds, become of special interest to
the odoiitologist. The wings were stretched membranes,
like those of a bat, and the measurement across their tips
in some of the largest must have been twenty-five
feet ; but most of those known were much smaller, from

s 2

260 A MANUAL OF DENTAL ANATOMY.

10 to 15 inches in total length of body. In the Pterodactyls
the jaws are furnished with long, slender, sharp teeth in
their whole length : but in Ramphorhynchus the anterior
extremities of the jaws are without teeth, and it has been con-
jectured that these portions were sheathed in horny beaks.

And Prof. Marsh (American Journal of Science, 1876,) has
discovered, in the same formation in which he found the
toothed birds, several species of Pterodactyls wholly without
teeth, for which the generic name Pteranodon is proposed.

The jaws, which are more like those of birds than those of
any known reptile, show no traces of teeth, and the pre-
maxillaries seem to have been encased in a horny covering.

THE TEETH OF BIRDS.

Prior to the discovery by Professor Marsh of Yale College,
in 1870, of the remains of birds with teeth in the cretaceous
formations of Western Kansas, little was with certainty
known about the existence of teeth in any bird, although
one or two fossils, leading to the suspicion that birds might
have possessed teeth, were known. The state of knowledge
up to that time has been clearly summarised by Mr. Wood-
ward (Popular Science Review, 1875,) to this effect : that
it had been long supposed that no examples of teeth were to
be met with amongst the birds, although some, such as the
Merganser, have the margins of the bill serrated, so that the
functions of teeth are discharged by this horny armature of
the jaws.

It is noteworthy that the margin of the bone of the jaws
is also serrated, each serration corresponding to a similar
serration in the bill. In the fossil bird described by Pro-
fessor Owen, from the London clay, under the name of
Odontopteryx toliapicus, the form of the bill is not known,
but the margins of the jaws are furnished with strong bony
prominences, far more conspicuous than those of the

THE TEETH OF BIRDS. 261

Merganser. And Geoffrey St. Hilaire had described a series
of vascular pulps as existing on the margin of the jaw of
parroquets just about to be hatched, which, though destined
to form a horny bill, and not to be calcined into teeth, yet
strikingly recalled dental pulps. Then there is also the
famous fossil Archseopteryx, an anomalous oolitic bird, with
a long and jointed tail, which is by many zoologists believed
to have possessed teeth. There is a flaw in the evidence,
however, inasmuch as the toothed jaw is not in situ, and
therefore may possibly have belonged to some other animal
than that perpetuated in the rest of the fossil impression,
though probability is altogether in favour of its really
belonging to the Archseopteryx.

In successive expeditions, conducted under great difficulties
owing to the extremes of heat and cold, and to the hostility
of the Indians, the remains of no less than one hundred
and fifty different individuals referable to the sub-class
ODONTORNITHES have been obtained by Prof. Marsh ; they
are classified under nine genera, and twenty species.

They are referable to two widely different types, one
group consisting of comparatively small birds, with great
power of flight, and having their teeth implanted in distinct
sockets (Odontotorna}, illustrated by the genus Ichthyornis
as a type) ; the other group consisting of very large swimming
birds, without wings, and having teeth in grooves (Odontolcae,
type genus Hesperornis).

In Ichthyornis the teeth were about twenty-one in number
in each ramus, all sharp and pointed, and recurved ; the
crowns were coated with enamel, and the front and back
edges sharp but not serrated.

They are implanted in distinct though shallow sockets,
and the maxillary teeth are a little larger than those
opposing them ; the premaxillaries were probably edentulous,
and perhaps covered with a horny bill.

In the lower jaw the largest teeth occur about the middle

262

A MANUAL OF DENTAL ANATOMY.

of the ramus, those at its posterior end being materially
smaller; and the sockets are deeper
and stronger than in the upper jaw.
The succession takes place vertically,
as in Crocodiles and Dinosaurs.

The genus Hesperornis, probably
diving birds, includes species 6 feet in
length : as has already been mentioned
the teeth are not implanted in distinct
sockets, but lie in a continuous groove
like those of Ichthyosaurus ; slight pro-
jections from the lateral walls indicate
a partitioning off into sockets, but
nothing more than this is attained, and
after the perishing of the soft parts the
teeth were easily displaced, and had
often fallen out of the jaws. The pre-
maxillary is edentulous, but the teeth
extend quite to the anterior extremity
of the lower jaw : in one specimen
there are fourteen sockets in the max-
illary bone, and thirty-three in the
corresponding lower ramus.

The successional tooth germs were
formed at the side of the base of the
old ones, and causing absorption of the
old roots, migrated into the excavations
so formed, grew large, and ultimately expelled their prede-
cessors, as is seen in the accompanying figure.

In structure these teeth consist of hard dentine, invested
with a rather thin layer of enamel, and having a large axial pulp
cavity. The basal portion of the roots consists of osteodentine.

(]) Mandible of Ichthyornis (after Prof. Marsh). A. Side view, show-
ing the teeth in situ. B. View of upper surface, showing the sockets in
which the teeth were implanted.

THE TEETH OF BIRDS.

The outer side of the crown is nearly flat, the inner strongly
convex : the junction of these surfaces is marked by a sharp
ridge, not serrated.

In form the teeth of Hesperornis present a close resem-
blance to those of Mosasaurus, a great extinct lizard.

Indeed, as Prof. Marsh observes, "in all their main

features the teeth of Hesperornis are essentially reptilian,
and no anatomist would hesitate to refer them to that class,
had they been found alone. Combined with the other
reptilian characters of Hesperornis .... they clearly
indicate a genetic connection with that group."

In the dentine contour lines are abundant ; the enamel
is so dense as to appear structureless, and there is no coronal
cementum.

The foregoing account is condensed from the magnificent
volume published by the United States Government Geo-
logical Exploration. (Odontornithes, a monograph, &c., by
0. C. Marsh, Prof, of Palaeontology, Yale College.)

With these notable exceptions, the jaws of all known birds
are toothless, the horny cases forming their beaks taking the
places and fulfilling the functions of teeth.

(*) (After Prof Marsh. ) A. Hesperornis regalis, with successional tooth
in an excavation at its base ; enlarged eight diameters. B. Tooth of Mos-
asaurus princeps, half natural size.

CHAPTER VIII.

THE TEETH OF MAMMALS.

THE class Mammalia is divided into three groups : —

I. Ornitliodelpliia.

Animals with a common genito-urinary chamber, and separate
coracoid bones ; no vagina ; no teats ; comprises a single order,
Monotremata, which contains only two genera, the Ornithorhynchus
and the Echidna.

II. DidclpMa.

Animals with a vagina, &c. ; of which the young are born in an
exceedingly early condition, probably without the formation of any
placenta, and are transferred to the nipple of the mother, where,
in almost all, they are protected by a fold of the abdominal integu-
ment, which forms the marsupium, or pouch ; comprises the single
order Mwsupialia, animals now most largely represented in Aus-
tralia and its zoological region ; some few exist also in America.

The kangaroos, wombats, opossums, &c., are familiar examples
of Marsupials.

III. MonodelpMa.

Placental mammals : i.e., animals in which the foetus acquires a
connection with the parent through the medium of a vascular pla-
centa, by means of which it is nourished for a long time, and is
ultimately born in an advanced condition.

The relations which the different orders of placental Mammalia
bear to one another are rather complex, and it is not possible to
place them satisfactorily in a consecutive series, because many of
the orders present affinities with, and are indeed linked by transi-
tional forms to, not one. but several other orders. Professor Flower
(Osteology, page 6,) has arranged them in the following tabular
manner, each order being placed near to those to which it presents
most resemblance.

THE TEETH OF MAMMALS. 265

Relation of the existing Mammalian orders to one
another : —

Hominina.

PRIMATES.

Simiina.
Lemurina.
CHIROPTERA.

INSECTIVORA. CARNIVORA.

Fissipedia, Pinnipedia.

HYRACOIDA. CETACEA.

RODENTIA. SlRENIA.

DINOCERATA (?) Perissodactyla.

PROBOSCIDEA. UNGULATA.

Suina. Tylopoda.

Artiodactyla.
Tragnlina. Pccora.

EDENTATA.

Primates include man and the monkeys, the Lemurs connecting
them with both the Insectivora and Chiroptera.

Chiroptera — Bats.

Insectivora — Moles, Hedgehogs. &c.

Carnivora fissipedia — Cats, Dogs, and Bears, &c.

Carnivora pinnlpcdi a — Seals, Walrus, &c.

Cetacca — Whalebone Whales, Sperm Whales, Porpoises, &c.

Sirenm (Herbivorous Cctacea) — Manatee, Dugong, &c.

Ungulata (Hoofed Mammals) —

(i.) Perissodactyla, or odd-toed — Horse.

Tapir, Rhinoceros, &c.

(ii.) Artiodactyla, or those with an even number of toes — Pigs
and their allies, Camels, Ruminants, &c.

Dinoccrata — Gigantic Fossil Mammals, somewhat intermediate be-
tween Perissodactyl Ungulata, and Proboscidea.

llyracoidca — The anomalous Hyrax (Biblical " Cony ") alone.

Proboscidea — Elephants, extinct Mastodons, &c.

liodent'la — Hares, Rabbits. Rats, &c.

Edentata — Sloths, Armadillos, Ant-Eaters, &c.

To illustrate the meaning of the table, the gap existing between
the typical Camivora and the Cetacea is bridged over by the seals,
which, though true carnivora, are some of them near the Cetacea

266 A MANUAL OF DENTAL ANATOMY.

in many particulars ; or, again, the gap between the Monkeys and
the Insectivora is bridged over by the Lemurs, which are inter-
mediate forms.

We do not yet know enough of extinct Mammalia to feel quite
sure of the true line of affinities between all the orders, but the
foregoing table serves to give a more true idea of our present know-
ledge than any arrangement in linear series can convey. There is
no animal to which we can point and say that we know its whole
line of descent ; but the ancestry of some of the Ungulata has been
greatly elucidated of late years, and the chain of progressive modi-
fication by which so highly specialised a form as the Horse has been
arrived at, starting from a very much more generalised form, is now
pretty complete.

In a treatise dealing only with the teeth, in which the orders
must necessarily be taken in succession, it will be convenient to
deviate somewhat from the natural order for the sake of taking
first those animals whose dentitions are of the simplest character.
Thus it is convenient to describe in succession the Edentata and
the Cetacea, which have little or nothing to do with one another,
because they alike have teeth of simpler form than the rest of the
Mammalia. But, as far as possible, the arrangement indicated in
the table, which the student will do well to impress upon his mind,

will be followed in these pages.

•

INTRODUCTORY REMARKS.

Not many years ago it was customary to explain the
various facts which were revealed by the study of com-
parative anatomy upon the supposition that there was some
sort of type or standard organization, and that all others
were arrived at by modifications and departures from this
type, these modifications being introduced with a direct
purpose in view, in order to fit the creature to a special
habit of life.

Among the matters which this " type " theory sought to
account for was this : when an animal possesses some pecu-
liar organ, it is found on close examination that it, however
specialised, is after all only something which allied animals
also possess, only it has been exaggerated or developed in an
unusual manner and degree ; or, on the other hand, that
when an organ is wanting, the suppressed organ is not
absolutely abolished, but is to be found stunted and in a

THE TEETH OF MAMMALS. 267

rudimentary condition, instead of in its ordinary size and
functional activity.

This is as true of teeth as of any other organs ; indeed
the study of odontology reveals many admirable examples
of the law.

Thus the tusks of the boar or of the Sus babirussa, large
and peculiar though they be, are not new developments, but
are merely the canine teeth which in these species attain to
unusual dimensions. In the same way the enormous
straight tusk of the Narwal (see Fig. 133) is nothing more
than an incisor tooth of one side, the fellow to which has
been checked in its development ; but this is not missing, for
it remains throughout the life of the animal buried within
its socket. In the female Narwal both of the teeth, being
rudimentary, are permanently enclosed within the sockets,
and are of course not of the smallest service to the animal,
directly or indirectly ; furthermore, as has been shown by
Professor Turner, in young specimens, a second pair of
rudimentary aborted incisors are to be found, which in the
adults have disappeared.

The modern school of biologists, rejecting this "arche-
type " theory as a far-fetched and unsatisfactory hypothesis,
refer these resemblances detected between dentitions upon the
whole dissimilar to one another to a more intelligible cause,
namely, inheritance. Assuming, as the balance of evidence
compels us to assume, that the many divergent forms which
we observe have been derived by progressive modifications
and differentiations from fewer ancestral forms, we shall have
no difficulty in seeing how, by such processes as we full well
know to occur, namely, the dwindling of disused organs and
the exaggerated development of those used in an unusual
degree, great differences may ultimately result.

To illustrate what is meant by this so-called " adaptive
modification," this suppression of things that are not
needed, and increased development of those most used, we

268 A MANUAL OF DENTAL ANATOMY.

may recur to the dentitions of non-venomous and venomous
snakes.

In these we saw, in the non-venomous snakes, the max-
illary bones covered by a row of teeth sub-equal in size ;
then in the ' Colubrine ' poisonous snakes the front tooth of
those standing upon the maxillary bone taking upon itself a
special and important office, namely, the conveyance into a
wound of a poisonous saliva, and coincidently with this
tooth attaining its increased size and importance, the teeth
behind it on the maxillary bone reduced both in number
and in size. Going a step further, to the Viperine poisonous
snakes, the now useless small maxillary teeth have all disap-
peared, leaving the poison fang alone, and of vastly increased
dimensions, to occupy the whole bone.

But in many poisonous colubrine snakes three or four
small and useless teeth lingering upon the maxillary bone,
though their function was gone, seemed to indicate to us
in some measure the gradual process by which that singu-
larly perfect adaptation of means to an end, the poison
apparatus of the viper was arrived at.

It would be impossible in these pages to go through the
arguments by which Mr. Darwin has established his main
propositions ; it must suffice to say here, that he has fully
convinced all those who are not in the habit, from the fixity
of early impressions, of putting many matters upon another
footing than that established by the exercise of reason, that
any modification in the structure of a plant or an animal,
which is of benefit to its possessor, is capable, nay, is sure
of being transmitted and intensified in successive genera-
tions, until great and material differences have more or less
masked the resemblance to the parent form.

Just as man, by favouring the breeding of those modifica-
tions of form, &c., that please him best, has been able, in
the course of a few years — in a length of time altogether
infinitesimal, as compared with the time during which the

THE TEETH OF MAMMALS. 269

surface of land and sea has been of pretty nearly its present
form, to say nothing of the enormously longer earlier geo-
logical epochs — to profoundly modify the breeds of dogs, of
horses, of numbers of plants, all of which are absolutely
known to have had a common origin, so in nature forces are
and ever have been in perpetual operation, which effect the
same thing.

A pigeon fancier wants a pigeon of particular plumage,
with a few feathers a little different from any pigeon he has
ever seen or heard of^1) he knows by experience that little
variations are for ever arising, and that by watching a
sufficient number of young ones, and rigorously picking out
those which at all tend in the direction of what he wants,
he will get what he wants, and will even tell you with confi-
dence that in so many years he will make a breed with the
peculiarity desired. And exactly as the plumage that was
wanted is got, so in nature the tooth that is "wanted,"
i.e., the dentition that is excellently well adapted to do its
work is manufactured by the operations of that law known
as " survival of the fittest."

It is quite enough that one of the small variations for
ever arising in animals shall be of advantage to it, for us to
see that the peculiarity is likely to be transmitted and
intensified in successive generations.

The question has been well presented by Mr. Wallace,
who points out that we must not think so much of varia-
tions in individuals as in groups of individuals : for instance,
it is a familiar fact that people vary in height, so that any
hundred persons may be divided into fifty taller and fifty
shorter. Now if a little extra height were of advantage,
many or most of the fifty would experience it ; though some
might not. In the same way if we grouped one hundred

(*) An eminent pigeon fancier, Sir J. Sebright, told Mr. Darwin that he
could produce any given feather in three years.

270

A MANUAL OF DENTAL ANATOMY.

animals whose teeth varied a little in respect of strength into
the fifty weaker and the fifty stronger, it is easy to see that
the stronger fifty would get the better of the others in the
struggle for existence on the whole, and would be more
certain to propagate their kind, and would repeat in most of
their progeny those peculiarities which had helped them-
selves to live.

Thus the doctrine of natural selection or survival of the
fittest, is as fully applicable to the teeth of an animal as to

FIG. 113 !.

any part of its organisation, and the operation of this natural
law will be constantly tending to produce advantageous or
" adaptive " differences. On the other hand, the strong
power of inheritance is tending to preserve even that which
in the altering conditions of life has become of very little
use, and thus rudimentary teeth we may understand to be
teeth which are in process of disappearance, having ceased
to be useful to their possessors, but which are still for a
time lingering upon the scene. Some teeth have disap-

(*) Skull of a placental rodent (Capybara), showing general character of
a rodent's dentition.

THE TEETH OF MAMMALS.

271

peared utterly; thus the upper incisors of Ruminants are
gone, and no rudiments exist at any stage (l) (see page 334);
others still remain in a stunted and dwindled form, and do
not persist throughout the life-time of the animal, as for
instance the first premolars of a horse, or two out of the
four premolars of most bears.

Before leaving this section of our subject, an instructive
illustration of the operation of these agencies may be given.
FIG. 114 (2).

It is very easy for us to see how a "rodent" type of
dentition is beneficial to its possessor by rendering acces-
sible articles of food wholly unavailable for creatures which

(*) Statements to the contrary have been made, and copied from book to
book without verification.

(2) A. Milk teeth of the Lemurine Cheiromys, with the permanent in-
cisors just coming into place. It differs from any Rodent by having many
milk teeth. i. Permanent incisor. i 2. Posterior deciduous incisor.
c. Deciduous canine, d, d 2. Deciduous molars. 1. Lower permanent
incisor. I 2. Lower deciduous canine, da, db. Lower deciduous molar.
B. Reduced outline figure of its permanent dentition, in which it closely
mimicks the true rodents.

272 A MANUAL OF DENTAL ANATOMY.

have no means of gnawing through a shell or other hard
body. Now it happens that in three regions of the world,
pretty completely cut off from one another, three animals,
in parentage widely dissimilar, have arrived at dentitions of
" rodent " type.

Thus in Australia, a region practically wholly monopo-
lised by Marsupials ; a marsupial, the Wombat, has a den-
tition very much like an ordinary placental Rodent. In the
island of Madagascar, one of the very few parts of the globe
without indigenous rodents, except a few Muridse, a Lemurine
animal, the Cheiromys, has a dentition modified in a similar
direction, though probably employed to get at a different
food ; and elsewhere, scattered all over the world, we have
the ordinary Rodents.

In fact, three creatures, as widely different from each other
in parentage as they well could be, have been modified by
natural selection until they have dentitions, not identical,
but for practical purposes not unlike.

It is impossible to conceive that these three creatures
have had anything in the way of common origin : their
ancestry must have been widely different, the regions in
which they live have been isolated from one another for
countless years, and yet they have each got to a "rodent"
type of dentition. Of extinct Lemurs little is known, and
of the ancestry of Cheiromys nothing ; but in the compact
group of Marsupials, still living in Australia, we are able to
dimly see some of the progressive steps which seem to tend
towards a rodent form of dentition. In Australia, roughly
speaking, there were nothing but Marsupials ; in Madagascar
more Lemurs than anything else ; and in each case out of
the material at hand, natural selection manufactured its
" rodent " dentition.

At the same time the force of inheritance is seen in each
of them retaining characteristics of the groups whence they
have been derived, so that underlying the pritnd facie

THE TEETH OF MAMMALS. 273

resemblance in the teeth, there are points in their several
dentitions whereby the wombat shows its marsupial affini-
ties, and the Aye-aye its quadrumanous affinities.

In addition to those modifications which are of direct use
to the individual in the way of assisting in the procuring of
food, &c., any character which would enable one male to get
an advantage over other males, and so render him more
certain to propagate his kind, will be sure to be transmitted
and intensified.

Thus we can understand how the males of some species
have become ornamented; how the males of many birds
have come to sing : and, what is of more immediate concern
to us, how the males of some animals have become possessed
of weapons which the females have not. The possession of
weapons by the male is strikingly exemplified in the teeth
of animals. The males of many frugivorous monkeys have
canine teeth much larger than those of the females ; they
are cut late coincidently with the attainment of sexual
maturity, and are useful to their possessors as weapons in
their combats with other males. The male narwal has its
single elongated tusk; the male dugong has tusk-like
incisors; in the respective females these same teeth are
insignificant.

But the most striking instance of the teeth being modi-
fied, so as to serve as weapons for sexual combat, is afforded
by some members of the group of ruminants, amongst whom,
as Cuvier long ago pointed out, those which are armed with
horns have no canine teeth, and vice versd — a generalisation
which, although subject to slight exceptions, remains upon
the whole true.

The male musk-deer (Moschus moschiferus) has canine
teeth of enormous length, while it is quite without horns
(see fig. 115); the female has no canine teeth. The male
muntjak, which has very short horns, has canine teeth, but
of much smaller size than those of the musk-deer. Other

T

274 A MANUAL OF DENTAL ANATOMY.

examples of hornless deer furnished with canine teeth are to
be found in Swinhoe's water-deer (Hydropotes inermis) and in
the Elaphodus cephalophus (which has very small antlers),
a Chinese deer more recently discovered, and in the Tragu-
lidce. It is obvious that males furnished with weapons
more powerful than their fellows, will be more likely to

FIG. 115 (J).

prove victorious in their battles, to drive away the other
males, to monopolise the herd of females, and so to transmit
their own peculiarities to offspring, which will again be
favoured in the same way. Thus it is very easy to see how,
amongst gregarious animals, the development of teeth
serving as sexual weapons is likely to be favoured, genera-
tion after generation, until canines as highly specialised as
those of the musk-deer, or the wild boar, are attained to.

It will suffice to indicate to the reader that he must be
prepared to find that the teeth are profoundly susceptible
of modification, but that, amid all their varied forms, the
evidences of descent from ancestors whose teeth departed
less from the typical mammalian dentition are clearly trace-
able by the existence of rudimentary teeth and other such

(!) Cranium of Moschus, showing the long canine tooth.

MAMMALS.

characters. And, although it is by no means probable that
we have recognised more than a part of the agencies which
are at work, natural selection and sexual selection appear
to be competent to produce most of the phenomena of
modification observed. There remains one other influence,
much more obscure in its nature, to be touched upon,
namely, "correlation of growth" or "concomitant varia-
tion." When we find that when horns are developed, canine
teeth are absent ; or that, after a boar has been castrated,
his tusks cease to grow, although we may be quite unable
to conceive the precise manner in which the one thing in-
fluences the other, we can see that there is a consistency in
the development of the sexual weapon ceasing coincidently
with the destruction of the sexual apparatus, or in the fact that
two kinds of weapon are not developed in the same animal.

But there are some correlations of growth of a still more
recondite nature, in which the connection is less obvious.
Of this nature is the relation which exists between pecu-
liarities of the skin and of the teeth : the Edentata, abnormal
in their skins, are different from most other Mammalia in
their teeth ; foetal whales, yet more aberrant in the nature
of their skins, have only rudimentary teeth, in the place of
which, after birth, plates of whalebone are found.

Mr. Darwin ("Animals and Plants under Domestication,")
has collected a number of curious instances of relations exist-
ing between hair and teeth. In general terms it may be said
that any great abnormality in the hair goes hand in hand
with an abnormality of the teeth. Thus, there is a breed of
dogs found in Turkey which are almost hairless, and which
have very few teeth, their dentition being reduced to a single
molar on each side, together with a few imperfect incisors ;
and in the human subject inherited baldness has been found
to be associated with inherited deficiency of the teeth.

But we must not go further than to say, that great
abnormality of hair goes hand in hand with abnormality

T 2

276 A MANUAL OF DENTAL ANATOMY.

of teeth, for examples have just been given of absence of
hair and absence of teeth ; and, on the other hand, redund-
ance of hair has in several cases been accompanied by
absence of teeth.

Thus, in the case of the now famous hairy family of
Burmah, the peculiarity of silky hair being developed over
the face was transmitted to a third generation, and in each
case the teeth were very deficient in number. A year or
two ago a hairy man and his son, said to have come from
the interior of Russia, were exhibited in London, and they
were also almost toothless.^)

A good many years ago a hairy woman (Julia Pastrana)
was exhibited in London, of whom it has commonly been
reported that she had an extensive number of teeth. Certain
it is that her mouth was very prominent, and that she was
described as " dog-faced " and " pig-faced," but models have
been presented to the Odontological Society by Mr. Hepburny
which are indisputably known to be models of her mouth,
and these do not show any excessive number of teeth. The
teeth, at least such of them as can be seen, are enormously
large, but the mouth is affected with general hypertrophy
of the gums and alveolar processes to such a degree, that
only a few of the teeth can be made out.

But this does not make her case the less interesting to
the odontologist, for in the huge teeth, the enormous palilla?
of the gum, and the redundant hairs on the face, we have
evidence of a disposition to hypertrophies of the integument
affecting in different places the different tegumentary appen-
dages which happen to be there. And that the teeth are

(a) The man's mouth exemplified the dependence of the growth of the
jaw upon the presence of teeth. Ordinarily the increase in size between
childhood and adult age takes place by a backward elongation, which
allows for the successive development and eruption of the molars behind
the space occupied by the temporary teeth. But this man never had any
true molars, and no such backward elongation of the jaw had ever taken
place, so that, though he was a full-sized man, his jaw was no larger than
a child's.

THE TEETH OF MAMMALS. 277

dermal appendages lias been sliown at a previous page (see
page 2).

He would indeed be a rash man who ventured to assert
that we had recognised all the agencies which are at work
in the modelling of animal and vegetable forms ; but it is
safe to say that, at the present time, we are acquainted with
" natural selection," or " survival of the fittest," an agency
by which variations beneficial to their possessors will be
preserved and intensified in successive generations; of
" sexual selection," which operates principally by enabling
those possessed of certain characters to propagate their race,
while others less favoured do not get the opportunity of so
doing ; of " concomitant variation " between different parts
of the body, an agency much more recondite in its opera-
tions, but by which agencies affecting one part may second-
arily bring about alterations in some other part.

And operating in the contrary direction, we have a certain,
fixity of organisation, so that the power of inheritance is
constantly asserting itself by the retention of parts which
have become useless, for a time at all events, and by the
occasional reappearance of characters which have been lost.

Allusion has been made to these great biological questions
with the view of helping the student to have patience to
master descriptions of minute points, of wThich he does not
at the moment sec the bearing, by giving him confidence
that there are no characters so trivial but that they may
throw very important light upon the remote parentage and
the line of descent of the creature under examination. And
as a further incentive to painstaking and minute observa-
tion, it may be added, that things which are rudimentary,
and therefore inconspicuous, are often just the things which
happen to teach us most ; for being of no present use, they
are not undergoing that rapid change in adaptation to the
creature's habits which may be going on in organs which are
actively employed.

278 A MANUAL OF DENTAL ANATOMY.

THE HOMOLOGIES OF THE TEETH.

A superficial survey of the teeth of those mammals which
possess two sets of teeth (diphyodonts) will indicate that,
notwithstanding the apparent anomalies brought about by
adaptive modifications, a close correspondence between the
several teeth of different animals exists. That is to say, we
can generally identify incisors, premolars, and molars ; nay,
more, when an animal has less than the full typical number
of a particular class of teeth, we can ordinarily say with
certainty which of them it is that are absent.

As it is impossible, or at least inconvenient, to avoid the
use of the term "typical" dentition, it will be well to explain
at the outset what is, and what is not, meant by it.

That the great majority of biologists reject utterly the
" archetype " theory, by which all those resemblances which
really exist were referred to the influence of a sort of gene-
ralised " pattern " animal, according to the model of which
all other animals were fashioned, has already been mentioned :
this, then, is what is not meant by a "typical" dentition.
What is meant, is a form, so simplified, so little modified in
in any special direction, that we can conceive it to be near
to a common parent form whence, by progressive modification
in successive generations, other forms have been derived.
We cannot point to any mammalian dentition at present
known to us, and say this may have been the parent;
this is a typical form of mammalian dentition ; but we
do know many fossil forms which approximate to it far
more closely than do any at present in existence, and as
transitional forms of animals, and animals of highly gene-
ralised characters, are every day coming to light, we do not
doubt that such forms once did actually exist, and may
one of these days be found. Absolute proof would be ob-
tainable only if we could refer to its place every mammal
that had ever existed, and show every step in the series

THE TEETH OF MAMMALS.

of modifications by -which the ultimate divergence of den-
tition was effected. But evidence far short of absolute
ocular demonstration serves to satisfy us on most points,
and there is sufficient evidence available to enable us to
say with some confidence that our "typical" or parent
mammalian dentition was, so far as the numbers of the
several kinds of teeth go,

i ' c -— prm -. m ' -= 44.

And when there are less than forty-four teeth, as has been
already mentioned, we can in most cases say which they
are that are absent.

Thus, taking a certain bear and a baboon (each having
two premolars only on each side), we are able to decide, by
comparison with allied creatures, that, in the case of the
bear, it is the second and third premolars which are wanting,
the first and fourth remaining ; while in the baboon it is the
first and second which are wanting, the third and fourth
being present. By hoinology we mean such correspondence
as is above indicated ; a correspondence which might almost
be expressed as a relationship by descent.

Homology, then, is almost equivalent to identity of origin,
or, at all events, to similarity of origin ; but it by no means
necessarily involves identity or even similarity in the pur-
pose to which a thing is ultimately applied — a fact which will
be further illustrated in speaking of canine teeth.

The homologies of the teeth may be treated under two
heads : the one, the. homologies of the teeth in their relation
to other parts of the body, and the other, their more special
homologies, or their relation to one another.

The relation of the teeth to the skin, which we express by
calling them " dermal appendages," as well as the epidermic
nature of the enamel, and the dermic nature of the dentine,
have been sufficiently discussed at former pages, so that

280 A MANUAL OF DENTAL ANATOMY.

we may at once pass to the homologies of the teeth with
one another.

Teeth are divided into incisors, canines, premolars, and
molars, but these classes do not all admit of quite satis-
factory definition. Incisors are denned as teeth implanted
in the intermaxillary bone, a definition which has the merit
of being precise ; and on the whole there is a certain resem-
blance running through incisor teeth in most animals, but
the definition of lower incisors as being the corresponding-
teeth in the lower jaw is a good deal less satisfactory, because
they are not situate upon any distinct bone. And it has
even been denied that there can be a true homology between
a maxillary and a mandibular tooth.

Molars are teeth at the back of the mouth, which come
up behind the milk teeth (when there are any), and which
are generally subservient to grinding the food.

Premolars are teeth in front of the molars, usually differ-
ing from them by being more simple in form and being-
smaller, and in most animals by having displaced deci-
duous predecessors. But they are not always simpler in
form, nor smaller (e.g., the horse, fig. 138), nor do they
always displace deciduous predecessors (e.g., they do not all
do so in the Marsupials), so that this definition is not abso-
lutely precise. Still, as a matter of practice, it is usually
easy to distinguish the premolars, and the division into
premolars and molars is useful.

Any objection that can be raised to the name of premolar
on the score of a short logical definition being impossible,
applies with tenfold force to the canines. (Cf. Messrs.
Mosely and Lankester, Journ. Anat. and Physiology, 1869.)

The nearest approach to a good definition is that which
describes the canine as the next tooth behind the intermax-
illary suture, provided it be not far behind it; and the
lower canine as the tooth which closes in front of the upper
canine.

THE TEETH OF MAMMALS. 281

A great deal of confusion has arisen out of the twofold
sense in which the word " canine " is used : if it were always
applied to designate the first tooth in the maxilla of the
typical mammalian dentition quite irrespective of its size,
<fec., and of the lower tooth closing in front of it, no objec-
tion to its employment could be made, inasmuch as it would
designate truly homological organs.

But it so happens that the tooth in question is, in a very
large number of familiar animals, developed to a large size
and sharply pointed for use as a weapon, and so with the
word canine there comes to be associated a teleological
idea; and hence we are dissatisfied with calling the first
maxillary tooth "canine," when it is some other tooth
which is doing its work.

On the other hand, if we are to leave out of court all
considerations as to size, purpose to which it is to be
applied, and so forth, there is nothing left to make it
deserving of a name distinguishing it from the four teeth
behind it. So we must be content with some such state-
ment as the following.

A very large number of animals, notably the Carnivora,
have one tooth, situated a little way from the front of the
mouth, developed to an unusual length and sharply pointed,
for use as a weapon. The tooth which has undergone this
adaptive modification is usually the first which lies in the
maxillary bone ; in fact, the foremost of the premolar
series ; but it occasionally happens that it is some other
tooth which has undergone this modification. When we
use the term canine wre should generally mean a tooth so
modified, and generally, but not always, should be alluding
to the same tooth, i.e., to the tooth which in the typical
mammalian dentition comes next behind the outermost or
incisor — the first of the premolars, if we allow five premolars
instead of four.

It would practically be very inconvenient to abolish the

282 A MANUAL OF DENTAL ANATOMY.

term canine ; but it should be borne in mind that its signi-
ficance is merely equivalent to " caniniform premolar," and
that in describing the dog's dentition (fig. 163) we should be
less liable to be misinterpreted, were we to say that it has
five premolars, of which the first is caniniform. To those
who accept the doctrine of evolution it is not needful to say
more, as it is hardly possible to resist the conclusion that
the teeth of the parent forms were, like those of the present
monophyodonts, not much differentiated from one another.
Then, as animals diverged and became modified in accordance
with their requirements, their teeth would become so far
differentiated that they would admit of being classified.
Thus the Carnivora would have attained to a stage of
differentiation in which the canine is functionally certainly
deserving of a distinction, whereas along other lines of
descent, differentiation having not proceeded so far, or having
proceeded in a somewhat different direction, it would not
merit a distinctive appellation.

But as it so happens, that all the works on odontography
have started upon the basis that there was a " type " denti-
tion, and as a canine figured in that dentition, it will be
necessary to point out a few instances of the propositions to
which those anatomists are committed who call some tooth
a " canine " in every case where a tooth is situated in the
maxillary bone, close behind the suture which connects it
with the intermaxillary bone, whether that or any other
tooth be large and pointed, " caniniform " or not.

In typical Ruminants, the upper jaw lacks both incisors
and canines (with certain exceptions, for which see p. 335), but
in front of the lower jaw there are grouped together eight
teeth, closely fitted together, and of almost exactly similar
size and shape. The outermost pair of these teeth are
called canines, because (i.) in some allied species the tooth
in this situation is more pointed; (ii.) because this tooth
shuts in advance of the upper canine when the mouth is

THE TEETH OF MAMMALS.

283

closed (in those allied creatures which have an undoubted
upper canine) ; (iii.) because it is cut later than the others
(Owen).

These three reasons are weak, because (i.) form is a
veiy unsafe guide to homology, and as to the lateness of its
development (iii.), it succeeds to the third incisor, by Pro-
fessor Owen's own showing, after about the same lapse of
time which separated the eruption of the second and third

incisors. Moreover, Oreodon, an extinct Ruminant with
caniniform teeth, has the eight incisors in the lower jaw in
addition to a caniniform tooth, which is the fifth tooth counting
from the front. With reference to the relative positions of
the upper and lower teeth, determining which is and which
is not "the canine," (ii.) no one, looking at the dentition
of Oreodon, would be inclined to hesitate which teeth he
should call " canines ; " yet the lower caniniform tooth
shuts behind the upper, and therefore, according to this
test, it is not a true canine.

In the Lemurs there are similarly eight procumbent teeth

(!) Oreodon Culbertsonii (after Leidy). It will be observed that in the
upper jaw the four premolars of the typical mammalian dentition are
behind the "canine," but that in the lower jaw the tooth which would
fulfil the functions of a canine is the first of these four, and therefore is
not the corresponding tooth to the " canine" in the upper jaw.

284 A MANUAL OF DENTAL ANATOMY.

occupying the front of the lower jaw, of which the outer-
most pair are called canines, although not in the smallest
degree meriting that name for any other reason than that
they close in front of the caniniform tooth of the upper jaw,
for they are just like the other incisors.

But it is in the Insectivora that the greatest difficulties
occur.

To the mole no less than four dental formulae have been
assigned, all turning upon the identification of the canine.
The difficulty is this : The upper tooth, which looks like a
canine, has two roots, and is implanted (and its deciduous
predecessors also lie) (Spencer Bate) within the limits of
the premaxillary bone. And besides this, the lower tooth,
which answers the purpose of, and looks like, a canine,
closes behind instead of in front of the great upper tooth.

FIG. 117 0.

Nal~. Size.

Erlculus has a sharp, long, two-fanged tooth, in pattern of crown,
an enlarged premolar, in position of upper canine, and no canini-
form tooth in lower jaw.

Centetcs has typical canines, like a Carnivore.

Ilemicentctvs, the so-called canine, differs in no respect from the
premolars behind it.

Erinaceus. So called upper canine two-rooted, and like the pre-
molars which follow behind it.

0) Upper and lower teeth of the common mole. In it, just as in
Oreodon, the teeth which fulfil the functions of canines are not corre-
sponding teeth in the upper and lower jaws.

THE TEETH OF MAMMALS. 285

(rymmtra. Upper canine-like tooth has two roots ; a single-rooted

lower pointed tooth closes in front of it.
Macroscelis and Petrodromm. The third or outermost incisor is

two-rooted, long-, and sharp, and plays the part of a canine.
Potamoyalc. A small tooth, in no respect different from the other

premolars, is called a " canine."

In some of the groups no tooth has been lengthened and
pointed, so as to serve as a canine ; in others it is the wrong
tooth, i.e., not the same tooth as in the Carnivora, or as in
other Insectivora. Consequently, in the Insectivora the
elevation of a tooth into caniniform length and character is
a mere adaptive modification, which may affect an incisor,
or a premolar, or no tooth at all.

It appears to me that the result of all investigations into
the homologies of mammalian teeth may be summed up
somewhat in the following manner.

The evidence of a common pattern, which is traceable in
incisors, canines, premolars, and molars (see page 8), would
seem to indicate that their special forms have been all
derived from modifications of some much more simple form,
and that if we are ever to find what might be called a parent
mammalian dentition, it will be nearly "homodont:" that
is to say, the several teeth will not differ much from one
another in size and shape, just as we see to be the case in
the dolphin (see fig. 131), or the armadillo.

It becomes open to question whether the term canine is
desirable in dental formula) or in homological determinations ;
if we put on one side its functional modifications there is
nothing left to distinguish it from other premolars : if we
bear in mind its functional development we encounter the
anomalies noticed above.

If we were able to place in unbroken series all the den-
titions through which, by progressive modification, the
original almost homodont dentition had passed into a
highly specialised dentition, like that, say, of the cat, it
would be a matter of impossibility to fix upon any point

286 A MANUAL OF DENTAL ANATOMY.

where we should be justified in asserting that here the
homodont dentition has recently become heterodont : at
this point, for the first time, we have incisors, canines,
molars.

As a matter of fact, a large number of extinct Ungulata
had the full typical number of mammalian teeth, viz., forty-
four, and in some the individual teeth, incisors, canines,
premolars, and molars, passed into one another by insensible
gradations, and contiguous teeth were but little differen-
tiated from one another. Professor Flower has described
afid figured such an extinct Ungulate under the name of
Homalodontotherium (Philos. Trans., 1874). It is exceedingly
interesting to find that back in geological time the dentitions
were more generalised, both carnivorous and herbivorous
mammals of the Eocene period usually possessing the full
typical number of teeth, and displaying less of special
modification ; but the few forms of life which have been
handed down in a fossil state do not as yet offer us by any
means an unbroken chain of forms differing from one another
by progressive modification, except in a few cases : thus the
ancestry of the horse is now comparatively completely known
to us. Bearing in mind that the several kinds of teeth have
probably a common origin, the homological differentiation
in the incisors, premolars, and molars may be advantageously
admitted, and made use of as a basis for comparing and
classifying the teeth of different animals. It is usually said
that when incisors are missing from the full typical number,
they are lost from the outer end of the series : that
is to say, if there is but one incisor it is I1 ; if two, Ix
and I0.

There are many exceptions to this : e.g., the first incisor
is the first to disappear in the otter, walrus, and some few
others.

When premolars are missing, it is said that they are lost
from the front of the series. This is generally true, but the

THE TEETH OF MAMMALS. 287

following exceptions may be given. In bears the second
premolar is often absent, the first being very constant ; the
same thing is true of many bats ; in Dasyurus the third, or
hindmost (it being a Marsupial) is absent, the first two being
present. (J)

A difficulty at times occurs in deciding whether a tooth
is to be regarded as a premolar, or as a milk tooth, as there
are many so-called permanent teeth which are lost early in
the lifetime of the animal.

Professor Flower gives an instance of this in the hippo-
potamus : the first premolar appears with the milk teeth ;
it probably has no predecessor, and is shed in middle life.
But in allied forms the corresponding tooth remains in place
throughout the creature's life.

The wart-hog is a conspicuous example of the early loss
of teeth which clearly belong to the permanent series (see
page 328), all the teeth (premolar and molar) in front of the
last great molar being cast off, and the dentition ultimately
reduced to —

.21 1

i c — m —

3 1 1

That general correspondence, which is found to exist
between the dentitions of various animals, extends also to
the patterns of individual teeth, so that we are able to trace
out the various stages by which complexity of pattern has
been arrived at.

In what might be termed a typical tooth we should have
a single central pulp cavity surrounded by a body of hard
dentine ; over the crown this is coated by enamel, whilst the
whole, crown and root, would be invested by a layer of
cement.

The layer of coronal cement may be so thin as to be

(!) This is ascertained by the examination of allied forms, in which the
hird premolar is found to be so small as to be rudimentary.

238 A MANUAL OF DENTAL ANATOMY.

merely rudimentary, as in Man or the Carnivora ; or the
investment with enamel may be only partial, as upon the
front of a Rodent incisor; or a tooth may be composed
solely of a mass of hard unvascular dentine, as in the teeth
of the Wrasses.

And just as endless varieties of teeth may be produced
by the suppression, or partial suppression, of certain of the
tissues, so differences may be brought about by the occur-
rence of other than the three usual tissues. Thus the
remains of the central pulp cavity often becomes occupied
by calcined pulp, forming " osteodentine ; " this, which
occurs in man as an almost pathological condition, is per-
fectly normal in many animals; in the sperm whale, for
instance, or in the constantly growing teeth of the sloth, the
central axes of which are occupied by dentine permeated by
medullary canals.

It is not so much the complexities induced by variation
in minute structure that concerns us here, as those brought
about by the arrangement of the different tissues.

If we take a simple conical tooth with one cusp, such as
a canine, and grind or wear down its apex till the terminal
portion of enamel is removed, its blunted end will present a
more or less circular area of dentine, surrounded by a rim
of enamel. If we imagine a tooth with four long similar
cusps, we shall at a certain stage of wear have four such
areas, while eventually, as the tooth gets worn down below
the level of the basis of the cusps, there will come to be a
single larger area of dentine surrounded by enamel. Thus
in those teeth the grinding surfaces of which are rendered
complex in pattern by the presence of several cusps, the
pattern changes from time to time as the tooth wears down ;
while the addition of thick cementum filling up the inter-
spaces of the cusps, adds a further element of complexity, as
is seen in the teeth of most herbivorous creatures. The
change of pattern induced by the wearing down of the surface

THE TEETH OF MAMMALS. 289

to a lower level is well and simply illustrated by the
" mark " of the incisor teeth of a horse.

In an uncut, and therefore perfectly unworn tooth, such
as is represented in the figure, the condition of the apex
may be compared to the finger of a glove, the tip of which

FIG. 118 .

has been pushed in or invaginated. The depression so
formed is, like the rest of the surface, coated with enamel,
and with a thin layer of cementum.

When the tooth is worn down to a considerable extent,
we have a field of dentine, in the centre of which is an
oval ring of enamel; within this a space filled with the
debris of vfood, &c. This constitutes the mark (see next
page), and as the tooth becomes further worn down, below
the level of the bottom of the pit, the mark disappears,
and a plain area of dentine results.

JSTot only may inflections of the surface and of the enamel
take place from the grinding surface, but they also abun-
dantly occur upon the sides of the tooth. The inflection
of the surface, which in the incisors of the horse is of the
simplest possible form, may be cruciform, or variously
waved and broken up, thus leading to all sorts of com-
plications of surface. As the tooth becomes worn, the
longitudinal inflections, running in from the sides, may also
be oblique, or variously waved, or they may extend through
the entire width of the tooth, thus cutting it into a series

(J) Apex of crown of an upper incisor of a Horse, not yet completely
formed.

290

A MANUAL OF DENTAL ANATOMY.

of plates of dentine and enamel, fused into one tooth by
the cementum (see fig. 125).

Interesting as have been the discoveries made of late

FIG. 119

FIG. 120 (2).

years in Mammalian paleontology, it is not as yet by any
means possible to determine from what common pattern or
patterns all complex mammalian teeth may be considered
to have been derived; though the pattern of some, for
example, of the molars of the horse, may be traced back
in increasing simplicity through a number of parent forms.
Enough has, however, been done to indicate that by careful
study many complexities of pattern may be referred to a
few particular types, and thus may be simplified by a com-
parison with other allied forms, in which essential charac-
teristics are not masked by minor complications. Starting

(!) Horse incisor, in longitudinal section.

(2) Horse's incisors, showing the mark at various ages.

THE TEETH OF MAMMALS. 291

from the human tooth, as being familiar to us all, a quadrate
crown, with four cusps at its corners, is common to many
animals ; the oblique ridge, present in some apes, not in
others, is met with also in some Insectivora, e.g. the Hedge-
hog. Around the neck of many teeth runs a pronounced
ridge, the " cingulum," and this may be produced up into
additional cusps.

A very instructive series of comparisons of the molar
teeth of Insectivora has been made by Mr. Mivart (Journal
of Anat. and Physiol. : 1868); pointing out that within
the limits of this group a great variety of patterns is met
with, the several modifications being connected by transi-
tional forms.

It would appear that upon the molar teeth (upper) of
Insectivora there are four principal cusps (lettered a, b, c, d,
in the figure) which are more or less connected by ridges ;
such simple teeth are met with in the elephant mice
(Macroscelides), and hedgehog. The cingulum is well
developed in most of the group, and the further complexity
of the crowns, which often bristle with sharp points, is
brought about by the elevation of the cingulum into long
sharp points, equalling, or exceeding in length, the principal
cusps of the tooth.

Thus in Urotrichus, a Japanese creature having affinities
with the mole, the external cingulum is elevated into three
distinct pointed cusps, united by ridges with the two prin-
cipal cusps, an arrangement which gives a sort of W pattern
to the surface, while to the inner side the cingulum forms
another cusp, so that there are in all eight cusps; the
common mole has the third cusp developed from the outer
cingulum, but its two inner principal cusps are fused together
and lose their distinctives. The suppression and fusion of
cusps is carried to a much greater extent in the compressed
teeth of the iridescent mole (Chrysochloris), but there are
intermediate forms which render it easy to identify its

u 2

292

A MANUAL OF DENTAL ANATOMY.

reversed part with those corresponding to them in the mole
or in Urotrichus.

Speaking generally, it may be said that new cusps are
added to the number already existing, by the cingulum

FIG. 121

becoming elevated into points ; it is not very unusual to
see subsidiary cusps, obviously originating in this way, upon
human molars.

Ridges may variously connect the cusps ; and the coales-
cence of two or more cusps to form an exceedingly elevated
point is illustrated by the Carnassial tooth of carnivora ; to
this transformation certain marsupial teeth form the clue, as
they afford unquestionable evidence of such coalescence by
a gradational series of small modifications in this direction
occurring in allied creatures.

A simple pattern of tooth is formed by the junction of
the two anterior and two posterior cusps by simple ridges ;
and the cingulum may connect the outer ends of these two
ridges; such a tooth is seen in the Tapir and in the
Palseotherium. By the varied obliquity of these ridges,

(x) Upper molar teeth of (A) Urotrichus ; (B) Mole ; and (C) Chryso-
chloris. The four principal cusps are lettered a, &, c, d, in each of the
figures. In A the cingulum has been elevated so as to form four additional
cusps on the exterior of the tooth, and one additional cusp on the interior.
B and C show the fusion of certain of these cusps, and the consequent
diminution in their number. (From Mivart.)

THE TEETH OF MAMMALS. 293

and by the introduction of secondary inflections, patterns
apparently dissimilar are arrived at.

In the molar tooth of the horse, arrived at by a modifi-
cation of the Palseotherium type, we have a surface con-
stantly kept rough by the varying hardness of its different
constituents.

In a worn tooth, we have upon a general field of dentine

FIG. 122

two islands of cementum, bounded by tortuous lines of
enamel, and on the inner side a sort of promontory of
dentine, bounded by enamel. The tortuous lines of enamel
by virtue of their hardness will, at all stages of wear, be
more prominent than the dentine or the cementum, and
will hence maintain the efficiency of teeth as grinders.

The -patterns of grinding surface thus produced, are very
constant for allied species, so that an individual tooth of a
herbivore may sometimes be correctly referred to its genus,
and always to its family.

But as it will be necessary to recur to this subject from
time to time, it will suffice for the present to point out that
such correspondences do exist, and that all the complexities
of pattern found, may, in practice, be reduced to some few
types.

(!) Molar tooth of Horse, showing the characteristic pattern of its
grinding surface.

294 A MANUAL OF DENTAL ANATOMY.

The development of additional cusps from up-growths of the
cingulum, and the suppression or fusion of pre-existing cusps,
may be traced by a comparison of the teeth of allied animals,
and thus connecting links are found between patterns at
first sight very dissimilar. The order Proboscidea affords,
however, so instructive an example of the manner in which
an exceedingly complex tooth has been derived from a
simple one, that it may be mentioned in this place as an
example.

The tooth of the elephant is so strikingly unlike other
teeth that it might at first sight be supposed that it is
more essentially different than is really the case. The clue

to its nature is afforded by the teeth of an extinct Pro-
boscidian, the Mastodon. If we take as our starting point
the second true molar of one of the Mastodons (Tetralo-
phodon) we find its crown to be made up of four strongly
pronounced transverse ridges, the summits of which are
made up of rounded eminences (whence the name Mastodon,

(*) Second upper molar of Mastodon (longirostris), from Falconer.
About one-eighth natural size. The four transverse ridges, b, c, d, e, are
seen to be, to some extent, divided into outer and inner divisions by a
longitudinal cleft, much less deep than the transverse indentation. At
the front there is a slight elevation of cingulum into a " talon '' (a), and a
similar one at the back of the tooth ; by its further elevation additional
ridges or cusps would be formed.

THE TEETH OF MAMMALS. '295

from HCICTTOS, a nipple). The three transverse ridges coalesce
at their bases, and the crown is supported upon a number
of roots corresponding to the ridges.

If we take the next tooth, or the third true molar, the
general character remains the same, save that there are five
ridges, and indications of as many roots ; still the general
correspondence of the ridges with the cusps of less aberrant
teeth is obvious.

The crown is coated by enamel, over which there is a
thin layer of cement, which does not fill up the whole
interval between the ridges.

Thus the tooth is not a very aberrant one • it is obviously
nothing more than a tooth in which the somewhat numerous
cusps are connected by transverse ridges, and are very long
and strongly pronounced.

To convert the tooth of a mastodon into that of an

FIG. 124

elephant, we should have to multiply the number of ridges,
to further increase their depth, to fill up solidly the inter-
spaces between them with cementum, and to stunt the
roots. The completed tooth of an elephant is a squarish
or rather oblong mass, from the base of which spring con-
tracted and stunted roots. It consists of a common pulp
cavity, small in proportion to the bulk of the tooth, and

(]) Molar tooth of an Asiatic Elephant, showing the transverse plates of
dentine bordered by enamel.

296 A MANUAL OF DENTAL ANATOMY.

deep down in the mass, from which many thin laminse are
sent up towards the surface, each consisting of an oblong
area of dentine enclosed by enamel ; and the interspaces of
these exaggerated cusps are solidly filled in by cementum.

Between the Mastodon and the Indian Elephant are a
number of transitional forms in which we are able to trace
the gradual modification of the not excessively aberrant
tooth of the Mastodon into the very peculiar huge molar
of the Indian Elephant.

The numerous transverse plates of the elephant's grinders-
are united by dentine at their bases, and a common pulp
cavity and truncate roots are formed ; but in this last
respect the molar teeth of the capybara depart still farther
from the ordinary type, for being molars of persistent

FIG. 125 !.

growth, their numerous transverse plates of dentine and
enamel do not become continuous, and there is no common
pulp cavity. It is as though in an elephant's grinder the
plates, which are for a long time distinct, never coalesced,
but continued to grow on separately, being united with their
fellows by cementum only.

It has been suggested (J. A. Ryder, Proc. Acad. Nat,
Sciences, Philadelphia, 1878), that the pattern of the molar
teeth of herbivora is the result of the extent and direction
of the excursions of the mandible when it is in use, and so
depends upon the form of the glenoid cavity and of the
condyle, and that hence the greatest modification is to be

(*) Molar of Capybara, showing the transverse plates of dentine and
enamel united to one another !>y cementum.

THE TEETH OF MAMMALS. 297

found nearest to the articulation, where the greatest force
is exerted.

Thus " bunodont " animals, i.e. those that have rounded
conical cusps upon their short-rooted teeth, have a cylindrical
condyle ; selenodonts, or those with crescentic ridges on the
molars, have a condyle which is expanded and plane, while
lophodonts, or those with transversely ridged teeth, have a
globular condyle.

This correspondence pointed out between the condyle, the
movements of the jaw, and the form of the teeth does exist,
but it is less easy to see how it is brought about. The
simple mechanical explanation that the teeth are drawn out
into these forms, hardly conveys much information, seeing
that the tooth before it is subjected to these influences, is
quite finished, and its form, such as it is, is unalterable :
while to effect an alteration in the form of a masticating
surface an influence must be brought to bear upon the
tooth germs at an exceedingly early period. It might with
equal justice be said that the crown of the tooth being-
formed thus had influenced the excursions of the jaw, and
so modified the condyle.

THE MILK DENTITION.

Some thirty years ago Professor Owen called attention to
the fact that those mammals in whom thd teeth situated in
different parts of the mouth were alike in form (homodonts),
developed only one set of teeth, and to indicate this charac-
teristic he proposed for them the term " monophyodonts."
Those, which, on the contrary, had teeth of different size
and form in various parts of the mouth (heterodonts), de-
veloped two sets of teeth ; a " milk " set, which was dis-
placed by a permanent set, and this peculiarity he expressed
by the term " diphyodonts." As originally set forth, the
terms homodont and monophyodont were interchangeable,

298 A MANUAL OF DENTAL ANATOMY.

for they designated the same groups of animals ; in the
same way heterodont was an equivalent for diphyodont.

But although this is true of a large number of animals, it
is not true of all, and it becomes necessary to note some of
the exceptions.

The nine-banded armadillo (Tatusia peba) is a true homo-
dont : its teeth are all very nearly alike, they are simple in
form, and they grow from persistent pulps. Yet it has
been shown by Rapp, Gervais, and Professor Flower, to
have a well developed set of milk teeth, retained until the
animal is of nearly full size.

Thus it is a true diphyodont, at the same time that it is
a true homodont mammal. But no milk dentition has been
observed in the sloths, nor indeed at present has it been
seen in any other armadillo (except the doubtfully distinct
T. Klapperi) ; nor have milk teeth been found in any ceta-
cean, so that the rest of the homodont animals are, so far as
we know, really rnonophyodont.

Nor is it absolutely true that monophyodonts are all
homodont : thus the rudimentary teeth of baleenoptera are
heterodont (see p. 311).

Upon the whole, our information respecting the " milk "
or deciduous dentition is defective ; but much light has
been thrown upon the subject by the investigations of
Professor Flower (Journal of Anatomy and Physiology,
1869, and Transactions Odontological Society, 1871), of
whose papers I have made free use in this chapter.

The perpetual replacement of teeth lost, or shed in regular
course, which characterises the dentition of fish and reptiles,
finds no parallel in the case of mammals, none of whom
develop more than two sets of teeth.

Just as homodont mammals as a rule develop but one set
of teeth, so heterodont mammals as a rule develop two sets
of teeth, though exceptions to this rule may be found.

The deciduous or milk set of teeth may be of any degree

THE TEETH OF MAMMALS. 299

of completeness ; the milk teeth in man answer the require-
ments of the child up to the age of seven years, and in the
Ungulata they commonly remain until the animal has
assumed its adult proportions. On the other hand, in
many " diphyodont " animals the milk teeth disappear very
early indeed, as in the mole (see Fig. 117) ; whilst there are
many instances of the milk teeth being absorbed in utero.
So that in the extent to which the milk teeth are developed,
the greatest variability is found to exist.

A perfectly typical milk dentition represents, upon a
reduced scale, the adult dentition of the animal, with the
exception only that sexual differences are but feebly marked,
if indeed they are at all present.

Thus, as a general rule, the' hindmost of the milk teeth
bear more resemblance to the true molars which come up
behind them, than they do to the premolars which come up
from below to displace them, which latter are generally of
simpler form.

In what may be termed the normal arrangement, each
tooth of the milk series is vertically displaced by a tooth of
the permanent series ; but plenty of examples may be
found of particular milk teeth which have no successors,
and, on the contrary, individual permanent teeth which
have never had a deciduous predecessor.

It has already been mentioned that amongst homodonts
no succession of teeth has been observed in the Cetacea,
nor in any other of the Edentata, save the armadillo ;
amongst heterodonts there are several Rodents which have
no deciduous teeth, e.g., the rat ; the dugong has probably
deciduous incisors, but no other milk teeth ; the elephant
has no vertical succession, save in the incisors.

Among Marsupials, which are true heterodonts, there is
only one milk molar on each side in each jaw; this is always
displaced by the third or last premolar ; but the milk tooth
varies in the extent to which it is developed from being

300

A MANUAL OF DENTAL ANATOMY.

rudimentary in Thylacinus, probably absent altogether in

Dasyurus, and Phascolarctus, to being a large tooth retained

in full use till the animal is nearly full grown in Hypsiprymnus.

Within the group Carnivora, the dog and many others

FIG. 126

FIG. 127 (2).

have a thoroughly well developed set of milk teeth, which
do service for some time ; in the bear the milk teeth are
relatively smaller, and are shed very early ; in the seal the
milk teeth are rudimentary, functionless, and are absorbed
before birth, so that in the specimen figured the milk incisors
had already disappeared (see Fig. 127).

(*) Permanent and milk dentitions of a Dog ; the latter was well de-
veloped. Nat. size.

(2) Permanent and milk dentition of a seal (Phoca Greenlandia).
Nat. size.

THE TEETH OF MAMMALS.

301

In the elephant seal the milk teeth are yet more rudi-
mentary, and the difference between its dentition and that
of the monophyodont homodont cetacean (Grampus) is not

FIG. 128 f1).

great ; an observation which is the more interesting, inas-
much as this seal in other characters than its teeth ap-
proaches towards the cetacean group. From these facts,
which are well indicated in the accompanying figures, Pro-
fessor Flower argues that the permanent set of teeth of

(*) Permanent and milk dentition of an Elephant Seal (Cystophora
proboscidea).

(2) Teeth of the truly monophyodal Grampus (Orca capensis). (These
four figures are copied from Prof. Flower's paper).

302 A MANUAL OF DENTAL ANATOMY.

diphyodonts correspond to the single set of nionophyodonts,
so that the milk dentition, when it exists at all, is some-
thing superadded.

Whether this be so is a question difficult to determine ;
from the facts advanced by Professor Flower, while they
stood alone, most people would, with little hesitation, concur
with his conclusions ; but the history of the development of
the teeth interposes a fresh difficulty.

The tooth germ of the milk tooth is first formed, and the
tooth germ of the permanent tooth is derived from a portion
(the neck of the enamel germ) of the formative organ of the
milk tooth (see Fig. 67). Again, in most of those animals
in which there is an endless succession of teeth, such as the
snake, the newt, or the shark, each successive tooth germ is
derived from a similar part of its predecessor, the natural
inference from which would be that the permanent set,
being derived from the other, was the thing added in the
diphyodonts.

The question cannot be finally settled until we know more
of the development of the teeth of the monophyodont cetacea :
thus it might turn out that in them also there are abortive
germs of milk teeth formed, which do not go on so far as
calcification, but which do bud off, as it were, germs or per-
manent teeth ; if such should prove to be the case, this
would bring their teeth into close correspondence with those
of the elephant seal.

The investigation of these questions is further complicated
by the fact that there are quite numerous instances of " per-
manent " teeth, that is teeth unquestionably belonging to
the second set, which are shed off early, and do not remain
in place through the lifetime of the animal ; an example of
this is to be found in the Wart Hog (Phacochserus), which
loses successively all its premolars and the first and second
true molars, the last true molar alone being truly persistent.

Sometimes nothing but a careful comparison of the teeth

THE TEETH OF MAMMALS. 303

of allied creatures will enable us to decide whether a,,parti-
cular tooth is to be referred to the milk or to the permanent
series ; as occasionally teeth of the latter set are cut very
early, at a time when the milk teeth are all in place, and
are shed during adult life. Professor Flower gives as an
example of this, the first premolar of the hippopotamus.

CUVIER. Dents des Mammiferes.

DE BLAINVILLE. Osteographie. 1839—1864.

OWEN. Odontography. 1845.

GIEBEL. Odontographie. 1855.

FLOWER. Lectures on Odontology (British Med. Journal, 1871).

CHAPTER IX.

THE TEETH OF MONOTREMATA, EDENTATA, AND CETACEA.
MONOTBEMATA.

THE Echidna, or Spiny Ant-eater has no teeth whatever,
and the strange Ornithorhyncus (duck-billed Platypus) is
also destitute of true calcined teeth.

In the place of teeth its flattened bill is furnished with
eight horny plates, two on each side of each jaw. We
may therefore pass at once to the orders Edentata and
Cetacea, which it is convenient to take first, as their
dentitions are of that simple form designated by the term
"Homodont."

THE TEETH OF EDENTATA (BRUT A).

Sloths, Armadillos, Ant-eaters.

The term Edentata was applied to the animals of this
order to indicate the absence of incisors (teeth in the inter-
maxillary bone) : though this is true of most of them, a few
have some upper incisors, but the central incisors are in all
cases wanting.

Some of them are quite edentulous ; this is the case in
the Mutica, or South American Ant-eaters (Myrmecophaga
and Cyclothurus), in which the excessively elongated jaws
cannot be separated to any considerable extent, the mouth
being a small slit at the end of the elongated muzzle.
Food is taken in by the protrusion of an excessively long,
whip-like tongue, which is covered by the viscid secretion

THE TEETH OF EDENTATA. 305

of the great sub-maxillary glands, and is wielded with
much dexterity. The Manis, or Scaly Ant-eater is also
edentulous.

The Edentata belong to the monophyodont or homodont
section of Mammalia ; but, in some, certain teeth are more
largely developed than others, so that we have teeth which
might be termed canines ; and it has already been mentioned,
that one armadillo, at all events, is diphyodont.

The teeth are of simple form, and do not in any marked
degree differ in the different parts of the mouth, except
only by their size (to this the canine-like tooth of the two-
toed sloth is an exception). They are all of persistent
growth, and therefore no division of parts into crown, neck,
arid root is possible : they consist generally of dentine and
cement, with sometimes the addition of vaso-dentine, into
which latter tissue the central axis of the pulp is converted ;
while in some members of the order other peculiarities of
structure exist : thus in the Orycteropus (Cape Ant-eater),
dentine like that of Myliobates is found ; and in the Megathe-
rium hard dentine, a peculiar vaso-dentine, and richly
vascular cementum co-exist (see Fig. 43).

I am not aware that enamel has been seen upon the teeth
of any Edentate animal, but I found some years ago that
the tooth germs of the nine-banded armadillo were provided
with enamel organs ; this, however, proves nothing, for
(Philos. Trans., 1876) I believe the presence of enamel
organs to be universal and quite independent of any after
formation of enamel.

The teeth of the nine-banded armadillo (T. peba), will
serve to illustrate the character of the dentition of the
class. They are seven in number on each side of the jaw, of
roundish form on section, and those of the upper and lower
jaws alternate, so that by wear they come to terminate in
wedge-shaped grinding surfaces : before they are at all

306

A MANUAL OF DENTAL ANATOMY.

worn they are bilobed, as may be seen in sections of the tooth
germs.

In the accompanying figure the milk teeth are represented,
and beneath them their permanent successors : the divari-
cated bases of the milk teeth are due to the absorption set
up by the approach of their successors, and not to the

FIG. 130 0).

X2 N.S.

formation of any definite roots. Successional teeth have
been detected in this armadillo only (except also in T.
kappleri, which is perhaps a mere variety) ; but material
does not exist in our museums which would enable us to
positively deny their occurrence in other forms.

Professor Flower has failed to discover any succession of
teeth in the sloths, and I have myself, through the kindness of
the late Professor Garrod, examined microscopically the jaws
of a foetal Choloepus, in which the teeth were but little
calcified, and failed to detect any indication of a second set
of tooth germs. The probability is, therefore, that they are
truly Monophyodont.

In the armadillos the teeth are always of simple form and
about thirty- two in number, except in Priodon, which has as
many as a hundred teeth, a number altogether exceptional
among mammals.

Sloths have fewer teeth than armadillos, and these softer
in character, the axis of vaso-dentine entering more largely

(:) Lower jaw of a young Armadillo (Tatusia peba), showing the milk-
teeth (a) in place, and their successors (6) beneath them. From a specimen
in the Museum of the Royal College of Surgeons.

THE TEETH OF GET ACE A. 307

into their composition, and forming as much as half the
bulk of the tooth.

The Orycteropus, or Cape Ant-eater, the peculiarities of
whose teeth have already been alluded to, has about twenty-
six teeth in all ; the true ant-eaters are edentulous. The
teeth of some of the gigantic extinct Edentates were a little
more complex in form and structure ; thus the teeth of the
Glyptodon were divided by longitudinal grooves, which in
section rendered it trilobed ; and the teeth of the Mega-
therium were likewise marked by a longitudinal furrow.

In their persistent growth, uniformity of shape, and
absence from the inter-maxillary bone, they strictly con-
formed with the teeth of recent Edentata.

THE TEETH OF CETACEA.

No cetacean is known to develop more than one set of
teeth, and these, when present in any considerable numbers,
closely resemble one another in form.

They are usually composed of hard dentine, with an in-
vestment of cement; after the attainment of the full
dimensions of the tooth what remains of the pulp is very
commonly converted into secondary dentine ; tips, and even
entire investments of enamel, are met with in many of the
order.

The dentine of many Cetaceans, e. g. of the sperm whale,
is remarkable for the very numerous interglobular spaces
which it contains ; these are clustered in concentric rows,
so as to give rise to the appearance of contour lines. The
cement is often of great thickness, and the lacunae in it are
very abundant ; its lamination is also very distinct.

In the dolphin the teeth are very numerous, there being
about 200 ; they are slender, conical, slightly curved in-

x 2

308

A MANUAL OF DENTAL ANATOMY.

wards, and sharply pointed ; as they iriterdigitate with one
another there is very little wear upon the points, which
consequently remain quite sharp. The largest teeth are
those situated about the middle of the dental series.

FIG. 131 (i).

Many variations in the number and form of the teeth
are met with ; the porpoise has not more than half the

FIG. 132 (2)

number of teeth possessed by the dolphin, while the gram-
pus has still fewer. The teeth of the grampus become
worn down on their opposed surfaces, and coincidently
with their wearing away the pulps become calcined. In
the Oxford museum there is a grampus in which, owing to
a distortion of the lower jaw, the teeth, instead of inter-
digitating, became exactly opposed to one another ; the
consequence of this was that the rate of wear was greatly
increased, and the pulp cavities were opened before the

(*) Jaws of a common Dolphin.

(2) Teeth of upper jaw of a Grampus (after Professor Flower).

THE TEETH OF GET AGE A. 309

obliteration of the pulps by calcification,1 so that the pulps
died and abscesses around the teeth had resulted.

In the sperm whale the teeth are numerous in the lower
jaw, but in the upper jaw there are only a few curved,
stunted teeth, which remain buried in the dense gum. The
teeth of the lower jaw are retained in shallow and wide de-
pressions of the bone by a dense ligamentous gum, which,
when stripped away, carries the teeth with it. Every inter-
mediate stage between this slight implantation and the
well-developed stout sockets of the grampus, is met with in
the Cetacea.

In the bottle-nosed whale (Hyperoodon bidens) the only
large teeth present are two conical, enamel-tipped teeth
(sometimes four) which remain more or less completely
embedded within the gum, near to the front of the lower
jaw: in addition to these there are 12 or 13 very small
rudimentary teeth loose in the gums of both jaws. (Esch-
richt, Lacepede.)

In the narwal (Monodon monoceros) two teeth alone per-
sist, and these are in the upper jaw. In the female the
dental germs become calcified, and attain to a length of
about eight inches, but they remain enclosed within the
substance of the bone, and their pulp cavities speedily fill
up. In the male, one tusk (in some very rare instances
both) continues to grow from a persistent pulp till it at-
tains to a length of ten or twelve feet, and a diameter of
three or four inches at its base. This tusk (the left) is
quite straight, but is marked by spiral grooves, winding
from right to left. It is curious that in one of the speci-
mens, in which the two tusks had attained to equal and
considerable length, the spirals on the two wound in the
same direction ; that is to say, as regards the sides of the
head, the spirals were not symmetrical with one another.

(l) Trans. Odonto. Society, 1873. When I published this paper I was
not aware that Eschricht had previously published a similar observation.

310

A MANUAL OF DENTAL ANATOMY.

The tusk of the male narwal may fairly be assumed to
serve as a sexual weapon, but little is known of the habits
of the animal.

FIG. 133 0).

iNatSi*

Professor Turner has lately noted the occurrence of two
stunted incisor rudiments in a foetal narwal : these ob-
viously represent a second pair of incisors, and attain to a
length of half an inch, but are irregular in form ; they are
situated a little behind the pair of teeth which attain to

(*) Cranium of Narwal (Monodon monoceros). a. Stunted tooth, with
its basal pulp-cavity obliterated, b. Long tusk. The small figure, giving
the whole length of the tusk, shows the proportion which it bears to the
rest of the skull.

THE TEETH OF CETACEA. 311

more considerable dimensions. All trace of this second pair
of incisors is lost in adult skulls.

The Cetaceans classed together as Ziphoids have no teeth
in the upper jaw, and in the lower jaw only two (in a
single species there are four) teeth which attain to any
considerable size, though other rudimentary teeth have been
formed in the dense gum.

The structure of these teeth is very peculiar : a tooth of
a species of Ziphius in the Oxford University museum,
which was described by Professor Ray Lankester, consists in
great part of cementum and osteo-dentine, the true dentine
being merely a little fragment situated at the top, and not
forming more than a tenth of the whole bulk of the tooth.

Ziphius Layardii has teeth nearly a foot long projecting
upwards from the lower jaw, which arch towards one
another, above the upper jaw, so that they must prevent
the mouth from being opened to any considerable extent.

The whalebone whales are, in the adult condition, des-
titute of teeth, but prior to birth the margins of both upper
mid lower jaws are covered with a series of nearly globular
rudimentary teeth, which become calcined, but are speedily
shed, or, rather, absorbed.

The foetal teeth of the Balsenoptera rostrata have been
carefully described by M. Julin (Archives de Biologic, 1880),
the Balcenopteridce having been previously supposed to be
without rudimentary teeth. The ramus contained 41 tooth
germs, each furnished with an enamel organ and dentine
bulb, with a slight capsule; these were lodged in a con-
tinuous groove in the bone above the vessels, thus re-
calling the condition of the parts in a human embryo at a
certain stage. A very small amount of calcification takes
place, a mere film of dentine being formed upon the dentine
bulb. But what is very remarkable is that the dentine
bulbs are simple • near the front, bifid in the middle, and
trifid at the back of the mouth ; in other words, these

312 A MANUAL OF DENTAL ANATOMY.

rudimentary teeth seem not to be rudiments of a homo-
dont dentition as might have been expected, but of a
heterodont dentition ; and it is suggestive of a resemblance
to such forms as Squalodon, an extinct cetacean, peculiar in
having heterodont teeth. At all events it seems to indicate
that the homodont dentition of Cetacea is a case of degrada-
tion from ancestral forms, a conclusion likewise pointed to
by the gradual suppression of milk dentitions (see p. 300).

From the upper jaw of an adult whalebone whale there
hang down a series of plates of baleen, placed transversely to
the axis of the mouth, but not exactly at right angles to it.
The principal plates do not extend across the whole width of
the palate, but its median portion is occupied by subsidiary
smaller plates. The whalebone plates are frayed out at
their edges, so as to be fringed with stiff hairs, and their
fringed edges collectively form a concave roof to the mouth,
against which the large tongue fits, so as to sweep from the
fringes whatever they may have entangled. The whale in
feeding takes in enormous mouthfuls of water containing
small marine mollusca ; this is strained through the baleen
plates, which retain the Pteropods and other small crea-
tures, while the water is expelled. Then the tongue sweeps
the entangled food from the fringe of the baleen plates and
it is swallowed. Each plate consists of two dense but
rather brittle laminae, which enclose between them a tissue
composed of bodies analogous to coarse hairs. By the pro-
cess of wear the brittle containing laminae break away,
leaving projecting from the edge the more elastic central
tissue, in the form of stiff hairs.

Each plate is developed from a vascular persistent pulp,
which sends out an immense number of exceedingly long-
thread-like processes, which penetrate far into the hard
substance of the plate. Each hair-like fibre has within its
base a vascular filament or papilla : in fact, each fibre is
nothing more than an accumulation of epidermic cells, con-

THE TEETH OF CETAOEA. 313

centrically arranged around a vascular papilla, the latter
being enormously elongated. The baleen plate is composed
mainly of these fibres, which constitute the hairs of its frayed-
out edge, but in addition to this there are layers of flat cells
binding the whole together, and constituting the outer or
lamellar portion. As has been pointed out by Prof. Turner
(Proc. Roy. Soc. Edinburgh, 1870), the whalebone matrix
having been produced by the cornification of the epithelial
coverings of its various groups of papilke, is an epithelial or
epiblastic structure, and morphologically corresponds not
with the dentine, but with the enamel of a tooth.

The whole whalebone plate and the vascular ridges and
papillae which form it may be compared to the strong ridges
upon the palates of certain Herbivora, an analogy which is
strengthened by the study of the mouth of young whales
prior to the cornification of the whalebone.

CHAPTER X.

THE TEETH OF UNGULATA.

IN the two orders just considered, the Cetacea and
Edentata, a single set of teeth would seem to be the rule,
and most members of these orders are, so far as is known,
both monophyodont and homodont. But in all orders that
remain to be considered a Diphyodont dentition, the milk set
varying from the merest rudiments to full development, will
be the rule ; and being diphyodont, they are for the most
part heterodont, that is to say, the teeth differ from one
another, and we can distinguish them into incisors, canines,
premolars, and molars. Hence we are able to assign to them
a dental formula, and an extended survey of mammalian
forms lends strong support to the idea that the typical dental
formula, in which the full normal mammalian number of
teeth is present, is

.31 43

1 "3" c T prm T m 3~

Very many have less than this full number : only a few
have more ; and it is not a little interesting to find that
among extinct mammalia, and especially among extinct
ungulata, the typical dentition was more often present than
amongst recent animals. Indeed it may be said that most
mammals of the Eocene period had the full typical mam-
malian dentition.

THE TEETH OF VNGULATA. 315

Ungulata, or hoofed animals are grouped thus : —

(i.) Artiodactyles, or ) Hippopotamus. Pigs, Anoplotherium,&c.,
even-toed Ungulata j Cows, Sheep, Deer, and other Ruminants.

(ii.) Perissodactyles or) Hoi-ses, Tapirs, Rhinoceros, PalaBotherium,
Ungulata with an /•
odd number of toes )

The distinction between the two groups is strongly marked, if
living animals alone be considered ; but, as Professor Huxley has
pointed out, increasing knowledge of fossil forms is tending to
break down the line of demarcation.

The recent forms bear, in all probability, but a very small pro-
portion to the extinct Ungulata, of which our knowledge is as yet
but fragmentary ; though the discoveries of Professors Marsh and
Cope in the " mauvaises terres " of Wyoming have brought to light
a very large number of strange and interesting ungulates ; and this
fragmentary condition of our knowledge makes it as yet impossible
to give a connected account of the dentition of ungulates, seeing
that the forms known to us are only isolated and often widely
separated links in the chain.

The Teeth, of Perissodactyle Ungulates. — Perisso-
dactyle (odd-toed) Ungulates are far less numerous than the
even-toed section, and among recent animals only comprise
the Horse, the Rhinoceros, the Tapir, and their allies. Their
premolars, or at least the last three of them, are equally
complex in pattern with the true molars ; and canines, tusk-
like but not very large, are of frequent occurrence. The
lower molars of almost all perissodactyles have a character-
istic form, their grinding surfaces being made up of two
crescentic ridges.

The ungulate animals are all possessed of molar teeth,
which are kept in an efficient state of roughness by the
enamel dipping deeply into the crowns ; by the cusps, in
fact, being of very great depth. It consequently happens
that after the immediate apex is worn away, the flattened
working face of the tooth is mapped out into definite
patterns, which, on account of the light thus thrown upon
fossil remains, often consisting of little else than the teeth,
have been studied with great care. The result has been to

316 A MANUAL OF DENTAL ANATOMY.

establish a general community of type, so that, dissimilar
as they at first sight appear, it is possible to derive all, or
almost all, the configurations of their crowns from one or
two comparatively simple patterns. But odontologists are
not yet agreed, or rather do not yet know enough of the
vast number of extinct Ungulates which there is reason to
believe once existed (of which many have lately been dis-
covered) to decide with certainty what the parent pattern
was.

Rhinoceros. — It is difficult to assign a dental formula to
this genus, as the incisors are variable in the different
species, but all agree in the absence of canines.

9 n 4. 3

i^c-p 1ml.

2 0 l 4 3

In the African Rhinoceros, in which the adult has no
incisors, the young animal has eight incisors ; other species
retain the incisors through life ; and it is noteworthy that

9

in the Indian Rhinoceros, which has i — , the outer incisors

in the upper jaw, are, as is usual, the ones that are absent, but
in the lower jaw it is the central incisors which are missing.
The first premolar, just as in the Horse, is small, has no
milk predecessor, and is not long retained ; the other pre-
molars do not markedly differ from the true molars. The
premolars and molar teeth, though not differing much in
character, increase in size from before backwards. The
crowns of the teeth are of squarish outline, larger on their
outer than their inner side, and are implanted by four roots.
The pattern of their grinding surfaces is very characteristic;
but it will be best understood by first digressing to say a
few words on the dentition of the Tapir.
Tapir. — The dental formula is

3143

THE TEETH OF PERISSODACTYLE UNGULATA. 317

In a brief survey, like that to which the present work is
necessarily confined, it will suffice to mention that there is
no great peculiarity about the incisors, or the canines, save
that the lower canine ranges with the lower incisors ; behind
the canine comes an interval, after which come the pre-
molars and molars, which are interesting, as being of simpler
pattern than those of most Ungulates, and it will be necessary
to very briefly allude to the various patterns characteristic
of ungulate teeth, with a view of showing how they may
have been derived the one from the other.

In the Tapir four cusps are traceable, but ridges uniting
the two anterior and the two posterior cusps are strongly
developed, at the cost of the antero-posterior depression, i. e.
of one of the arms of the cross which separates the four cusps
in other quadricuspid molars. There is therefore left only a
deep transverse fissure (hence it is called a bilophodont
tooth), and the quadricuspid form is disguised. A low wall
on the outside of the tooth connects the two ridges.

In the Hog we have a simple four-cusped molar, with a

Fro. 134 J.

crucial depression separating the cusps ; in the Hippopot-
amus the same pattern is repeated, though not quite so
simply, as each cusp is fluted in a definite manner.

In Rhinoceros the two external cusps are united by a

(J) Grinding surfaces of upper molar series of a Rhinoceros, a. Posterior
sinus, which at a' has become an island, c. Posterior ridge, d. Anterior
ridge.

318 A MANUAL OF DENTAL ANATOMY.

longitudinal ridge, possibly the ciugulum, and the transverse
ridges become oblique ; consequently the valley between
the ridges c and d is also oblique in direction, and a
second valley " a " behind the posterior ridge is introduced
(Fig. 134).

The simplicity of the pattern is also departed from by
the margins of the ridges, and therefore the boundaries of
the depressions, being waved and irregular.

The lower molars of the Rhinoceros are made up of two
crescentic ridges, one in front of the other, with the hollows
turned inwards. It is less obvious how this pattern is
derived from that of the Tapir, but it may be that the trans-
verse ridges of the Tapir type of tooth may have become
curved and crescentic, so that the original outer edge of the
posterior ridge abuts against the exterior of the ridge in
front of it. The valleys between the processes of enamel
and dentine of the tooth of the Rhinoceros, termed " sinuses,'7
are not filled up solidly with cementum. The more complex
pattern which characterises the molar of the Horse may be
derived from a further modification of the Rhinoceros molar.

To use the words of Professor Huxley : " Deepen the
valley, increase the curvature of the (outer) wall and laminae
(transverse ridges), give the latter a more directly backward
slope ; cause them to develop accessory ridges and pillars ;
and the upper molar of the Tapir will pass through the
structure of that of the Rhinoceros to that of the Horse."

By a further increase in the obliquity of the ridges and
in their curvature (c and d), they become parallel to the
external or antero-posterior ridge (wall), and bend round
until they again touch it, thus arching round and completely
encircling the sinuses (a and the space between c and d) in
the Rhinoceros tooth. In this way the unsymmetrical pattern
of the Rhinoceros tooth may be supposed to become trans-
formed into the comparatively symmetrical one of the Horse
or of the ruminant.

THE TEETH OF PERISSODACTYLE VNGULATA. 319

The outer ridge or wall is in the upper molar of the horse
doubly bent, the concavities looking outwards. The trans-
verse ridges start inwards from its anterior end and from its
middle, and they curve backwards as they go to such an
extent as to include crescentic spaces (between themselves

FIG. 135 0).

and the outer wall). To this we must add a vertical pillar,
which is slightly connected with the posterior end of each
crescentic edge (this pillar is in Hipparion quite detached).

The lower molars of the horse present the double crescent,
just like those of the rhinoceros, save that vertical pillars
are attached to the posterior end of each crescent, thus
slightly complicating the pattern of the worn surface. The
interspaces of the ridges and pillar are in the horse solidly
filled in with cementum. The extinct ancestors of the horse
have the molar pattern considerably simplified, but yet
recognisable as being built up on the same model.

But in an elementary handbook such as this it will only
serve to perplex the reader to enter into a discussion of
the relative probabilities of the various and incompatible
explanations given of the homologies of the parts of the
ungulate molar : suffice it that such correspondences do exist,
and if we had before us perfect chains of molars from every

(*) Molar tooth of a Horse, showing the pattern of its grinding surface.

320 A MANUAL OF DENTAL ANATOMY,

ungulate which ever lived, there would be no doubt as to
the relationship of the various patterns : as it is, we are em-
barrassed by the lack of the material, which leaves gaps too
great to bridge over without some amount of speculation.
As it is, Professor Flower divides the principal varieties
(Phil. Trans., 1874,) into three :—

(i.) That in which the outer wall is feebly developed,
and transverse ridges become the prominent features, as in
the tapir.

(ii.) That in which the outer wall is greatly developed
and more or less smooth, the transverse ridges being oblique,
as in the rhinoceros.

(iii.) That in which the outer surface and edge of the
outer wall is zigzagged, or bicrescentic, as in the horse and
palseotherium.

Eqims. — The horse is furnished with the full mammalian
number of teeth, the dental formula being —

.3143

The canines, however, are rudimentary in the female, whilst
in the male they are well developed (in the gelding they are
of the same size as in the entire horse) ; and the first pre-
molar, which has no predecessor, is also rudimentary, and is
lost early. A considerable interval exists between the
incisors and the premolars and molars, which latter are very
similar to one another, both in shape, size, and in the
pattern of the grinding surface.

The incisors of the horse are large, strong teeth, set in
close contact with one another ; the teeth of the upper and
lower jaws meet with an "edge to edge bite," an arrange-
ment which, while it is eminently adapted for grazing, leads
to great wearing down of the crowns. An incisor of a horse
or other animal of the genus may be at once recognized by
that peculiarity which is known as the " mark."

THE TEETH OF PERISSODACTYLE UNGULATA. 321

From the grinding surface of the crown there dips in a
deep fold of enamel, forming a cul de sac. As this pit does
not extend the whole depth of the crown, and the in-
cisors of a horse are submitted to severe wear, the fold
eventually gets worn away entirely, and the worn surface
of the dentine presents 110 great peculiarity. But as this
wearing down of the crown takes places at something like a
regular rate, horse dealers are enabled to judge of a horse's
age by the appearance of the mark upon the different incisors.
The " mark " exists in Hipparion, but not in the earlier
progenitors of the horse.

FIG. 136 0).

FIG. 137

A. horse attains to its adult dentition very slowly ; the
first permanent incisors appear about the end of the third
year, and the other two pairs follow at intervals of about

Incisors of the Horse, showing the marks at various stages of wear.

322

A MANUAL OF DENTAL ANATOMY.

six months. As the rate of wear is equal, the mark gets
worn out soonest upon the central incisors (about the sixth
year) • in the middle incisors next (about the seventh), while
it has totally disappeared by about the eighth year.

After the " mark " is worn away the centre of the tooth
is marked by a difference of colour, due to the presence of
secondary dentine, into which the remains of the pulp has
been converted.

FIG. 138 (»).

The molars of the horse are remarkable for their great
length ; they do not grow from persistent pulps, but never-
theless they do go on growing until a great length of crown
of uniform diameter is made, subsequently to which the
short and irregular roots are formed. As the upper working
surface of the crown becomes worn, the tooth rises bodily in
its socket, and when by an accident its antagonist has been
lost, it rises far above the level of its neighbours. This
elevation of the tooth takes place quite independently of

(l) Side view of the dentition of a Stallion. At a short interval behind
the incisors are seen the canines ; then, after a considerable interval, the
premolar and molar series.

THE TEETH OF PERISSODACTYLE UNGVLATA. 323

growth from a persistent pulp, and, in fact, happens after
the formation of its roots.

The pattern of the horse's molar has been already de-
scribed ; it should be added that the last molar differs from
the rest in its posterior moiety being less developed than in
the other teeth.

As each ridge and each pillar of the tooth consists of den-
tine bordered by enamel, and the arrangement of the ridges
and pillars is complex ; as, moreover, cement um fills up the
interspaces, it will be obvious that an efficient rough grinding
surface will be preserved by the unequal wear of the several
tissues.

When a bit is put into a horse's mouth it rests in the
interval, or diastema, which exist between the incisors and
the commencement of the molar series, and the great con-
venience of the existence of such a space has led many
authors to assume that the horse was moulded in accordance
with man's special requirements, so that it might be suited
for its subserviency to his wants.

But the wide diastema appeared in the remote ancestors
of the horse long ages before man's appearance on the earth,
and the advocates of this theory of design would, as Professor
Huxley suggests, have to tell us what manner of animal rode
the Hipparion.

The milk teeth of all the Ungulata are very complete,
and are retained late ; they resemble the permanent teeth
in general character, but the canines of the horse, as might
have been expected, their greater development in the male
being a sexual character, are rudimentary in the milk
dentition.

To the Perissodactyle Ungulates which are specially inte-
resting on account of their dentition, must be added Homa-
lodontotherium, a tertiary mammal, the remains of which were
described by Professor Flower (Phil. Trans., 1874).

It had highly generalised characters ; its teeth were

Y 2

324 A MANUAL OF DENTAL ANATOMY.

arranged without any diastema, and the transition in form
from the front to the back of the mouth was exceedingly
gradual, so that no tooth differed much from those on either
side of it. Taking the pattern of its molar teeth alone into
account, it would have been without hesitation declared to
be very nearly allied to rhinoceros, on which type they
are formed, but the resemblance fails in the canine and
incisor region, and it must be considered to be one of those
generalised types related to rhinoceros, to Hyracodon and
perhaps connecting them with such aberrant forms as
Toxodon.

The largest of Perissodactyles equalled the elephant in
size, and have been named by Prof. Marsh BrontotJieridce.
The dental formula was

21 43

1 _ o _ pm _m_.

The incisors were small and sometimes deciduous, and the
canines short and stout, the lower being the more conspicuous
owing to its being separated by a slight diastema from the
premolars, which is not the case in the upper jaw.

The premolars in both jaws increase in size from before
backwards, and do not differ from the molars next them. In
the lower jaw the premolars and molars all consist of two
crescents, save the last, which have three crescentic cusps.
The molar teeth stand apart from those of any recent peris-
sodactyles in their huge size, the squarish last upper molar,
for example, measuring four inches antero-posteriorly and
more than three transversely (Prof. Marsh, American Journal
of Science and Arts, 1876).

THE TEETH OF ARTIODACTYLE UNGULATA. 325

THE TEETH OF ARTIODACTYLE UNGULATA.

Artiodactyle, or even-toed Ungulata, comprise pigs, hippo-
potami, camels, sheep, oxen, &c., amongst living animals.

They are divided into Ruminant and Non-ruminating animals :
the latter group, equivalent to the "Suina" of the table on
page 265, includes the Pigs (Shtidai), Hippopotanndce , and Ano-
plotheridfs.

The Ruminants are divided into three groups : (i.) The Tragu-
I'ldce (small deer of Southern Asia), which form a connecting link
between the Anoplotherium (itself a link between the Pigs and the
true Ruminants) and the Pecora ; (ii.) Pecora (sheep, oxen, &c.) ;
(iii.) Camelidce.

In Artiodactyle Ungulata the premolars differ markedly
both in size and pattern from the true molars.

Of those Artiodactyle Ungulates which are not ruminants
the common pig may be taken as an example.

3133
The dental formula is i — c — p — m — .

The position of the upper incisors is peculiar, the two
central upper incisors, separated at their bases, being inclined
towards one another so that their apices are in contact ; the
third pair are widely separated from the inner two pairs of
incisors. The lower incisors are straight, and are implanted
in an almost horizontal position : in both upper and lower
jaws the third or outermost incisors are much smaller than
the others.

The lower incisors are peculiar in having upon their upper
surfaces a strongly pronounced sharp longitudinal ridge of
enamel, which gets obliterated by wear.

An interval separates the incisors from the canines, which
latter are very much larger in the male than in the female,
and in the wild boar than in the domesticated animal.
Castration arrests the further development of the tusks ; the
peculiarities as to size and direction which characterise the

326

A MANUAL OF DENTAL ANATOMY.

tusks of the adult animal are not represented in the canines
of the milk dentition, about which there is not much that

is noteworthy, save that the young pig has dec. m — , of

which the first remains in place till the permanent dentition
is nearly complete, and then falls out without having any
successor ; or it may perhaps be regarded as a permanent
tooth which has had no predecessor.

The form and direction of the canines are alike peculiar ;

FIG. 139

i Nat. Size

the upper canine, which in its curvature describes more
than a semicircle, leaves its socket in a nearly horizontal
direction, with an inclination forwards and outwards. After
rounding past the upper lip its terminal point is directed
upwards and inwards. The enamel upon the lower surface
of the tusk is deeply ribbed : it does not uniformly cover the
tooth, but is disposed in three bands. The lower canines
are more slender, of much greater length, and by wear
become more sharply pointed than the upper ones : they
pass in front of the latter, and the worn faces of the two
correspond.

The lower canine is in section triangular, one edge being

(i) Upper and lower teeth of Wild Boar (Sus scrofa). In this specimen
the tusks are not so largely developed as they sometimes may be seen to be;

THE TEETH OF ARTIODACTYLE UNGULATA. 327

directed forwards, and its sides being nearly flat. Enamel
is confined to the internal and external anterior surfaces ;
the posterior surface, which plays against the upper canine,
is devoid of enamel ; the tooth is kept constantly pointed by
the obliquity with which its posterior surface is worn away.
The tusks of a boar are most formidable weapons, and are
capable of disembowelling a dog at a single stroke, but they
are greatly exceeded by those of the African wart-hog (Phaco-
chcerus), which attain to an immense size.

In the domestic races the tusks of the boars are much
smaller than in the wild animal, and it is a curious fact
that, in domestic races which have again become wild the
tusks of the boars increase in size, at the same time that
the bristles become more strongly pronounced. Mr. Darwin
suggests that the renewed growth of the teeth may perhaps
be accounted for on the principle of correlation of growth,
external agencies acting upon the skin and so indirectly
influencing the teeth.

As in most artiodactyles, the teeth of the molar series
increase in size from before backwards : thus the first pre-
molar or milk molar has a simple wedge-shaped crown, and
two roots ; the second and third by transitional characters
lead to the fourth premolar, which has a broad crown with
two principal cusps, and has four roots.

The first true molar has four cusps divided from one
another by a crucial depression ; and the cingulum in front,
and yet more markedly at the back, is elevated into a
posterior transverse ridge. In the second molar the trans-
verse ridge is more strongly developed, and the four cusps
are themselves somewhat divided up into smaller accessory
tubercles.

The last molar measures, from front to back, nearly twice
as much as the second ; and this great increase in size is
referable to a great development of the part corresponding
to the posterior ridge or cingulum of the second molar,

328

A MANUAL OF DENTAL ANATOMY.

which has become transformed into a great many subsidiary
tubercles.

That such is a correct interpretation of its nature is indi-
cated by our being able to trace the four principal cusps,
though modified and not divided off, in the front part of the
tooth, of which, however, they do not constitute more than
a small part. Those Ungulates in which the surfaces of the
molar teeth are covered by rounded or conical cusps, are
termed " bunodonts," in contradistinction to those which
present crescentic ridges on the masticating surface of their
molars, and which go by the name of " selenodonts."

In the Wart-hog (Phacoehrerus), the genus with very large

canines, the disproportion between the last true molar and
the other teeth is yet more striking.

(*) Upper and lower teeth of Phacochoerus. In the tipper jaw, the last
two premolars, and the much- worn first true molar remain. In the lower
all have been shed off, save the last two true molars. From a specimen
in the Museum of the Royal College of Surgeons.

THE TEETH OF ARTIODACTYLE UNGULATA. 329

In antero-posterior extent the third molar equals the first
and second true molars and the third and fourth premolars
(the whole number of teeth of the molar series possessed by
the animal) together.

When a little worn its surface presents about thirty islands
of dentine, surrounded by rings of enamel, the interspaces
and the exterior of the whole being occupied by cementum.
Of course, prior to the commencement of wear, each of these
islands was an enamel-coated cusp.

The Wart-hog's dentition has, however, another instructive
peculiarity ; the first true molar is in place early, and be-
comes much worn down (this is true, in a less degree, of the
common pig, and indeed^of most Ungulata). Eventually it
is actually shed ; the same fate later befalls the third pre-
molar and second true molar, so that the dentition in an
aged specimen is reduced to the fourth premolar and the
third true molar alone, and eventually to the last true
molars alone. Thus, both in the great complexity of the
back molars and the fact that the anterior teeth are worn
out and then discarded, the Wart-hog affords a parallel to
the anomalous dentition of the elephant.

As has already been noticed, the upper canines in the
boar turn outwards and finally upwards, so as to pass out-
side the upper lip ; this peculiarity in direction, yet more
marked in Phacochcerus, attains its maximum in the Sus
babirussa.

This creature, strictly confined to the Malay Archipelago,
where it frequents woody places, has (in the male) the upper
and lower canines developed to an enormous extent. The
upper canines are turned upwards so abruptly that they
pierce the upper lip, instead of passing outside it as in
other Suidce, preserve a nearly upright direction for some
little distance, and then curve backwards, so that their
points are directed almost towards the eyes.

The lower canines are less aberrant in direction and in

330 A MANUAL OF DENTAL ANATOMY.

shape, being somewhat triangular in section, but they also
are of very great length, and pass upwards, far above the
level of the snout ; their points are also directed backwards,

FIG. 141 (').

but have in addition an outward inclination. The canines
are devoid of enamel, and grow from persistent pulps, a
fact which sometimes has a disastrous result, for the tip
of the tooth, occasionally taking a wrong direction, re-enters
the head or the jaws of the animal.

Their length is very great; the animal is smaller than
the domesticated pig, but its canines attain a length of
eight or ten inches. Their use is a matter of conjecture;
the position of the upper tusks has suggested the idea that
they may serve as a protection to the creature's eyes, as it
seeks its food, consisting of fallen fruits, amongst the
brushwood. But were that the case the female also would
probably have them, which is not the case ; and although

(*) Skull of Sus babirussa (male). The upper incisors have been lost
from the specimen figured : they are much like those of a pig.

THE TEETH OF ARTIODACTYLE UNGULATA. 331

in old animals they are often broken off, it is not certain
that they are much employed in fighting. Its other teeth
are in no respects remarkable.

Hippopotamus. — The dental characters, as well as others,
indicate the affinity of the Hippopotamus to the Suidce.

The incisors are tusk-like, and bear but little resemblance to
those of most other mammalia ; they are nearly cylindrical,
bluntly pointed at their apices by the direction of wear,
which is in some measure determined by the partial distri-
bution of the enamel, which is laid on in longitudinal
bands in the upper teeth, but merely forms a terminal
cap on the lower incisors.

The upper, standing widely apart, are implanted nearly
vertically : the lower incisors, of which the median pair are
exceedingly large, are implanted horizontally.

The canines are enormous teeth; the lower, as in the
Hog, is trihedral, and is kept pointed in the same manner ;
the upper canines are not so long, and the portion exposed
above the gum is but short.

The incisors and canines are all alike teeth of persistent
growth.

The premolars, of which the first is lost early (being
perhaps a milk molar like the similar tooth in the pig)
are smaller and simpler teeth built up on the same type
as the true molars.

These latter, especially when worn, have a very charac-
teristic double trefoil pattern ; the four cusps, in the first
instance, were separated by a deep longitudinal and a still
deeper transverse groove; each cusp was, moreover, tri-
lobed; the first result of wear is to bring out the appear-
ance of four trefoils ; next, when the longitudinal furrow is
worn away, two four-lobed figures result ; and finally all

332 A MANUAL OF DENTAL ANATOMY.

pattern becomes obliterated, and a plain field of dentine
surrounded by enamel alone remains.

The teeth of the Hippopotamus are subject to a great
amount of attrition, as is well shown by a specimen pre-
sented to the museum of the Odontological Society, in which
the molar teeth are all excessively worn. The Hippopotami
use their incisors and canine tusks for the purpose of up-
rooting aquatic plants, of which their food mainly consists :
the roots of these are of course mixed up with much sand,
which wears down the teeth with great rapidity. The larger
incisors and the canines are, and for centuries have been,
articles of commerce, the ivory being of very dense substance
and useful for the manufacture of small objects.

Anoplotherici.se are an extinct (Eocene and Miocene)
family, linking together the Pigs and the Pecora.

Fio. 142 (i).

Anoplotherium is a genus of interest to the odontologist
because fcit possessed the full typical mammalian dentition,
as far as the number of the teeth went; the teeth were
of nearly uniform height, none strongly differentiated from
those nearest to them ; and they were set in close contiguity
with one another, so that there was no " diastema."

The lower molar teeth of the anoplotherium are built up

J) Side view of the dentition of Anoplotherium (after Owen).

THE TEETH OF ARTIODACTTLE UNGULATA. 333

on the same type as those of the rhinoceros (page 317),
and present the double crescent ; the upper molars are also
referable to the same fundamental forms, though the dif-
ference is greater. The laminse (transverse ridges) oblique
in the rhinoceros, are in anoplotherium still more oblique, so
that they become more nearly parallel with the outer wall,
and an accessory pillar is developed at the inside of the
anterior laminae.

Not very widely removed from the anoplotherium is the
Oreodon, an Ungulate of Eocene age.

Like a good many tertiary Ungulates (both artiodactyle
and perissodactyle) it had the full typical number of teeth,
forty -four ; but its interest to the odontologist is enhanced
by the Co- existence of strongly marked canines with molars
very much like those of ruminants, a group almost always
devoid of canines.

In the upper jaw oreodon had

.31 43
i — c pni m —

i.e. the typical number of each kind of teeth. But in the
lower jaw the first four teeth are like incisors, and the
tooth which is like a canine is not the tooth corresponding
to the upper canine, but to the small upper first premolar.

(*) Upper and lower teeth of Oreodon Culbertsonii after Leidy (Smith-
onian Contributions, 1852).

334 A MANUAL OF DENTAL ANATOMY.

This is a fair illustration of the fact that although in
nature it is generally the same tooth which is modified to
perform the function of a canine, it is not invariably the
same ; for here in the same animal are two different teeth
in the upper and lower jaw thus respectively modified.

And as they are different teeth, it happens that the upper
canine closes in front of the lower.

There is reason to believe that there was some difference
in the size of canines between the male and female oreodon.

The hollow-horned ruminants (sheep and oxen and ante-
lopes), and likewise almost all the solid horned ruminant
(deer) have the following dental formula : —

.00 3 3
1 3 C TiP3-m3-

The lower incisors are antagonised not by teeth, but by
a dense gum which clothes the fore part of the upper jaw ;
if a sheep is watched as it feeds, it will be seen to grasp the
blades of grass between the lower teeth and the gum, and
then to tear them off by an abrupt movement of the head,
as it would be impossible for it to, strictly speaking, bite
it off.

The anomaly of the entire absence of upper incisors was
held to have been diminished by the statement of Goodsir,
who believed that uncalcified tooth germs were to be found
in the foetuses of many species. As this was precisely what
might have been expected, it has since that time passed
current as an established fact ; but recently M. Pietkewickz,
working in the laboratory of M. Ch. Robin, has absolutely
denied the occurrence of even the earliest rudiments of
tooth germs in this situation, after an examination of a
series of foetuses of the sheep and cow, ranging even from
the earliest periods. (Journal d' Anatomic, par C. H. Robin,
1873, p. 452.) Since meeting with this statement I have
had no opportunity of verifying this matter myself.

THE TEETH OF ARTIODACTYLE UNGULATA. 335

Grouped with the six incisors of the lower jaw, and in no
respect differing from them, rise the pair of teeth which are
very arbitrarily termed " canines." As I cannot attempt to
do more in these pages than give the most bare outline of
generally well-known facts, I have retained the usual dental

formula, i -~ c ~-r ; though under protest, as I do not con-
o 1

sider the " canine " to have any such distinct existence as
would justify our calling a tooth which is so obviously refer-
able to the incisors by any distinctive name.

Although the absence of upper canine teeth is a very
general characteristic of ruminants, rudimentary canines
exist in some deer, and I am indebted to the kindness of
Sir Victor Brooke, a high authority upon the Cervidce, for
the following : —

" The upper canines are present in both sexes in all the
species of cervidce, with the exception of Alces, Rangifer,
Dama, some smaller species of Rusa, Axis, Capreolus, Caria-
cus, Blastocerus, Coassus, and Pudu. The upper canines,
when present, are with the notable exception of Moschus,
Elaphodus, Cervulus, and Hydropotes, small laterally com-
pressed rudimentary teeth. Their crowns are in about the
same stage of reduction as the crowns of horses' canines,
but their roots are relatively nmch more reduced." Hence
they are often lost in dried skulls, and it has generally been
supposed that but few deer possessed canines at all.

The hornless musk deer possesses upper canines of most
formidable dimensions, while the female has very small
subcylindrical canines.

The male pigmy musk deer (Tragulus) has large canines
of persistent growth, the female small canines with closed
roots.

The Indian Muntjac deer (Cervulus) has somewhat small
horns, which are perched upon persistent bony pedicles,
and it has upper canines which are curved outwards from

336 A MANUAL OF DENTAL ANATOMY.

beneath the upper lip, much as are the tusks of a boar ;
they do not, however, grow from persistent pulps, and are
absent in the female.

Cuvier first pointed out that there was a relation between
the presence of horns and the absence of canine teeth ; the
latter, serving as weapons for sexual combat solely, and

FIG. 144 (i).

being, probably, in no other way of service to the animal,
are not required by an animal provided with powerful antlers
or horns, whereas the absolutely hornless musk deer would
be totally unprovided with weapons of offence were it not for
his canines. To the musk deer and the muntjac must be
added Swinhoe's water deer, Hydropotes inermis, and Michie's
deer, Elaphodus cephalopus, another small hornless species,
of which the males are furnished with formidable canine
teeth.

Although, with the foregoing exceptions, all the deer,
oxen, sheep, antelopes, and the giraffe, animals constituting
the greater number of the " Ruminantia," are without canine
teeth, yet in the remaining family, the Camelidce, tusk-like
canines are met with.

It is a character of the Artiodactyle Ungulata that the

(*) Cranium of male Musk Deer (Moschus moschiferus).

THE TEETH OF ARTIODACTYLE UNGULATA. 337

premolar teeth are of decidedly simpler form than the
molars ; indeed in the ruminants the premolars may be
said each to correspond to one half of a true molar.

The dentition of the ordinary ruminant having been
more or less illustrated by the example of the musk deer
(minus its great canines), the Camel may be selected as
illustrating the peculiarities of the molar series.

The Camel is possessed of an upper incisor, and, as has
already been noticed, of canines.

The first two pairs of upper incisors are absent, but the

FIG. 145 I1).

third or outermost pair are present, and are rather caniniforni
in shape. In quite young skulls six upper incisors are present,
but the two inner pairs are lost very early. The canines are
strong pointed teeth, and the lower canine stands well apart
from the three incisors of the lower jaw, unlike the fourth
tooth in front of the mandible of typical pecora (see Fig.
115).

(*) Upper and lower teeth of a Camel.

338 A MANUAL OF DENTAL ANATOMY.

The first preinolars are absent altogether ; the second
premolars, following the canines after an interval, are
pointed caniniform teeth. The third premolar is some-
times lost early, but the fourth persists.

The molars of the Camel are of the " Selenodont " type ;
their derivation from forms already alluded to will be
sufficiently obvious to the reader who has mastered the
descriptions, and their double crescentic crowns, may
be taken as fair examples of simple ruminant patterns,
accessory pillars, &c., being added in some of the other
groups.

In all true Ruminants the last true molar of the lower
jaw has a third lobe (1), and the line of the outer surface of
the row of teeth is rendered irregular by the anterior edge
of each tooth projecting outwards slightly more than the
posterior border of the one in front of it. And the devia-
tions in the patterns of the surfaces of the molar teeth are
so constant and so characteristic that, although the common
ruminant pattern is preserved in all, it is often possible to
refer an individual tooth to its right genus.

The Ruminants all have a well-developed milk dentition,
which serves the animal for a long time, indeed until after
it has attained to its adult dimensions ; thus a sheep has not
completed the changing of its teeth till the fifth year, and
a calf till the fourth year. But the first permanent molar
is in them, as in so many other animals, the first of the
permanent set to be cut, and comes up in its place at the
sixth month (in the lamb), and hence has a long period of
wear before any of the other second teeth are cut. Conse-
quently the first permanent molar is, as is seen in Fig. 145,
invariably worn down to a much greater extent than the
other permanent teeth ; in the specimen figured it has been

(;) Sir Victor Brooke informs me that Neotragus hemprichii, a small
Abyssinian antelope, has only two lobes to the third lower molar.

TOXODONTIA. 339

worn down below the inflections of enamel, so that it has
lost its roughened grinding surface, and is reduced to a
smooth area of dentine.

Not much is known of the structure of the dental tissues of
the Ungulata which calls for mention in an elementary work.
The thick cement of the crown of the teeth of the Horse,
and indeed of most of the group which possess thick cement,
contains many " encapsuled lacunse," and is developed from
a distinct cement organ of cartilaginous consistence (see
page 144).

TOXODONTIA.

The existing ungulate animals form only a small propor-
tion of those once peopling the earth, and many extinct forms
have been discovered, which while having affinities with the
Ungulata, can yet hardly be classified under any existing
order. For example, Toxodon, a creature equalling the
Hippopotamus in size, which was discovered by Mr. Darwin
in late tertiary deposits of South America, has a dentition
recalling in some respects the Bruta, in others the Rodents.

It possessed in the upper jaw two pairs of incisors, the
median pair small, the outer exceedingly large, with per-
sistent pulps, and long curved sockets extending back to the
region of the molars, just as in existing Rodents.

In the lower jaw there were three pairs of incisors, sub-
equal in size, and growing from persistent pulps ; they
resemble the incisors of Rodents in having a partial invest-
ment with enamel, but differ from them in being prismatic
in section, and in having the enamel disposed on two sides
of the prism.

The molars were also very remarkable ; they grew from
persistent pulps, and had curved sockets, but the curvature
of these was in the reverse direction to that which obtains

z 2

340 A MANUAL Of DENTAL ANATOMY.

in Rodents, i.e., the convexity was outwards, and the apices
of their roots almost met in the middle line of the palate ;
it was this peculiarity that suggested the name.

Another peculiarity in the molar teeth, in which they
stand quite alone, is that, like incisors, they have a partial
investment with enamel ; those referred to the premolar
series having it confined to their outer surfaces, while the
three back teeth of the molar series had a plate also laid on
to their inner surfaces ; there were seven molar teeth above,
and six below.

In the interval between the incisor and molar series
canines have been found in the lower jaw ; they were sharp
edged, and had a partial distribution of enamel over their
surface. In an upper jaw alveoli for canines were found, but
the teeth themselves are not known.

DINOCERATA.

In the same region which yielded the toothed birds
(Eocene formations of Wyoming), the remains of many huge
animals have been discovered, for which new orders have been
proposed by Prof. Marsh (" American Journal of Science and
Art," 1876), it being impossible to classify them under any
existing order. The Dinocerata were creatures nearly as
large as Elephants, and presenting some general resemblance
to them in general form ; they were remarkable for the
relative smallness of their brains, which could apparently
have been drawn through the canal of the vertebral column.
They present points of resemblance to the Perissodactyle
Ungulata, and also to the Proboscidea, to which they were
at first referred, though their affinities are rather with the
former.

The dental formula was
.0

DINOCERATA.

341

In Prof. Marsh's words " The superior canines are long,
decurved, trenchant tusks. They are covered with enamel,
and their fangs extend upwards into the base of the maxil-
lary horn-core. There is some evidence that these tusks
were smaller in the females. Behind the canines there is a
moderate diastema. The molar teeth are very small. The
crowns of the superior molars are formed of two transverse
crests, separated externally, and meeting at their inner
extremity. The first true molar is smaller in this specimen
than the two preceding premolars. The last upper molar is
much the largest of the series.

" The lower jaw in Dinoceras is as remarkable as the skull.

FIG. 146

Its most peculiar features are the posterior direction of the
0) Upper and lower jaws of Dinoceras (Marsh).

342 A MANUAL OF DENTAL ANATOMY.

condyles, hitherto unknown in Ungulata, and a massive
decurved process on each ramus extending downward and
outward below the diastema.

"The position of the condyles was necessitated by the long
upper tusks, as, with the ordinary ungulate articulation, the
mouth could not have been fully opened. The low position
of the condyle, but little above the line of the teeth, is also
a noteworthy character. The long pendant processes were
apparently to protect the tusks, which otherwise would be
very liable to be broken. Indications of similar processes
are seen in Smilodon and other Carnivores with long upper
canines. With the exception of these processes the lower
jaw of Dinoceras is small and slender. The symphysis is
completely ossified. The six incisors were contiguous, and
all directed well forward. Just behind these, and not
separated from them, were the small canines, which had a
similar direction. The crowns of the large molars have
transverse crests, and the last of the series is the largest."

It would appear possible that the eminences shown in the
figure, and spoken of as " maxillary horn-cores," may be
merely the extended sockets of the teeth, which would other-
wise have had an implantation inadequate to their length ;
they are, however, described as solid, except at their bases,
where they are perforated for the fang of the canine tusk,
which would look as though they were truly horn-cores ',
moreover the Brontotheridae had horn-cores equally peculiar
in position (i.e., on the maxillary bones).

Yet another new order, TILLODONTIA, comprising several
genera, has been proposed by Prof. Marsh for the Wyoming-
fossil remains, to receive forms which, though not amongst the
biggest, are " amongst the most remarkable yet discovered in
American strata, and seem to combine characters of several
distinct groups; viz., Carnivora, Ungulata, and Rodentia.
In Tillotherium, Marsh, the type of the order, the skull has
the same general form as in the Bear, but in its structure

DINOCERATA.

343

resembles that of the Ungulata. Its molar teeth are of the
ungulate type, the canines are small, and in each jaw there
is a pair of large scalpriform incisors, faced with enamel and
growing from persistent pulps, as in the Rodents. The
second pair of incisors are small, and have not persistent
pulps. The adult dentition is as follows —

" There are two distinct families, Tillotkeridce (perhaps
identical with Anchippodontidce), in which the large incisors

FIG. 147 i1).

571.5.

grew from persistent pulps, while the molars had roots ; and
the Stylinodontidas^ in which all the teeth have persistent
pulps.

One genus (Dryptodon), known only by the lower jaw, had
six teeth, described as " clearly incisors," the two inner pairs
of which are small and cylindrical, the outer of enormous
size, faced in front only with enamel, and with persistent
pulps carried back under the premolars.

C) Upper and lower jaws of Tillotherium (Marsh).

CHAPTER XL

THE TEETH OF SIRENIA, HYRACOIDEA, PROBOSCIDEA, AND
RODENTIA.

THE TEETH OF SIRENIA.

MORE nearly connected with the Ungulata than with any
other order, but still rather widely removed from them,
stands the limited order of Sirenia, aquatic mammals
formerly termed Herbivorous Cetacea, a term rather ob-
jectionable, as they are not very nearly allied to the true
Cetacea.

The order is now represented by two genera only, the
Dugongs (Halicore) and the Manatees (Manatus), but a
third genus (Rhytina) has only become extinct within about
a century. Their teeth, and other points in their organiza-
tion indicate that they are more nearly allied to the Ungu-
lata than to any other group, though their peculiarities are
such as to elevate them to the rank of a distinct order.
They are of large size, and frequent shallow water, such
as the mouths of great rivers, their food consisting of sea-
weed and aquatic plants.

The dentition of the Dugong is in several respects a very
interesting one : the front part of the upper jaw, consisting
in the main of the intermaxillary bones, bends abruptly
downwards, forming an angle with the rest of the jaw.
This deflected end of the jaw carries two tusks, of each of
which the greater part is buried within the alveolus. The
tusk has an investment of enamel over its front and sides,
but on the posterior surface of cementum only, so that in

THE TEETH OF SIRENIA.

345

the disposition of the three structures it recalls the charac-
teristics of a Rodent incisor, like which it is worn away
obliquely so as to keep a constantly sharp edge, and like
which it grows from a persistent pulp.

In the female, the tusks (incisors) do not project from the

FIG. 143 ]).

gum, their pulp cavities are closed, and the investment of
enamel is complete over the crown of the tooth.

The sloping surface of the upper jaw is opposed by the
region of the symphysis of the lower jaw, which is of un-
usual depth. In this deflected part of the lower jaw there
are eight, or ten (four or five on each side) shallow and
rather irregularly-shaped sockets, in which curved distorted

(!) Side view of cranium and lower jaw of a Dugong (Halicore Indicus).
From a specimen in the Museum of the Royal College of Surgeons. The
surface of the deflected portion of the lower jaw, with its sockets for
rudimentary teeth, shown both in front and in profile view, is indicated by
the letter a ; the corresponding surface of the upper jaw by the letter I.

346 A MANUAL OF DENTAL ANATOMY.

teeth may be found in a fresh specimen, but it must not
be from too aged an animal, as they become eventually eaten
away by a process of absorption.

These abortive teeth are excellent examples of rudi-
mentary teeth, as not only are they stunted, and even
ultimately removed by absorption, but they are actually
covered in by a dense horny plate which clothes this part
of the jaw, and so are absolutely functionless (J).

These horny plates, in their structure analogous to whale-
bone, are possessed also by the Manatee and Khytina ; on
the free surface they are beset with stiff bristles, and are
throughout built up of hair-like bodies welded together by
epithelium.

Behind the region covered in by the horny plates, the
Dugong has five molar teeth on each side, of simple form,
like those of the Edentata, and consisting of dentine and
cementum only.

By the time the last molar is ready to come into place,
the first of the series is being removed by absorption of its
root and of its socket. In aged specimens only two molars
remain on each side of the jaws.

The Dugong is also peculiar as having a single deciduous
tooth : namely, a predecessor to the incisive tusks ; but it
has been doubted whether it be not rather a rudimentary
incisor than a milk tooth.

The molar teeth of the Manatee are much more nume-
rous and more complex in form, and they approach to the
configuration of the teeth of the Tapir very closely.

The Manatee has as many as forty-four molars, which
are not, however, all in place at one time, the anterior ones
being shed before the posterior are come into place ; no

(*) Similar rudimentary teeth are found in the corresponding deflected
part of the jaw of the young Manatee, to the number of twelve. (Gervais,
"Histoire Nat. des Mammiferes," vol. ii., p. 312.)

THE TEETH OF SIRENIA. 347

vertical succession is known to occur among them. There
are no incisors nor canines, but there are horny plates in
the front of the mouth like those of the Dugong.

The extinct Rhytina, formerly abundant about Behring's
Straits, was altogether without teeth.

It has been mentioned that the teeth of the Manatee are
tapiroid in external form ; they also possess peculiarities in
minute structure, which are unusual in mammalian teeth, but
which are common to them and to the Tapirs. In examining
some teeth, I found that the dentine, to all intents and pur-
poses, of the hard unvascular variety, was permeated by a
system of larger, or "vascular" canals, which were ar-
ranged with much regularity, and passed out from the pulp
cavity to the periphery of the dentine, where they commu-
nicated with one another. The dentinal tubes did not
radiate from these vascular canals ; they, so to speak, take
no notice of them, so that there is an ordinary unvascular
dentine with a system of capillary- convey ing channels as
well. It is interesting to find that the primd facie external
resemblance of the teeth of the Tapir is fully borne out by
minute histological structure, and it certainly suggests that
the resemblance is not accidental, but has some deeper
significance.

The enamel of the Manatee is also somewhat remarkable
for the absolute straightness of its enamel prisms in many
parts of the tooth.

The molar teeth of the Dugong consist of a central axis
of vaso-dentine, a much larger mass of ordinary unvascular
dentine, and a thick layer of cementum, but they do not
share the peculiarities of the Manatee's tooth.

348

A MANUAL OF DENTAL ANATOMY.

THE TEETH OF HYRAC01DEA.

The Biblical coney (Hyrax), an animal as large as a
rabbit, must not be passed over without mention, as its
dentition has been indirectly the source of much contro-

versy. So far as the pattern of its molar teeth goes, it
corresponds closely with Rhinoceros, and was hence classed
in. close proximity to that genus by Cuvier. But a more
extended survey of its characters has led to its being placed
in a separate order; it is a good example of the danger
which attends relying iipon any single character, such as
the pattern of the teeth, as being alone a sufficient basis for
classification.

O Skull of the Hyrax.

THE TEETH OF PROBOSCIDEA. 349

All observers, however, are not agreed as to its position ;
it certainly presents affinities with Perissodactyla, and also
with the Rodents, also, perhaps, with the Insectivora. The

dental formula is i ~ c — prm — m — .

Seen from the side, the dentition bears some resemblance
to that of a Rodent, because the large size of its central
incisors, which grow from persistent pnlps, are chisel-edged,
and are furnished with a very thick coat of enamel on their
anterior faces : the second pair of incisors, which are small,
are soon lost. But Hyrax has the full typical number, pre-
molars and molars, and the patterns of the teeth are closely
similar to those of the Rhinoceros.

In the lower jaw the middle incisors are small, and the
outer ones largely developed, and all persist : their crowns
are in a manner trilobed, and they pass in ordinary closure
of the mouth behind the upper incisors, where they are met
by a dense pad of gum.

THE TEETH OF PROBOSCIDEA.

At the present day the Elephant stands alone, removed
by many striking peculiarities from the Ungulata, to which
it is more nearly allied than to other orders ; but in former
days the order Proboscidea was represented by a good many
genera, was widely distributed over the globe, and tran-
sitional forms linking the elephant with somewhat less
aberrant mammalia were not wanting. In this group the
incisors grow from persistent pulps, and form conspicuous

tusks ; the Elephant has i , the Mastodon has i — , the

Dinotherium i —

Two striking features characterise the dentition of the

350 A MANUAL OF DENTAL ANATOMY.

Elephant ; the enormous length of the incisor tusks, and
the peculiar displacement from behind forwards of the molar
teeth, by which it results that not more than one whole
molar, or portions of two, are in place at any one time.

The upper tusks are preceded by small deciduous teeth ;
this is well established, though it has been recently denied by
Sanderson ("Wild Beasts of India" ). When first cut they
are tipped with enamel, but the enamel cap is soon worn off,
and the remainder of the tusk consists of that modification
of dentine known as " ivory," and of a thin external layer of
cement.

In the Indian elephant the tusks are not so large as in the
African species : and the tusks of the female are very much
shorter than those of the male. In the African elephant, no
such difference in size has been established ; and amongst In-
dian elephants males are sometimes met with which have tusks
no larger than the females of corresponding size ; they go
by the name of " Mucknas." This peculiarity is not always
transmitted, and it is known that in Ceylon tuskless sires
sometimes beget " tuskers." Amongst the Ceylon elephants
the possession of large tusks by the male is an exceptional
thing, Sanderson stating that only one in three hundred has
them, while amongst 51 Indian elephants only five were
tuskless. The tusks are formidable weapons, and great
dread of a " tusker," is shown by elephants less well armed.

A male makes use of his tusks for all sorts of purposes ;
thus when a tamed one is given a rope to pull, he will, by
way of getting a good purchase upon it, pass it over one
tusk and grasp it between his molar teeth.

The largest tusks were possessed by the Mammoth, the
remains of which are so abundant in Siberia ; these, which
were strongly curved, and formed a considerable segment
of a circle with an outward inclination, so as to well clear the
sides of the head, attained the length of 13 feet, and a weight
of 200 Ibs. each.

THE TEETH OF PROBOSCIDEA. 351

A pair of African tusks exhibited at the Great Exhibition
of 1851 weighed 325 Ibs., and measured eight feet six inches
in length, and 22 inches in circumference, but the average
tusks imported from Africa do not exceed from 20 Ibs. to
50 Ibs. weight. Indian elephants seldom have tusks attain-
ing very large dimensions ; one was, however, shot by Sir
Victor Brooke with a tusk 8 feet long, weighing 90 Ibs.

The surfaces of the tusks of the female are often deeply
excavated about the level of the edge of the gum, and are
sometimes so weakened from this cause that they break off.
My friend Mr. Moseley tells me that he was informed by the
late Major Rossall, who as a sportsman had great knowledge
of Indian elephants, that the tusks of all the females he has
ever seen are so affected, and that the larvae or pupee of a
dipterous insect are often found bedded in the gum, and
attached to the surface of the tusk. There is a specimen of
a female elephant's tusk with the pupse attached in the
Museum of the Royal College of Surgeons. It would be a
matter of interest to ascertain whether the larva really eats
away the tusk, or whether the wasting of the tusk be due
to absorption set up by the irritated gum.

The tusks of the elephant are implanted in long and
stout sockets, and grow from persistent pulps throughout
the lifetime of the animal.

In the Indian elephant about one half of the length of the
tusk is implanted, and in young animals the pulp cavity
extends beyond the implanted portion, but in older animals
it does not extend nearly so far. A knowledge of its extent
is necessary, seeing that the tusks of captive elephants have
to be shortened from time to time ; this operation is by
some done frequently, by others only at long intervals, such
as ten years, in which case a large and valuable segment of
ivory is cut off, and the end of the tusk bound with metal
to prevent it from splitting.

Tusks sometimes exemplify on a large scale the results of

352 A MANUAL OF DENTAL ANATOMY.

injury to the growing pulp, as it is of no unfrequent oc-
currence that elephants which have been shot at and
wounded escape.

The thin walls of the tusk near to its open end do not
offer very much resistance to the entrance of a bullet ; the
result of such an injury is not, as might have been expected,
the death of the pulp, but in some cases abscess cavities
become formed in the neighbourhood of the injury, while
in others less disturbance is set up, the bullet becomes
enclosed in a thin shell of secondary dentine, or sometimes
lies loose in an irregular cavity, and round this the normal
" ivory " is deposited ; upon the outside of the tusk no indi-
cation of anything unusual is to be seen, so that the bullets
thus enclosed are found by ivory turners only when sawing
up the tusk for use.

As the tusk grows, that which was once in the pulp
cavity, and within the alveolus, comes to be at a distance
from the head, and in the midst of solid ivory.

As an example of the extent of injury from which a
tooth pulp is capable of recovery, may be cited a specimen
now deposited in the museum of the Odontological Society,
by Mr. Bennett, to whom I am indebted for permission to
figure it.

It is to be presumed that a trap was set with a heavily
loaded spear, or that it was dropped by a native from a
tree, with the intention of its entering the brain of the
elephant as it was going to water, both of these methods of
killing elephants being practised in Africa. But in this case
the spear penetrated the open base of the growing tusk,
which looks almost vertically upwards (see fig. 150), and
then the iron point appears to have- broken off. This did
not destroy the pulp, but the tooth continued to grow, and
the iron point, measuring no less than 7J by 1J inches,
became so completely enclosed that there was nothing upon
the exterior of the tusk to indicate its presence.

THE TEETH OF PROBOSCIDEA. 353

I ain told by Mr. Erxleben that he is acquainted with
another instance in which a spear head had become com-
pletely enveloped in ivory.

FIG. 150 (').

There is also a specimen of a javelin head solidly im-
bedded in ivory in the Museum of the Royal College of
Surgeons.

Ivory is one of the most perfectly elastic substances
known, and it is on this account that it is used for billiard
balls ; it owes its elasticity to the very small size of the
dentinal tubes and the frequent bends (secondary curva-
tures) which they make ; to the arrangement of the tubes
the peculiar engine-turning pattern of ivory is due. Tt
differs from other dentine in its containing from 40 to 43
per cent, of organic matter (human dentine contains only
about 25), and in the abundant concentric rows of inter-
globular spaces. Along these ivory when it decomposes
breaks up, so that a disintegrated segment of a tusk con-
sists of detached concentric rings ; in this condition many
mammoth teeth are found, although sometimes where they
have remained frozen and protected from the air until the
time of their discovery they are hardly affected by the lapse

(l) Iron spear-head, irremovably fixed in the interior of a tusk, believed
to be from an African Elephant. From a specimen in the possession of
Mr. Bennett.

354 A MANUAL OF DENTAL ANATOMY.

of the thousands of years which have gone by since their
possessors perished.

The trade in ivory is quite an important one, the Board
of Trade returns for 1879, giving 9,414 cwts., of the value of
.£406,927, as the quantity brought to this country.

The best ivory is that which comes from equatorial
Africa; Indian ivory, is not so highly esteemed, and Mam-
moth ivory is so uncertain in its degree of preservation
that it does not find a ready sale, even though some samples
almost attain the quality of recent ivory.

The last remains of the pulp are converted into dentine
in which a few vascular canals persist ; this of course occu-
pies the centre of the tusk, and is small in amount.

Six molar teeth are developed on each side of the jaw by
the elephant, and, arguing from analogy, they are some-

3 3

times classified thus — milk molars - true molars - ;

3 3

occasionally a rudimentary tooth in front brings up the
number to seven on each side. But the peculiarity of their
mode of succession renders such a classification merely
arbitrary, so far as the elephant itself is concerned, and it
depends upon analogy with the teeth of the mastodon.
Though the elephant has, during the course of its life,
twenty-four molars, they are not all in place, nor indeed
are they all actually in existence at the same time. Only
one whole tooth on each side, or portions of two (when the
front one of the two is nearly worn out), are in use at the
same time. After a tooth has been in use for some time,
and is worn down, a new tooth comes up to take its place
from behind it, and absorption in the old tooth being set up,
it is shed off, and the new tooth pushes forward into its
place (see fig. 151). Each successive tooth is of greater
size than its predecessor ; thus in the Indian elephant the
first tooth having, on an average, four transverse plates;
the second eight, the third twelve, the fourth twelve, the

THE TEETH OF PROBOSCIDEA.

355

fifth sixteen, the sixth from twenty-four to twenty-seven-
In the African elephant, in which the individual plates are
much broader, they are fewer in number (see page 361).

FIG. 151

10 flak Size.

A reference to the accompanying figure will indicate how
the succession takes place. The tooth in reserve occupies a
position at an angle to that in use ; as it moves forwards

(*) Side view of skull of young Indian Elephant. The teetli in use are
the second and third of the molars which displace one another from be-
hind forwards ; the anterior of these, corresponding to a milk molar in
other animals, is nearly worn out ; the residual fragment is separately re-
presented on the left. The tusk, of which only a short piece can be
shown, is indicated within the socket by dotted lines, by which also the
form of the pulp cavity is mapped out.

A A 2

356 A MANUAL OF DENTAL ANATOMY.

and (in the upper jaw) downwards its track forms almost
the segment of a circle. Thus its anterior corner is the
first to come into use, at a time when the position of the
whole tooth is still exceedingly oblique, and the greater part
of it is still within the socket.

The teeth as first formed consist of detached plates of
identinc coated with enamel, the tops of which are mammil-
lated ; these only coalesce after a considerable portion of
their depth has been formed, and that portion of the tooth
has been reached in which there is a common pulp cavity ;
here dentine is continuous from end to end of the tooth.

Just as the cusps of a human molar are separate when
first calcified, so these exaggerated cusps or plates of an
elephant's tooth are separate from one another till a great
part of their length is completed, and they only coalesce
when they reach the level ef the common pulp chamber ; in
point of fact the elephant's tooth is mainly made up of its
cusps, the remaining portion being insignificant.

Several of these detached plates, such as the one here
figured, are to be found at the back of the largest teeth
even at a time when the front corner has been erupted and
has come into wear.

That the tooth is thus being built up only as it is required
is of obvious advantage to the animal in diminishing the
weight to be carried, and is also an economy of space.

The teeth when they begin to be erupted do not at once
come into use over their whole surface, but they come for-
ward in an oblique position so that the front of the tooth
has been in use for some time, and its plates have been con-
siderably worn down, before the back of the tooth has
become exposed at all. Nay more, in the case of the larger
molars the front of the tooth is actually in use at a time
when its back is not yet completed.

In the elephant there is no vertical succession of teeth
whatever; the manner of succession usual amongst mammals

THE TEETH OF PROBOSCIDEA.

357

has in them given place to a succession from behind, the
older teeth being pushed out forwards. Had the elephant
always been as isolated a form as it now appears to be, it
would have been very uncertain how its six molars should

bo classified. But it happens that proboscideans formerly
existed in which this pecular succession from behind was to
be found, at the same time that the ordinary vertical suc-
cession was not quite lost, and amongst these creatures (the
mastodons) we are able to say with certainty which of the
teeth are milk molars, which are pre-molars, and which are
true molars. And as the mastodons pass by insensible gra-
dations into the elephants, so that the line of demarcation
between the two genera is an arbitrary one, we can tell
which of the mastodon's teeth correspond to each one of the
six molars of the elephant.

Mastodon. — In the later tertiary periods this genus, ap-
proximating in its dental and other characters to the true

(') Isolated plate (= exaggerated cusp) of an Elephant's tooth, prior to
its coalescence with neighbouring plates ; at the top are seen its terminal
mammillated processes, one of which has been cut off to show the central
area of dentine, surrounded by enamel ; at the base would be the open
pulp cavity, not shown in the figure.

358 A MANUAL OF DENTAL ANATOMY.

elephant, was widely distributed over the world. The dental
formula is not quite the same for all the genus, for in some
no premolars existed.

102 33

i. _ c. - prm. - milk molars _ m. r

The upper incisors formed nearly straight tusks, seven or
eight feet in length ; the lower incisors also grew out hori-
zontally from the front of the jaw, but in some species the
lower tusks are rudimentary, are lost early, or "are altogether
absent, thus more nearly approaching to the condition met
with in the elephant.

The several molar teeth of the Mastodon increased in
size from before backwards. The crowns were built up of
deep and strongly pronounced transverse ridges, of which
the last molar had the largest number. The apices of the
ridges, before being at all worn, were divided up into several
blunt nipple-like (mastoid) processes, upon which the
enamel was thick and dense, but the cement was thin, so
that the interspaces of the processes were not filled up level
by the latter tissue, as in the elephant.

Very definite roots were formed to the molars, the wear-
ing down of the teeth being met by the worn teeth being
shed off altogether from the front of the series, whilst new
teeth were added to the back. Thus, just as in the elephant,
the whole number of teeth were not in place at one time.
ISTot more than three were in use at one time, and by the
time the last and largest molar was cut, there was but one
tooth remaining in front of it, and even this was soon lost,
the dentition thus being reduced to a single molar on
each side.

As the succession of the molars in the Mastodon affords
a clue to the nature of the grinders of the elephant, it is
necessary to add a few words about it. Some Mastodons
had three milk molars, of which the last two were vertically
displaced by premolars, just as in most other mammals,

THE TEETH OF PROBOSCIDEA. 359

but the first milk molar was not so replaced (Mastodon
angustidens). There appear to have been Mastodons in
which no vertical succession at all took place, i.e., in which
there were no premolars, and others in which there was
but one.

No doubt can be entertained as to the homologies of the
teeth, even in those Mastodons which are not known to
have any vertical succession, because analogy with those
other species in which the second and third molars, counted
from the front, were vertically displaced by nearly func-
tionless premolars, tells us that the three front molars are
milk molars. Now elephants develop six molar teeth on
each side ; the elephant is in the same case, quoad its molars,
as the Mastodon Ohioticus, which had no vertical succes-
sion, so that we thus know the elephant's grinders to be

,33
dm.-m._

Dr. Falconer mentions an elephant from the Sewalik Hills
(E. planifrons) in which two rudimentary pre-molars, of no
functional importance, actually existed, and so the deter-
mination of the elephant's working teeth as

, 3 3

dm. - m. -

3 3

rests not only upon analogy, but upon actual observation.

The Dinotherium, a large animal, not unlike the Sirenia
in the character of its cranium, which was probably of
aquatic habits, was remarkable for possessing large tusks, by
analogy known to be incisors, in its lower jaw, none being
present in the upper jaw. The tusks projected downwards
at right angles with the body of the jaw, and were curved
backwards. The portion of jaw about the symphysis was
deflected downwards, so as to afford an adequate implanta-
tion for these anomalous tusks.

360

A MANUAL OF DENTAL ANATOMY.

The Dinotherium was as large as an elephant, and the
downward pointing tusks were about 2 feet in length ; as,
however, tusks of only half this length were found in some
jaws of identical dimensions and in other respects similar,
it is believed that the male Dinotherium had larger tusks
than the female. The molar teeth, much like those of a
tapir, need not detain us.

.002 3

The succession was vertical, as in other mammals, and it

3
had dna. 7

o

But the Dinotherium, Mastodon, and Elephant, present
us with a very instructive series of modifications in which
we see how the excessively complex grinder of the Indian
elephant was attained to by degrees.

The molar of the Dinotherium resembles that of a tapir
somewhat; it has not any very great exaggeration of its
cusps, and does not deviate very widely from the form of
many other mammalian teeth.

The tooth of Mastodon has its cusps or ridges more
numerous and more pronounced, as is seen in the accom-
panying figure.

FIG. 153

.A. IB

Other Mastodons have more numerous ridges upon the
teeth, and the African elephant has as many as ten upon

(l) Molar tooth of Mastodon.

THE TEETH OF PROBOSCIDEA.

S61

FIG. 154 (').

-j-N.S.

(*) Molar of African Elephant. E. Enamel. D. Dentine. C. Cementuni.

(2) Molar tooth of African Elephant, showing the form of its roots, &c.
a. Dentine, c. Ceinentum. e. Enamel.

(3) Molar tooth of an Asiatic Elephant, showing the transverse plates of
dentine bordered by enamel.

362 A MANUAL OF DENTAL ANATOMY.

its last or larger molar, although in it the ridges are in-
dividually wide and strongly pronounced.

In the Indian elephant the ridges or plates are still more
numerous, the roots very inconspicuous and the whole
formed into a solid block by cementum.

The gradual increase in complexity in the " ridge formula"
(or number of ridges in each tooth), of the molars, is well
seen in the following table, from Prof. Flower's Hunterian
lecture ("Nature," March 2, 1876); it is a corrected table
taken from Dr. Falconer's " Pakcontological Memoirs."

Milk Molars. ^rueMolars. Total.
I. II. III. I. II. III.

Dinotlierium giganteum . . . .123

Mastodon (Trilophodon) americanus ..123

„ (Tetralophodon) arvernensis .234

,, (Pentalophodon) sivalensis ..345

Elephas (Stegodon) insignis . . .257

322
334
445
556

7 8 10

(Loxodon) africanus . ..367 7 8 10

„ meridionalis . . 3 6 8 j 8 9 12

(Euelephas) antiquus . .. 3 6 10 j 10 12 16

13

16
22
28
39
41
46
57

primigenius . . . 4 8 12 ! 12 16 24 i 76
indicus . . . 4 8 12 12 16 24 j 76

Some variability exists in the number of ridges, especially
when they are very numerous, but the above may be taken
as averages ; and some species intermediate in the " ridge
formula " have been since discovered, thus M. pentelici and
M. andium bridge the distinction between Trilophodon and
Tetralophodon, and Elephas melitensis comes between
Loxodon and Euelephas (Flower).

It remains to describe, somewhat more in detail, the
structure of an elephant's tooth, and this has been deferred
till the last, because it can be the more easily understood
when the manner of its origin has been mastered. In the
Mastodon the molar consists of a crown with strong cusps,
standing apart, and with marked roots ; in the African
elephant that part which consists of cusps has become the

THE TEETH OF PROBOSCIDEA. 363

greater bulk of the tooth, the roots are comparatively in-
significant, and the interspaces of the cusps are filled up
with cementum. The molar of the Indian elephant con-
sists of a larger number of yet more elongated and flattened
cusps, so that the greater part of the tooth is made up of
these flattened plates, fused together with cementum, and so
forming a strong and solid mass ; the roots are compara-
tively inconspicuous.

When the tooth is a little worn each plate consists of an
area of dentine surrounded by enamel. The interspaces
of the series of plates are wholly filled up by cementum ;
the summits of each plate were originally maminillated,
and divided up into more numerous blunt processes than
the corresponding parts of the tooth of a Mastodon ; when
the tooth comes into use the rounded tips are soon worn
off, and the grinding surface of the tooth then consists of
narrow transverse bands of dentine, surrounded by enamel,
and of cementum in their interspaces. The difference in
hardness between these three tissues preserves a constant
rough surface, owing to their unequal rate of wear. In
their wild condition elephants eat trees with succulent juicy
stems, and oftentimes grass torn up by the roots, from
which they roughly shake out the adherent earth. In con-
finement, the food containing less that is gritty, the teeth
become polished by working against one another, but the
rate of wear is insufficient to keep their surfaces rough ; for
the softer cementum does not get worn down in the inter-
spaces of the plates of dentine and enamel, but remains on
a level with them.

Great though the size of the Proboscideans be, they have
some points of affinity with the Rodents in the great
development of the incisors, the vacant interval between
these and the molar teeth, and, as was pointed out by the
late Professor Rolleston, the enamel of the elephant's molar
having, in its inner portions, a pattern produced by the

364 A MANUAL OF DENTAL ANATOMY.

decussation of the prisms which is very similar to that de-
scribed by my father as characteristic of all the Rodents
save the Leporidse (Hares) and Hystricidae (Porcupines).

THE TEETH OF RODENTIA.

The animals belonging to this order, which is sharply
denned, are scattered almost all over the world ; the island
of Madagascar is, however, remarkable for being almost
without indigenous Rodents, as is the case also with Aus
tralia, two facts which are of no small interest to the
student of odontology.

For in each of these areas, out of the creatures which are
there (in the one Lemurs, in the other Marsupials), there
has arisen a form so modified as to mimic the dentition of
the true Rodents, viz., the Cheiromys in Madagascar, and
the Wombat in Australia.

The species of Rodents are exceedingly numerous, and
the great majority of them are of small size ; the aquatic
Capybara is far the largest of recent Rodents.

In general features the dentitions of the numerous
species comprising this order are very uniform ; the incisors,
(save in the hares and rabbits, in which there is an ac-
cessory small pair immediately behind the large ones) are
reduced to four in number, are of very large size, and grow
from persistent pulps. The jaws for some little distance
behind the incisors are devoid of teeth, while beyond the
interval the back teeth, generally not more than four in
number, are arranged in lines which diverge slightly as they
pass backward. The large scalpriform, or chisel-like incisors,
extend far back into the jaws, and are much curved, the
upper incisors, in the words of Professor Owen, forming a
larger segment of a smaller circle than the lower, which are
less curved. The length and curvature of these incisors

THE TEETH OF RODENTIA. 365

relieve from direct pressure their growing pulps, which
come to be situated far back in the jaw, the open end of the
lower incisor, for example, being in many species actually
behind the last of the molar teeth. The nerve going to
supply the persistent pulps is of very large size, and, owing
to the open end of the tooth having formerly occupied a
more anterior position in the jaw, runs forward beneath the

FIG. 157 -

tooth, and then bends abruptly backwards to reach the
tooth-pulp. In many Rodents the enamel of the front of
the large incisors is stained of a deep orange colour ; this
colour is situated in the substance of the enamel itself.

The scalpriform incisors terminate by cutting edges, the
sharpness of which is constantly maintained by the peculiar
disposition of the tissues of the tooth.

The investment of enamel, instead of being continued
round the whole circumference of the tooth, is confined to
its anterior and lateral surfaces, on the former of which it is
thickest.

It is said by HilgeudorfF (Berlin Akad. d. Wiss. Monats-
bericht, 1865), that the incisors of Hares differ from those of

*) Side view of skull of a Rodent, giving a general idea of the denti-
tion of the order.

366 A MANUAL OF DENTAL ANATOMY.

all other Rodents in having enamel all round them, although
it is very thin at the back. I have not been able to satisfy
myself that the thin clear layer at the back of the tooth is
enamel, and am disposed to regard it as cementum, the more
so as it seems to be continued a little way upon the enamel,
and in very young teeth the enamel organ is confined to the
anterior surface.

When a rodent incisor has been exposed to wear, the
anterior layer of enamel is left projecting beyond the level
of the dentine, and this arrangement results in a very sharp
.edge being constantly maintained. The dentine also is
harder near to the front of the tooth than towards the back
of the tooth.

A thin external coat of cement is found upon the back of
the tooth, but is not continued far over the face of the
enamel. In the marsupial wombat this layer of cement is
continued over the whole anterior surface of the scalpriform
incisors.

The molar teeth are not very numerous; the mouse

2

family have usually r ; the porcupines have constantly
o

j n

-, and the hares - ; the Australian water-rat (Hy-

4r 0

2
dromys) is altogether exceptional in having so few as X

Observation has established that the last three of these
teeth are always true molars, and that when there are more
than three, the rest are premolars, and have had deciduous
predecessors.

But the extent to which the milk teeth are developed
varies much. Mr. Waterhouse (Nat. Hist, of Mammalia —
Rodents, p. 4), has found the milk molar still in place in
the skull of a half-grown beaver, while in the hares they
are shed about the eighteenth day after birth, and in the
guinea-pig disappear before birth. Deciduous incisors have

THE TEETH OF RODENTIA. 367

not been found in any of the group, save in the hares and
rabbits.

In the hares and rabbits there are four incisors jnjjje
upper jaw, a small and apparently functionless pair being
placed close behind the large rodent incisors ; but in very
young specimens there are six incisors, of which the one pair
are soon lost.

Prof. Huxley (Nature, vol. 23, p. 228) has recently written
that " the deciduous molars and the posterior deciduous
upper incisors of the rabbit have been long known. But I
have recently found that unborn rabbits possess, in addi-
tion, two anterior upper and two lower deciduous incisors,
Both are simple conical teeth, the sacs of which are merely
embedded in the gum. The upper is not more than one-
hundredth of an inch long, the lower rather larger. It
would be interesting to examine frctal guinea-pigs in rela-
tion to this point ; at present they are known to possess
only the hindmost deciduous molars, so far agreeing with
the Marsupials."

Hares and rabbits have six milk molars in the upper and
four in the lower jaw, which come into use, but differ from
their successors in forming definite roots and not growing
from persistent pulps.

Other rodents, such as the rat, which has only three
teeth of the molar series on each side, and the Australian
water-rat ( Hydro mys) have no milk teeth, and are hence
truly Monophyodont.

More diversity exists in the premolar and molar teeth ;
in rodents of mixed diet, such as the common rat, the back
teeth are coated over the crown with enamel, which nowhere
forms deep folds, and have distinct roots, i.e., are not of
persistent growth ; the molars of the rat have some sort of
resemblance to minute human molars. In aged specimens
the enamel is consequently worn off the grinding surface of

368 A MANUAL OF DENTAL ANATOMY.

the crown, which comes to be an area of dentine, surrounded
by a ring of enamel.

But in those whose food is of a more refractory nature,
the molars, like the incisors, grow from persistent pulps (as
is exemplified in the Capybara here figured), and their
working surfaces are kept constantly rough by the enamel
dipping in deeply from the side of the tooth, as may also be
seen in the common water-rat. The inflection of enamel
may be so deep as to divide the areas of dentine completely
up, the result being a tooth like that of the Capybara, which

FIG. 158 !).

4&

NdbS&c

is composed of a series of plates of dentine, or f denticles,'
surrounded by layers of enamel, and all fused together by
the cementum. The result of this disposition of the struc-
tures is that the working surface is made up of enamel,
dentine, and cementum, three tissues of different hardness,
which will consequently wear down at different rates, and
so maintain its roughness. Various intermediate forms of
the molar teeth are met with ; thus there are some in
which complexity of the surface is maintained by folds of
enamel dipping in for a little distance, but which never-
theless after a time form roots and cease to grow. When
the molar teeth grow from persistent pulps, they are always
curved, like the incisors, with the effect of relieving the
pulps from direct pressure during mastication ; and the last

(x) Molar of Capybara, showing the transverse plates of dentine and
enamel united to one another by cementum.

THE TEETH OF RODENTIA.

369

remains of the pulps are converted into secondary or osteo-
dentine, which thus forms the central axis of the incisors, or
molars, as the case may be. In this tissue vascular tracts
sometimes exist, but it is altogether small in amount, the

Fro. 159 (').

formation of true dentine going on till the pulp at that
particular point is almost obliterated.

As has already been mentioned, when the molar series
consists of more than three teeth, those anterior to the
three true molars are premolars, which have displaced milk
teeth ; but they do not differ materially in size or form from
the true molars.

The form of the condyle and of the glenoid cavity in
Kodents are characteristic ; they are much elongated in an
antero-posterior direction, so that the range of backward

(t) Condyle and glenoid cavity of the Capybara, showing their longitu-
dinal direction.

R B

370

A MANUAL OF DENTAL ANATOMY.

and forward motion, made use of in gnawing, is very con-
siderable. The Leporidce are exceptional in having more
lateral play than most Rodents. And the power of the
teeth is marvellous ; rats will sometimes gnaw holes in
water-pipes, or in gas-pipes, in which they have heard water
bubbling.

The general character of a Rodent's dentition may be
illustrated by a description of that of the Capybara.

FIG. 160

The incisor teeth are squarish. They are wider than
they are deep, and are slightly grooved on their anterior
surface.

There are four grinding teeth on each side, of which the
first three are small, and with few cross plates of dentine
and enamel, but the fourth is a very complex tooth, with
twelve or more such plates, which are fused into a solid
mass by cementum.

The tooth being one of persistent growth, there is no
common pulp cavity, but each plate has its own.

It has already been mentioned (page 160) that the den-

(l) Cranium of Capybara.

THE TEETH OF RODENTIA. 371

tinal tubes at that part of the Rodent's incisor which has
come into use are much smaller than those near to its
growing base, thereby proving that they have undergone a
diminution in calibre at a time subsequent to their original
formation. Near to the surface actually in wear they
become cut off from the pulp cavity by the conversion of
what remains of the pulp into a laminated granular mass,
so that the dentine exposed on the surface of a Rodent's
tooth must be devoid of sensitiveness, and the contents of
the dentinal tubes must have presumably undergone some
change. But what the nature of the change in the contents
of dentinal tubes which have ceased to be in continuity with
a vascular living pulp may be, there are, so far as I know,
no observations to indicate.

As was shown by my father (Phil. Trans. 1850), the
enamel of Rodents is peculiar, and some little diversity in
the arrangement of the prisms exists in different families of
the order, their character being in many cases so marked,
that it is often possible to correctly refer a tooth to a par-
ticular family of Rodents after simple inspection of its
enamel.

In general terms it may be said that the enamel is divided
into two portions, an outer and an inner portion (this is
true of all save the hares and rabbits), and that the enamel
prisms pursue different courses in these two portions.

Thus in the enamel of the beaver, in the inner half, nearest
to the dentine, the prisms of contiguous layers cross each
other at right angles, whereas in the outer portion they are
all parallel with one another.

In the genera Sciurus, Pteromys, Tamias, and Spernio-
philus the enamel fibres, as seen in longitudinal section,
start from the dentine at right angles to its surface; in
Castor they incline upwards at an angle of GO0, but preserve
the distinction between the outer and inner layers very
distinctly.

B B 2

372

A MANUAL OF DENTAL ANATOMY.

In the Muridce the decussation of the layers in the inner
part, and their parallelism in the outer part of the enamel

are also found, but in addition to this the borders of the
individual prisms are slightly serrated, the serrations of
contiguous fibres interlocking.

In the porcupine suborder the fibres of the inner portion
of the enamel pursue a serpentine course, nevertheless
showing indications of a division into layers ; they become
parallel in the outer portions as in other Rodents. Small
interspaces are found amongst the enamel fibres of the
Porcupines.

In the hares (Leporida) the lamelliform arrangement, and
the division into outer and inner layers, alike disappear.

The peculiarities in the disposition of the enamel fibres,
which are so marked in the incisors, do not generally exist
in the molars of the same species.

Many minor differences in the arrangement of the enamel
prisms exist, for a description of which I must refer the
reader to the original paper, but in general terms it may be
said that the " enamel lamella) have a different and distinc-

(]) Transverse section of an incisor of a Beaver (Castor fiber). The
enamel prisms of superimposed layers cross each other at right angles in
the inner portion of the enamel, but all becorce parallel in the outer.

THE TEETH OF RODENTIA. 373

tive character in each of the larger groups, and that the
variety of structure is constant throughout the members of
the same group ; we may take, for example, the Sciuridce,
the Huridce, and the ffystrieidce, in each of which the
structure of the enamel is different ; and in each is highly
distinctive." And further, that the varieties in the struc-
ture of the dental tissue, so far as they are known, with a
few isolated exceptions, justify and accord with the classifi-
cation of the members of the order given by Mr. Waterhouse
in his Natural History of the Mammalia.

CHAPTER XII.

THE TEETH OF CARNIVORA.

THE animals grouped together under the name of
Carnivora are divided into two sections, the Aquatic and
the Terrestrial Carnivora.

The terrestrial Carnivora were formerly classed as " digitigrade 'r
and " plantigrade," a classification exceedingly inconvenient, as it
left the greater number of the animals to be classified in the de-
bateable ground between the two extreme types. As a linear
classification is impossible, they are now grouped around three
centres : the QEluroidea, or cat-like ; the Cynoidea, or dog-like ;
and the Arctoidea, or bear-like Carnivora ; and, instead of taking-
the Felidas, or Cats, as the type of the group, it is generally con-
sidered that the Dog tribe are the most generalised form, and that
the Cats are an extreme modification in one direction, the Bears in
another.

The Cynoidea comprise the Dog. and its immediate allies, the

Wolves and Foxes.
The CEluroidea, or cat-like Carnivora, comprise the Viverridae (Civets),

Hyasnas, and Cats.
The Arctoidea, or bear-like Carnivora, comprise the Mustelidge

(Weasels), Procyonidas (Racoons), and the true Bears.

The order Carnivora is a very natural one, and its name is, upon
the whole, fairly descriptive of the habits of the majority of its-
members ; though there are some creatures included in it which are
mixed feeders, and others which are purely vegetarian.

Iii carnivorous animals one tooth on each side of both
upper and lower jaws is of considerable length, is sharply
pointed, and is called a canine ; the upper canine is sepa-
rated by an interval from the incisors, the lower canine

THE TEETH OF VARNIVORA. 37;

being received into the vacant space or "diastema" so
formed.

The incisors are short, almost always six in number, and
stand nearly in a straight line, transversely across the front
of the jaw, the outermost upper incisor being sometimes
large and pointed so as to be like a small canine.

The incisors and canines may, on the whole, be said to be
tolerably uniform throughout the order, but the variations
in the premolar and molar teeth are both numerous and
interesting.

In the most purely carnivorous members of the order,
the Fdidce, the true molars are reduced to a minimum, and
the back teeth are thin edged, " sectorial " teeth ; in the

FIG. 182 (]).

bears, on the other hand, some of which are purely her-
bivorous, the molars arc little short of the full typical mam-
malian number, and are furnished with obtuse and broad
grinding surfaces.

The accompanying figure will serve to give the general
aspect of the teeth and jaws of a typically carnivorous

(l) Side view of the cranium of a Tiger, with the mouth slightly opened
to show the relative position of the great canines.

376 A MANUAL OF DENTAL ANATOMY.

animal, and to show the great development of the processes
for the attachment of muscles, and the stout wide arch of
the zygoma.

To a particular tooth in the upper jaw, and to its antago-
nist in the lower jaw, Cuvier gave the name of " carnassial ;"
these, conspicuous in the true flesh-feeders, become less dif-
ferentiated in the Arctoidea or bear-like Carnivora, and in
the bears themselves are indistinguishable from the other
teeth, save by a determination of their homologies by a
process of comparison with the teeth of intermediate forms.

The sectorial or carnassial tooth in the upper jaw is
always the fourth premolar ; its crown is divisible into two
parts, the one a thin sharp-edged blade, which runs in an
antero-posterior direction, and is more or less divided by one
or two notches into a corresponding number of cusps ; the
other part, the "tubercle," is a shorter and blunter cusp,
situated to the inner side of the anterior end of the blade
(see fig. 166). In those which are most purely flesh-feeders,
the " blade " is well developed, and the tubercle of small
size ; an increase in the tubercular character of the tooth is
traceable through those genera which are mixed feeders.

The lower tooth which antagonises the upper carnassial,
passing a little behind it, is the first true molar; in the
Felidce it consists solely of the blade, which is divided into
two large cusps, behind which is a very small and rudimen-
tary third division (which in the Hycenidce, for example, is
of conspicuous dimensions). In existing Carnivora but one
" sectorial " tooth is to be found on each side of the jaws, but
in the Hysenodon and some other extinct tertiary mammals
there were three teeth partaking of this character.

In a general sense we may say that the characters which
indicate a pure flesh diet are : the small size of the incisors
as compared with the canines, and their arrangement in a
straight line across the jaw ; the large size, deep implanta-
tion, and wide separation from one another of the canines ;

THE TEETH OF CARNIVORA. 377

the reduction in number of the molar series, those that
remain being without broad crushing surfaces, in the place
of which a pointed or sharp-edged form prevails.

Thus the more numerous the teeth of the molar series,
and the broader their crowns, the more likely it is that the
creature subsists upon a mixed diet ; and a gradation may
be traced even in individual teeth, such as the carnassials,
in which a gradual increase in relative size of the internal
tubercular cusps of the upper, and of the posterior tubercles
of the lower teeth, may be traced as we pass from the ex-
amination of the teeth of Felidce, to those of mixed feeders,
such as the Arctoidea.

It is a familiar observation that immature animals differ
less from their allies than do the respective adults, and this
is exemplified by the milk dentition of the present order.

With the exception of the Felidce, which have only two
lower milk molars, the terrestrial carnivora, so far as is
known, all have the same milk dentition.

.31 3
i - c - m _ .
3 1 3

Cynoidea. — The dog presents almost the full typical
number of teeth, one upper molar (present in an extinct
dog-like animal, the Amphicyon) alone being wanting.

.31 42

i _ c - pm - m _ .
3 1 J 4 3

The incisors are small, the outermost being the largest ;
the upper incisors have, as in a great many Carnivora, a
tri-lobed shape, the surface of the crown being marked by a
transverse groove into which the apex of the lower tooth
fits, and the anterior of the lobes thus formed being notched
so as to divide it into two.

The canines, large and conical, are somewhat compressed

378

A MANUAL OF DENTAL ANATOMY.

from side to side, and have an anterior and a posterior
sharp ridge ; they are also slightly flattened on their inner
surfaces.

The premolars are flattened from side to side, pointed,

FIG. 164 (2).

increasing in size from before backwards, and have small
basal accessory cusps (see fig. 163). The fourth upper pre-
molar is the sectorial tooth, and is very much larger than
the third premolar ; the blade is well pronounced, ' and the

(*) Dentition of Australian Dog (Canis dingo).

(2) Milk and permanent teeth of Dog (after Prof. Flower).

THE TEETH OF CARNIVORA.

tubercle small. The fourth lower premolar does not
greatly differ from the third. The two upper true molars
are blunt, broad-crowned tuberculated teeth, but the second
is very small.

In the lower jaw the first true molar or carnassial tooth,
has a well-marked blade, which articulates with the blade
of the upper carnassial tooth ; but towards the posterior
border there is a somewhat thick and blunt tuberculate
portion, barely represented in the corresponding tooth of
the Felicias; the tubercular portion articulates with the
broad flat first upper molar. The second lower molar is
smaller, not being one-fourth the size of the first ; the third
smaller still; both are blunt-crowned tuberculated teeth
(the third lower molar, rudimentary in all dogs, is alto-
gether absent in the Canis primsevus).

The dentition of the dog, closely similar as it is to that
of the wolves and foxes, is such as to allow of a considerable
range of diet, there being tubercular molar teeth in addition
to a full armament of such sharply-pointed teeth as are
characteristic of flesh-feeding animals.

Thus the Canidce, uniform as they are in dentition, have
somewhat different habits ; the Arctic fox, a flesh-feeder
purely, has a dentition indistinguishable from the North
Italian fox, which is reputed to be vegetarian in its diet ;
the Canis cancrivorus of Guiana eats small mammals, crabs,
and also fruit. Hence it is necessary to be very careful in
deducing from the character of the teeth what may pro-
bably have been the diet of the animal ; an approxi-
mate idea may often be reached, but the sources of
fallacy are sufficiently numerous to render the conclusion
uncertain.

Amongst the various breeds of dogs some slight differ-
ences exist. Thus in the long-muzzled races considerable
intervals exist between the premolars, as is to some extent
seen in C. Dingo (fig. 163), while in the short-muzzled races

380 A MANUAL OF DENTAL ANATOMY.

the teeth are in contact, and set somewhat obliquely, so as
to be almost imbricated.

On the whole it may be said that the teeth are less easily
susceptible of modification in size than are the jaws, so that
crowding of the teeth is induced by selective breeding aiming
at the production of short-muzzled varieties.

In some long-muzzled races supernumerary teeth are
sometimes found j thus De Blainville (Ostdographie,
Canidce) figures two examples, the supernumerary tooth
being in one case a premolar, in the other a true molar.

(Eluroidea. — With a dental formula not differing much
from the dog (and not all from Canis primsevus) the
Viverridce (Civet cats, Ichneumons, &c.) approach the more
typical carnivores in such points as the thinner and sharper
blades of the premolar teeth and the greater relative length
and sharpness of the canines.

The dental formula is

iiclplm!.

31*4 2

At the same time the lower carnassial tooth has no less
than six sharply pointed cusps, and it lacks the typical
character of a sectorial tooth, while the long pointed cusps
of the molars of some Viverridce recall the characters of
insectivorous dentitions rather than those of true flesh-
feeders ; furthermore, there are other Viverridce which are
not at all savage, and which subsist on a diet of fruits,
eggs, &c., such as the Binturong or the Paradoxurus, the
teeth of which have almost lost the carnivorous character.
Little use can therefore be made of the Viverridce as illus-
trating the transition between the dental characters of the
other families of the order ; they rather serve to exemplify
how, within the limits of a single family, with an identical
dental formula, the form and size of the teeth may vary so

THE TEETH OF CARNIVORA.

381

as to adapt its members to different forms of food and
habits of life.

Hycenidw. — In the Hyaena the jaw is short and stout;
the canines are set far apart, and the teeth of the molar
series are reduced in number.

.314 1

The incisors are short and stout, but the outermost
upper incisor is somewhat caniniform ; the canines are very
strong, but are not so long relatively to the other teeth as
in the Felidw.

The premolars are aH stout pointed teeth, with a very

FIG. 165 (').

well pronounced basal ridge or cingulum, serviceable in
protecting the gums when the creature is crushing up

(!) Upper and lower teeth of Hysena. The strongly marked cingulum
is seen upon the lower teeth. In the upper jaw the fourth premolar
(carnassial tooth) has a strong blade, divided into three cusps, and a
small tubercle opposite to and within the anterior cusp ; it is a good
typical carnassial tooth.

382 A MANUAL OF DENTAL ANATOMY.

bones ; they increase in size from before backwards in the
upper jaw, the fourth upper premolar being a well marked
carnassial tooth with its blade and tubercle.

The lower carnassial or first molar consists of little more
than the notched blade ; but the little posterior tubercle so
strongly pronounced in the dog, is in the hyaena distinctly
more marked than in the Fclidce (cf. figs. 165 and 166). The
only upper true molar is the rudimentary tooth, placed
inside the back of the fourth premolar.

The main feature of the dentition of the hyama is the
great stoutness and strength of the teeth ; they are admir-
ably adapted to the habits of the animal, which feeds rather
Tipon the portions of carcasses left by the fiercer carnivora
than upon those which it kills for itself, and consequently
bones form a large proportion of its food.

There is a curious hysena-like animal found at the Cape
(of which there are often specimens at the Zoological
Gardens) called Proteles or Aardwolf, in which the teeth
of the molar series are quite rudimentary. The incisors
(much worn in old animals) and the canines are fairly well
developed ; the molars and premolars quite stunted.

The deciduous dentition f dm. _ J is similar to the

adult, as respects the teeth being stunted. It is a cowardly
animal, and is supposed to feed on putrid flesh ; it is said to
eat young lambs, and to bite the large tails of the Cape
sheep, which are remarkable for containing an abundance of
semi-fluid fat.

. Felidce. — The dentition of this family is singularly
uniform.

. 3 1 31
i _ c - p _ m _ .

Thus the molar series is reduced below that of hycena

THE TEETH OF CARNIVOEA.

383

by the loss of a premolar in both jaws. The incisors are
very short, the canines very large, widely apart, and sharply
pointed, with a pronounced longitudinal ridge very charac-
teristic of the Felidce ; the premolars nearest to them are
quite short, so that they stand practically alone, and so can
penetrate the flesh of living prey more readily.

The first upper (really the second of the typical mam-
malian dentition) premolar is almost a rudimentary tooth ;

FIG. 166 ('). *

the second, a far larger tooth, is sharply pointed ; the third
is a well pronounced carnassial tooth, of which the "blade"
is divided by two notches into three sharp lobes, with the
middle one of which the " tubercle " is connected by a slight
ridge.

The solitary true molar is a small tooth, placed trans-
versely, and within the back of the premolar, so that looking
from the outside it is not visible at all.

In the lower jaw the carnassial (first molar) is reduced to

(*) Side view of lower, and palatal aspect of upper jaw (Leopard).

384 A MANUAL OF DENTAL ANATOMY.

the " blade " only ; it is divided by a V-shaped notch into
two lobes, and the posterior tubercle is hardly represented.

In an extinct feline animal, the Machairodus, found in
tertiary strata, and very widely distributed (in France,
Italy, India, Brazil, Buenos Ayres) the first of the pre-
molars left in the upper jaw of Felis, and there almost
rudimentary (see fig. 166), has disappeared; the dental
formula is thus :

? oi
3 1

Hmi

The upper canines are of immense length, and the ridge
of enamel which runs down the front and back surface of
the teeth is distinctly serrated ; hence the name of saw-
toothed Tiger which has been given to the animal.

FIG. 167 0).

The lower canines were quite small, and ranged with the
incisors. The enormous length of the upper canine renders
it difficult to see in what manner it was made use of, as the

(1) Side view of the jaws and cranium of Machairodus (Drepanodon)
after Owen.

THE TEETH OF CARNIVOUA. 385

mouth could hardly have been opened to an extent sufficient
to enable its point to do more than clear the lower jaw.

The extinct Hysenodon had feline affinities, but differed
in that it presented the typical mammalian formula of

.3 1 4 3

i _ c - p - m - ,
3 1 i 4 5

its great peculiarity being that one and all of these teeth
were of " carnassial " form. Yet the elongated form of its
jaw is, so far as it goes, opposed to the idea of its having
been highly carnivorous ; its food at all events must probably
have consisted of animals very much smaller than itself.

Arctoidea. — Amongst the Carnivora grouped together by
many characteristics as 'bear-like/ a tolerably complete
gradation of character in the matter of dentition may be
traced.

Some of the group, such as the stoats and martins, are
very carnivorous ; others are mainly herbivorous. Of the
Mustelidee the dental formula is

.3 1 4 1

i _ c - p _ m -

3 1*4 2

There is a sort of primd facie resemblance to the feline
dentition, for the sectorials are very much like those of the
Felidae, but the last tooth in each jaw is a broad topped
tubercular molar, even in the most carnivorous members of
the group, while in those which are less so, such as the
badger, the molar teeth are very broad and obtuse, the
lower sectorial having a very small blade and a very large
tubercular posterior talon, so that, without having really
lost its typical formation, it comes practically to be a broad
grinding tooth.

In the Procyonida) (Racoons and Coatimundis, &c.), we

c c

386

A MANUAL OF DENTAL ANATOMY.

have a further departure from the carnivorous character,
in the increased development of the molar series : the dental
formula is

i* cl P4m!.
3 1 l 4 2

In the Coatimundi, for example, the upper sectorial
Fio. 16S ].

has a very large ' tubercle,' and posteriorly to this there is
a small additional tubercle; the 'blade' has no large or
conspicuous thin, flat, sharp edge, but presents two pro-
nounced cusps.

The lower sectorial is no longer recognisable as a car-
iiassial tooth, but all the true molars are broad teeth with
four or five cusps.

The canines are very peculiar, those of the upper jaw
being very straight and much flattened from side to side ;
those of the lower jaw strongly curved, and marked by a
deep groove near the front of their anterior surface.

(*) Upper and lower tooth of a Coatimundi (Nasua socialis). The fourth
upper premolar (carnassial tooth) has lost its sectorial character by the
Wade being much less, and the tubercle much more developed than in the
(Eluroidea ; there is an additional internal tubercle at the back of the
tooth.

THE TEETH OF CARNIVOEA.

387

In the Bears the teeth are yet further modified to suit
the requirements of mixed or vegetable feeders.
The dental formula is generally

3

The incisors of the upper jaw present the notch across
the crown, so common in carnivora, and the outermost is
large and not unlike a canine ; the canines are, relatively
to the other teeth, not so large as in dogs or Felicia) ; never-
theless they are stout strong teeth, upon which the anterior
and posterior ridges of enamel are well marked.

Tho first three premolars are small dwarfed teeth ; the

Fia. 169 (>).

first premolar is very close to the canine, and has a crown
of peculiar form, produced out towards the canine.

All four of the premolars seldom persist through the lifc-

(l) Teeth of a Bear (Ursus thibetanus). The figure is drawn from a
young specimen, in Avhich the canines have hardly attained to their full
length. In this bear the four premolars are all persistent.

c c 2

388 A MANUAL OF DENTAL ANATOMY.

time of the animal ; the first premolar, however, is rarely
(if ever in recent species) lost, the second being the first to
fall out, and then the third. As the fourth is never lost, in
most adult bears the first and fourth premolars are found,
with a wide interval between them. The premolars of
bears thus form an exception to the rule that when a tooth
is lost from the premolars, the loss takes place from the
front of the series.

The fourth upper premolar (carnassial tooth) retains
something of its carnassial character ; the first lower molar
very little, save that it is a narrower and more elongated
tooth than the other true molars.

The true molars are squarish or oblong teeth, raised into
blunt tubercular cusps ; they vary in different species.

In the sloth bear (Ursus labiatus) the incisors are small
and the median pair are lost early ; it is variously stated
to be frugivorous and to feed on ants, the latter probably
being the more truthful account.

CARNIVORA PINNIPEDIA (SEALS).
The aquatic Carnivora are divided into three families : —

I. The Otariida3, or Eared Seals, comprising- the single genus Otaria

known as Sea Lions, or Sea Bears. These are the " fur Seals,'"
from which seal skin is procured, and they are less removed
from the terrestrial carnivora than are the other seals : the
limbs are better adapted for walking, there are external
ears, &c.

II. The Phocida3, to which family the Seals of our own coasts

(Phoca greenlandica, &c.) and the Great Proboscis Seals of the
southern seas (Cystophora) belong.

III. The Trichechidas, or Walruses, an aberrant Arctic family con-
sisting of one genus only.

The dentition of the seals is less highly specialised than
that of other carnivora, in some cases approximating to that
of homodont cetaceans.

The canines are generally well marked by being larger

THE TEETH OF CARNIVORA.

than the other teeth, but the molars and premolars are
very similar to each other, and are simple in pattern. The
milk dentition is very feebly developed in the seals ; in the
Otaria (fur seal), which of all the seals most approaches to

Fio. 170 0).

the terrestrial carnivora in other characters, the milk teeth
arc retained for a few weeks, but in most others they are

(l) Jaws of Otaria, in which the teeth are affected by the form of erosion
alluded to in the text. After Dr. Muric. Odcnt. Soc.'kTrans., 1870.

390 A MANUAL OF DENTAL ANATOMY.

shed about the time of birth (of. page 300). Thus Pro-
fessor Flower tells us that in a Phoca greenlandica a week
old scarcely a trace of the milk teeth was left.

The teeth of Otaria and of some other seals become
much worn down, and they also seem to become eroded at
the level of the gums, as they are often deeply excavated at
points which seem unlikely to have been exposed to friction,
but the nature of this erosion has not been adequately in-
vestigated.

The common seals (Phoca) have. a, dental formula

.3 1 4 1

i _ c -p _ m - .
3 1 l 4 1

The incisors are of simple form, and the outer are the
larger. The canine is a strong recurved tooth, with a
large root ; behind it follows a series of molars, each of

FIG. 171 I1).

which (with the exception of the first) bears a central
principal cusp, with a smaller accessory cusp before and
behind it. The forms of the crowns vary a good deal in
different genera, in some the cusps being much larger,
more deeply separated from one another and recurved ; and
in others the accessory cusps being multiplied, so that the

({) Teeth of Phoca greenlandica.

THE TEETH OF CARNIVORA.

391

name of ' saw-toothed seal ' has been given to their pos-
sessors.

In the Hooded seals (Cystophora) the incisors are re-
duced to one in the lower jaw and two in the upper ; the

FIG. 172 (').

canines are of great size, but the molars are small and
simple in form, so as to approximate to the teeth of true
Cetacea.

The walrus (Trichechus rosmarus), an aberrant Arctic
form, is possessed of enormous upper canines, which pass
down outside the lower lip, and are of such dimensions as
to materially modify the form of cranium by the size of
their sockets ; they grow from persistent pulps, and are
composed of dentine with a thin investment of cement.

The great tusks are employed to tear up marine plants
and to turn over obstacles, the walrus feeding upon Crus-
tacea, and also iipon seaweed, &c. ; they are also used to
assist the animal in clambering over ice : as they are of
almost equal size in the female, they cannot be regarded as
weapons of sexual offence, but they are undoubtedly used
in the combats of the males.

") Permanent and milk teeth of Elephant Seal (Cystophora proboscidea).

392

A MANUAL OF DENTAL ANATOMY.

In addition to the great tusks the walrus ordinarily has a
row of four or five short simple teeth, worn down to the
level of the gums ; of these, the one placed immediately
within the base of the great canine is in the inter-maxillary
bone, and is an incisor : the ordinary dental formula is given
by Professor Flower as

il dp*.

0 1 l 3

But there is some difficulty in assigning 'a definite dental
FIG. 173

formula : for in front of the solitary incisor are often the

(*) Side view of upper and lower jaws of a Walrus (Tricheclms rosmarus).
The upper jaw has been tilted a little to one side, in order to bring into
view the molar teeth at the same time with the long tusks. The deter-
mination of the teeth being open to question, they have been simply
umbered.

THE TEETH OF CARNIVORA. 393

sockets (or even the teeth themselves) or two others, which
are for various reasons rather to be regarded as non-per-
sistent teeth of the permanent set than as milk teeth ; and
there are also small teeth sometimes to be met with behind
the molars, which seem to be rudimentary permanent teeth.
The teeth above alluded to may persist through life, and
probably often do ; but they are sure to be lost in macerated
skulls, as they have but little socket. Of the milk den-
tition four teeth have been traced in each jaw; they are
rudimentary, are lost about the time of birth, and corres-
pond in position to the more largely developed teeth of the
adult. Hence the question if those small rudimentary
teeth above alluded to are to be regarded also as milk
teeth which are long retained, or as rudimentary permanent
teeth ; at present this requires further elucidation.

CHAPTER XIII.

THE TEETH OF INSECTIYORA, CHIROPTEEA, AND PRIMATES.

The insect! vora form rather a heterogeneous order of Mammals.
and embrace very various forms. All of them are of rather small
size, and some are very small indeed. Their diet consists for the
most part of insects, and their teeth are generally adapted for thi^
by being furnished with many points. The best known animals in
the order are the Hedgehogs, the Shrews, and the Moles ; to these
are to be added the Galeopithecus, or '• Flying Lemur," and the
Macroscelidae (Elephant mice). Insectivora are more abundant in
Africa, Asia, and South America than in Europe. The Shrews
approximate in some measure towards the Rodents, and the Tupaia
is very lemurine in its characters.

THE common English Hedgehog (Erinaceus) has the dental
formula

.30 43
4c.pm.m-.

In the upper jaw there is a wide interval between the
first pair of incisors, which are much the largest, and are
caniniform in shape. The next two teeth (incisors) are
quite small, and resemble premolars in their form. The
next tooth has two roots, and a crown with one cusp, and is
also like the premolars behind it. This tooth, the root
of which shows indications of division, is sometimes called
a canine, because it comes next behind the intermaxillary
suture ; behind this come two small premolars.

The fourth upper premolar is totally different in size and
form from the third : its crown is large, squarish, and

THE TEETH OF IN SECT IVOR A.

395

furnished with four cusps, of which the antero-extcrnal one
is far the longest and sharpest.

The first upper true molar has a square crown, upon
which are four sharp cusps : it is implanted by four roots.

FIG. 174(').

The second true molar is also square, quadricuspid, and
has four roots ; but it is much smaller than the first, while
the third upper true molar is quite a small, compressed,
double-rooted tooth, with a thin-edged crown.

In the lower jaw the first incisors, less widely separated
than the upper, are also the largest ; then follows another
tooth termed incisor, on account of its relation to the upper
incisors when the mouth is closed. The third tooth is
much larger, and of peculiar form. The fourth tooth from
the front is a small single tooth, like the third, but upon a
smaller scale. Next behind it, comes a tooth which is very
much larger, and its crown carries two principal cusps with
a small subsidiary cusp. The next tooth (first true molar)
has an oblong crown beset with five sharp cusps, of which
four are arranged at the corners of a square, while the fifth,
obviously an elevation of the cingulum, lies a little in front
and towards the inside of the tooth. In the second true
molar the fifth cusp is but little indicated, while the last

0) Upper and lower teeth of the Hedgehog.

396 A MANUAL OF DENTAL ANA TOM Y.

true molar is a dwarfed tooth \vith but one cusp. Several
dental formula) have been assigned to the Hedgehog : there
is little room for difference of opinion as to the nomen-
clature of its upper teeth : though some authors (e.g.*
Professor Mivart) prefer to call the first premolar a canine.
But in the lower jaw some authors give i -c- pm _,

others i _ c _ pm -, and others again, i - pm -. The

last given seems the least artificial, and corresponds best
with the relations between the upper and lower teeth when
the mouth is closed.

Rousseau describes the existence of twenty-four milk

teeth, which he classifies thus : (i - dm -) ; that is to say,

all the teeth in front of the true molars had deciduous pre-
decessors, but his grouping of them into incisors and molars
is quite arbitrary.

The milk teeth are not shed and replaced until the
animal has attained to almost its full dimensions, and all
three true molars are in place.

The teeth of the Hedgehog fairly represents sonic of the
features of Insectivorous dentitions, for the forcep-like in-
cisors, the stunted or non-developed canines, and the molars
bristling with pointed cusps, are common to very many
Insectivora.

The Shrews have numerous sharply-pointed teeth, the
points interdigitating and fitting very closely together
when the mouth is shut. There is no tooth either in the
upper or lower jaw which is so elongated as to deserve the
name of canine ; but between the incisors and the true
molars are several small teeth which, by analogy, are called
prernolars. The true molars are not very different in
pattern from those of the mole (B in Fig. 176), and present
the W-contour so common in the molars of Insectivora.

The most marked peculiarity in the dentition of the

THE TEETH OF INSECTIVORA. 397

Shrews lies in the form of their incisors. The first upper
incisor is always very large indeed : it looks vertically
downwards, is a little hooked, and has a notch, and a second
low cusp behind the principal long pointed cusp. The tip
of the lower incisor fits into this notch. The lower incisor
is also very large ; it lies nearly horizontally, though the
point is bent a little upwards. Along its upper edge there
are, in most species, three or four small cusps, while its
lower border is curiously prolonged outside the bone of the
jaw, so as to in some measure encase this latter. The lower
incisor is at least one-third as long as the whole alveolar
border. The incisor teeth of the Shrew would appear to
form a very efficient pair of pincers, with which to pick up
the minute creatures on which it feeds. Of the milk teeth
of Shrews little is known : they are said to be absorbed
before birth, but accurate observations upon them are much
needed, their very existence being doubtful.

The dentition of the Mole (Talpa) has been the subject of
much controversy, the determination of its canines, <fec.,
presenting such difficulty that no less than five different
dental formula) have been assigned to it,

In the front of the upper jaw come three small teeth, the
first being somewhat the largest, which are well within the
limits of the intermaxillary bone, and are doubtless incisors.
But the next tooth, which is very big, also appears to be
implanted in the intermaxillary bone, the suture passing
across its socket close to the back of its posterior root.
According to its implantation it therefore would be an
incisor (!) but it is very unlike an incisor ; and it is two-

(l) I confess I cannot follow Mr. Spence Bate when, in his valuable
paper on the milk teeth of the mole, he says, " This tooth is implanted
within the limits of the premaxillary bones, the suture separating them
from the maxillary, passing through the posterior portion of its alveolus :
thus demonstrating that this deciduous tooth is the true homologue of that
of the canine in the mammalian type." Surely it would go to prove the
contrary, if accepted as evidence at all upon this point.

398 A MANUAL OF DENTAL ANATOMY.

rooted, a thing anomalous either in an incisor or a canine,
though found in the canine of Gymnura, which is beyond
question in the maxillary bone.

Next come three minute premolars, and a fourth, which
is much larger than the others : these all have single crowns,
consisting of little more than a single sharply-pointed cusp.

The first two upper molars are large teeth bristling with
cusps : the third is much reduced in size and simplified

•j- Aw I; Sice

in pattern. In the lower jaw the four front teeth arc all
small, but the fourth or outermost of these incisors is called
by some writers the lower canine, because, when the teeth
are closed, it passes in front of the upper caniniform tooth.

Nevertheless the tooth which does the work of a canine in
the lower jaw is the fifth counting from the front : this is a
two-rooted tooth, and conforms so closely with the three
teeth behind it in configuration, that it is obviously only one
of these premolars developed to a greater length than the
others. It closes behind the caniniform upper tooth, so
cannot on this ground be called a canine by those who
attach importance to the term.

(!) Upper and lower teeth of the common Mole. The functionless milk
teeth (after Spence Bate) are placed above the permanent teeth which dis-
place them.

THE TEETH OF INSEOTIVORA. 399

The remaining three premolars are rather small and
single ; the true molars are of considerable size, and their
points are very long and sharp.

Ihave purposely avoided giving any dental formula for
the Mole : everything turns upon the value which we
attach to the term canine ; and I have already given reasons
(p. 284) for attaching but little homological importance to
its determination.

Mr. Spence Bate's paper (Trans. Odontol. Society, 1867),
valuable as it is in contributing to our whole knowledge of
the niilk dentition of the creature, does not appear to me to
help us much in the matter of determining the homologies
of the canine.

In a Mole 3J inches long he found eight milk teeth on
each side of both upper and lower jaws, as is indicated in
Fig. 175. The milk incisors were about one-twentieth of
an inch in length, and one two-hundredth in diameter, and
were rudimentary in form, consisting of long thin cylindrical
tubes surmounted by slightly expanded crowns. All the
milk teeth were of this simple form, save only the last in
each jaw, which presented crowns with two cusps, and had
their roots to some little extent divided into two.

At the time when these teeth are present the intermaxil-
lary suture is very distinct, and there is no doubt that the
fourth upper milk tooth, the predecessor of the caniiiiform
tooth, is in the intermaxillary bone.

The teeth had not fairly cut the gum, and the advanced
state of the permanent teeth beneath them make it doubtful
whether they ever do become erupted. At all events, they
can be of no use.

In many of the order Insectivora the milk dentition is
unknown, but we have exemplified amongst them every
grade of completeness in its development. Thus in the
Hedgehog and Centetes (an allied animal from Madagascar)

400

A MANUAL OF DENTAL ANATOMY.

the milk dentition is tolerably complete, while in the
Shrews it has all but, or quite, disappeared.

The W-pattern characterising the molars of Insectivora
has already been alluded to ; it is well exemplified in the
molar of Urotrichus.

In this tooth, as has been clearly shown by Prof. Mivart
(Osteology of Insectivora, Journ. of Anat., 1868), the four
cusps of the typical teeth (a, I, c, d) have been added to
by the elevation of the cingulum into three or four external,
and one internal cusp, making up the total number to nine.
Thus it is that the molars of this order often fairly bristle
with cusps.

In the Mole the number of cusps is diminished by the
coalescence of b and d into a ridge, and the disappearance of
FIG. 176 (').

l'' \3|T ** \jii^r' a

1> I"

AFC

the inner cusp of the cingulum, while the simplification is
carried yet further in the Cape Mole (c in Fig. 176).

It would be impossible to notice the somewhat varied
dentitions of other Insectivora in these pages, but mention
must be made of the very anomalous teeth of the Galeo-
pithecus, formerly placed with the Lemurs under the title of
"Flying Lemur."

Its lower incisors are divided by a number of vertical
divisions running down through a great part of the length
of the crowns, so that they can be compared to combs, or to

(*) A. Upper molar of Urotrichus ; B. Mole ; C. Cape Iridescent Mole,
(Cbrysochloris).

THE TEETH OF INSECTIVORA. 401

hands with the fingers slightly separated. What the pur-
pose served by these comb-like teeth may be remains
uncertain : no other animal has similar teeth. Ualeopithe-
cus has a well developed milk dentition, the milk teeth
being very similar to their successors.

The teeth of Insectivora are remarkable for the thickness
of their enamel, which in the Shrews is to some extent
penetrated by the dentinal tubes. The enamel is deeply
coloured in some Shrews, the pigment being actually in the
substance of the enamel, and not in any distinct layer.

THE TEETH OP CHIROPTERA.

The Bats, sharply distinguished from all other mammals
Toy the possession of wings, are divided into two groups,
respectively insectivorous and frugivorous.

The insectivorous Bats, by far the most numerous section,
are for the most part possessed of small incisors, rather
large canines, and premolar and molar teeth which bristle
with sharp cusps, and generally present the W-pattern. In
fact, in general character, their teeth resemble those of the
Insectivora.

The incisors are sometimes reduced in number, and
spaces left between them ; and some, as for example, the
Vampire (Desuiodus) have teeth specially modified to accord
with their blood-sucking habits.

This Bat has only one permanent incisor on each side, and
this is a large but thin and sharp-edged tooth, with which the
wound is made ; the lower incisors are small teeth with feebly
notched edges. The canines are large, and the molar
series, which is not required in an animal existing upon
blood, is stunted. The molar teeth are, however, sharp,
though small, and there is no marked distinction into
molars and premolars.

The frugivorous bats (of which the Pteropus, or flying

D D

402 A MANUAL OF DENTAL ANATOMY.

fox, is an example) have much larger muzzles, and the molar
teeth are set with intervals between them.

212 3

The dental formula is i - c - p - m -, but in some the
2i 1 o o

molar series is reduced below this number.

The incisors are small, and the canines rather large.

Both molars and premolars are of somewhat simple form,
being long, and compressed from side to side. The outer
borders of the crown of the molars are elevated into distinct
but not exceedingly sharp cusps, which become worn down
by use.

The insectivorous character of the presence of many
sharp cusps upon the teeth is not to be found in any of the
frugivorous bats. All the Pteropi have deciduous canines,
and four deciduous molars, of simple pointed form, but the
number of deciduous incisors is very variable.

The milk dentition of bats has been very carefully and
thoroughly investigated by Leche (Lund's Universit. Ars-
skrift, Tom. XII. and XIV., 1878), and at the present the
Megadermata are the only family in which the milk teeth
are unknown. The milk teeth are not of much functional
importance, as they are shed soon after, if not absorbed
before, birth, and they are not therefore implanted in very
definite sockets.

In their slight cylindrical elongated roots, surmounted by
expanded crowns, these milk teeth often recall those of the
Mole.

Sometimes the milk teeth are to be found even after the
permanent teeth are in situ; in other instances, as for
example the deciduous molars of Molossus, they never cut
the gum. The milk dentition of the Vampire (Desmodus) (l)

(J) In a skull of Desmodus, in the possession of Mr. E. F. Tomes, the
third milk tooth appears to correspond in position to the permanent
canine ; the same is the case in the specimen figured by Messrs. Gervais
and Castelmain (Exped. dans les part. cent. d'Amerique du Sud).

THE TEETH OF PRIMATES. 403

appears to consist of incisors only, or of incisors and canines ;
though the absence of observed molars may be due to the
fact that they are, as in Molossus, shed very early.

It has, near to the front of the upper jaw, six teeth, each
of which is very long and slender, and has a strongl}- hooked
point : it has been suggested that these feeble hooked teeth
may assist it in holding on to the mother.

THE TEETH OF PRIMATES.

The order Primates embraces Man, Monkeys, and the

Some naturalists have been disposed to separate the Lemurida3
from the rest of the Primates, on the ground that some Lemurs
approximate rather closely to the Iiisectivora, while again the order
Insectivora contains some forms which recall the Lemurs.

But although the Lemuridog are undoubtedly inferior to the
Monkeys, and stand apart from them more widely than do the
Monkeys from Man, most authors now place them in the order
Primates, which is to be divided as follows : — _

( Lemuridaj. Lemurs.

Primates < Simiada3. Old and new world Monkeys.

( Anthropidfe. Man.

Lenmridce. — The Lemurs for the most part are found in
Madagascar, and to a less extent on the mainland of Africa
and in southern Asia. In their dentition, just as in other
characters, they differ somewhat from the true monkeys,
though, on account of there being several very aberrant in
form, it is difficult to give any general account of them.
Most of them have the upper incisors very small, and widely
separated from one another ; in the lower jaw these are
antagonised by six long, thin, narrow procumbent teeth,
generally regarded as being two pairs of incisors and the lower
canines : in both upper and lower jaws the next tooth is
large and pointed like a canine, but the lower caniniforin
tooth bites behind the upper, and therefore is held not to

D D 2

404 A MANUAL OF DENTAL ANATOMY.

correspond to it, but to be the first premolar (cf. page 283).
The premolars are compressed from side to side, and are
very sharp : the molars are armed with long sharp cusps,
which are worn down in old animals.

The upper molars in many lemurs are armed with four
cusps, connected by an " oblique ridge " like those of man
and the anthropoid apes.

There is a very aberrant lemur, the Aye-aye (Cheiromys),
which in its dentition imitates the rodents.

ijogpmjmj.

In both upper and lower jaws the incisors form a single
pair of large curved teeth, growing from persistent pulps,
and wearing obliquely so as to constantly preserve a sharp
cutting edge. The enamel is very much less thick, if not
altogether absent, upon the backs of these incisors.

After a considerable interval, which is devoid of teeth,
there follow four upper and three lower teeth, which are
not of persistent growth, but have definite roots, .and re-
semble the molars of many omnivorous rodents.

Being a somewhat rare and strictly nocturnal animal,
little is known of its food ; some have believed that it made
use of its rodent incisors to cut away portions of wood in
order to get at the grubs contained in it, drawing them out
of their hiding place by means of its curiously elongated
finger, whilst others believe that it gnaws the sugar cane.
But whatever the nature of its food may be, it is certain
that its scalpriform incisors arc put to hard work, and so
kept worn down, for in a specimen kept for a time in the
Zoological Gardens, which was supplied with soft food,
the incisor teeth grew to an excessive length, and ultimately
caused the animal's death by the points of its lower incisors
perforating the palate. The accompanying figure represents
the muzzle of this specimen, and although the upper teeth

THE TEETH OF PRIMATES. 405

have grown to an inordinate length, and have diverged from
one another, it will serve to show the rodent-like aspect of
its mouth.

Although, functionally, its teeth are those of a rodent,
yet despite this adaptive resemblance, the milk dentition

FIG. 177 (').

retains certain characters which indicate the lemurine origin
of the creature.

In the upper jaw the milk dentition consists of two small
incisors, a canine and three molars ; in the lower jaw of two
small incisors and two small molars ; it is said that in an
early stage a third milk incisor is to be found.

The permanent incisors push their way up between the
first and second milk incisors ; at a certain stage all three

(!) Aye- Aye (Cheiromys), which died in the Zoological Gardens (after
Dr. Murie). The upper incisors, from want of sufficient use, have grown
long and diverged from the middle line.

406

A MANUAL OF DENTAL ANATOMY.

are to be seen at once, but the large size of the permanent
incisors causes the speedy loss of the milk incisors.

No known rodent has so many milk teeth, nor indeed any

FIG. 178 (').

milk incisors at all ; the aye-aye thus affords an excellent
example of a milk dentition preserving characters which arc
lost in the extremely modified adult dentition.

The special interest which attaches to the dentition of
Cheiromys has been already alluded to (see page 271) ; to
briefly recapitulate, it is this : in Madagascar, an isolated
area separated by a wide tract of deep sea from other areas,
true rodents are almost absent, but lemurs abundant. But

i1) Upper and lower jaws of Cheiromys. A. Milk dentition, witli the
permanent incisors just emerging, i, I. Upper and lower permanent in-
cisors. i2, 12. Upper and lower milk incisors, c. Milk canines, dl,
d 2, da, d b. Upper and lower milk molars. (Twice natural size.) B.
Reduced figure of permanent teeth (after Peters).

THE TEETH OF PRIMATES. 407

one of the lemurine animals which are to be found there has
been so modified that its teeth to all intents and purposes
are those of a rodent. Yet with all this modification it
retains characters (notably its milk dentition) which are
quite unlike those of true rodents, but which recall the
manner of its origin from higher lemurine forms.

SimiacUe. — The true monkeys are divided into two
great divisions, the new world monkeys and the old world
monkeys. The former differ in many respects from the
latter ; for the most part they have prehensile tails, and
their nostrils are set somewhat widely apart, whence
they are called Platyrrhine, or wide-nosed monkeys, and
they differ also in their dental formula, which is —

.2 1 3 3 0/,
i-_orP-m- = 36.

The little marmoset monkeys have only 32 teeth, but they
agree with the other new world monkeys in having three
premolars on each side, the molars being reduced to two in
number. The upper molars of many new world monkeys
have the antero-internal and extero-posterior cusps joined
by an oblique ridge, a character which is shared in the old
world groups by man and the anthropoid apes only.

All the Quadrumana have a well developed milk den-
tition.

Old world or Catarrhine monkeys all have the same
dental formula as man —

.212 3

As an example the Macaque monkey may be taken. The
upper and lower incisors, but especially the former, are
directed obliquely forwards, and the^ lateral incisors are very
much smaller than the centrals. In the upper jaw a con-
siderable interval separates the incisors from the canine,

408

A MANUAL OF DENTAL ANATOMY.

which is a very large tooth, somewhat triangular in section,
with a sharp edge directed backwards, and with a deep
groove on its anterior surface.

The upper premolars are implanted by three distinct
roots, as arc also the true molars ; the latter are quadri-
cuspid, but lack the oblique ridge.

Neil, Size ,

The lower canine is a sharp and powerful tooth, though
it is very much smaller than the upper; the first lower
premolar, by its front surface, articulates with the upper
canine, and is of curious form. It is implanted by two
roots, but the anterior root is produced forwards, so that the
antero-posterior extent of the tooth is much increased.

The apex of the cusp of the tooth is almost over the
posterior root, and from this point the crown of the tooth
slopes obliquely forwards down to its anterior root. This
peculiarity in the form of the first lower preniolar is eminently

0) Upper and lower teeth of a Monkey (Macacus nemestrinus, male).
The length and sharpness of the canines, and the peculiar form of the
anterior lower preniolar, contrasts with the aspect of the corresponding
teeth in the Anthropoid Apes or in Man.

THE TEETH OF PRIMATES. 409

characteristic of the baboons. There is nothing to note of
the second premolar save that it is implanted by two roots,
like the true molars, which are quadricuspid ; of them the
third is larger than the first two, and is quinquicuspid.

There is considerable difference in the size of the canine
in the two sexes, that of the male being very much the
larger ; this difference does not exist in the deciduous den-
tition, in which the canines are relatively small.

The Anthropoid Apes are the Gibbons (Hylobates), the
Chimpanzee (Simia Troglodytes, or Troglodytes niger), the
Orang (Simia or Pithecus Satyrus), and the Gorilla (Troglo-
dytes Gorilla).

Upon the whole the gibbons are the lowest, and the
gorilla the highest of the anthropoid apes, which are all
confined to tropical areas. Thus the gorilla and chimpanzee
are confined to tropical Africa, and the orang is limited to a
part of the Malay archipelago. The gibbons are more
widely distributed over the Malay archipelago and tropical
Asia.

Although upon the whole the gorilla approaches most
nearly to man, this can hardly be said to be the case with
its dentition. The jaws are very square, and there is a large
diastema in front of the upper canine, which in the male
gorilla is of great size and strength, its top descending far
below the level of the alveolar border of the lower jaw when
the mouth is shut.

In the lower jaw there is no diastema, but the teeth are
all in contact with one another ; the first of the pre-molars
is a very strong pointed cone, showing plainly the close
relationship between canines and premolars alluded to at a
previous page (p. 17).

The molars increase in size from before backwards, the
third molars attaining to a vejry large size.

Nevertheless, though the teeth are coarser and stronger,
there is a general resemblance to those of man.

410

A MANUAL OF DENTAL ANATOMY,

It has been pointed out by the late Professor Rolleston that
the canine tooth of the male anthropoid apes is a little later
in coming into place than in the female. Thus in the male
chimpanzee and orang, it is not cut until after the third
molars (wisdom teeth) are in place, whereas in the female
it follows the second, but precedes the third molars. The
sexual difference in the canine teeth is very well marked in
all the anthropoid apes, and its later eruption in the males

FIG. 180 0).

f Nat. Sir

is explicable both upon the ground that, being a sexual
weapon, it is not needed prior to the attainment of sexual
maturity, and also that being of very large size its formation
might be expected to take a longer time. No such difference
pertains to the milk dentition, in which the order of eruption
is exactly that met with in man.

Dr. Magitot (Bulletin de la Societe d'Anthropologie de
Paris, 1869) combats the idea that there is any difference in
the order of the eruption of the permanent teeth between
man and the anthropoid apes, but, while his observations
have been both careful and widely extended, he lays much

C) Upper and lower teeth of an Anthropoid Ape (Shnia Satyrus, or
Orang Outan).

THE TEETH OF PRIMATES.

A '

411

.stress upon an observation made upon a, female gorilla skull,
in which, as has just been mentioned, the order of succession
is not quite the same as in the male.

The dentition of the orang approaches tolerably closely to
that of man, and the points of resemblance and of difference
may be fairly well seen in the accompanying figure.

The central upper incisors are similar to those of man,
but are larger ; the laterals are, relatively to the centrals,
much smaller, and are very caniniform in shape, both inner
and outer angles of their cutting edge being sloped off to
such an extent that a central pointed cusp remains, in
place of a thin cutting edge. The canines are strong,
pointed teeth, the cingulum and the ridge joining it with
the apex of the cusp being well marked upon their inner
sides. In the female the upper canine is about half as
long again as any of the other teeth; in the male it is
longer.

The first bicuspid is a little more caniniform than that
of man ; its outer cusp is long and pointed, and a ridge
unites it with the anterior part of the inner cusp, which is
feebly pronounced; the second is a blunter and broader
tooth. The premolars are implanted by three roots. The
molars are not unlike the human teeth in pattern.

In the lower jaw the incisors are large and stout ; the
canines sharply pointed, with a well marked cingulum,
and a well marked median ridge on the inner side of the
crown. The first premolar is a shorter, stouter, and
blunter copy of the canine, and can hardly be said to
have an inner cusp. In the second premolar the inner
cusp is as high as the outer, and the cingulum is elevated
both before and behind till it almost forms two additional
cusps.

Indeed, I am not acquainted with any dentition which
exemplifies the transition from incisors to canines, from

412

A MANUAL OF DENTAL ANATOMY.

canines to premolars, and from premolars to true molars,
better than that of the orang.

FIG. 181 I1).

FIG. 182

FIG. 183 (3).

The lower molars resemble those of man, save that their

(') Skull of a young male Orang. The upper canine does not nearly
reach to the lower alveolar border.

(2) Skull of adult male Orang, in which the canine is largely developed.

(3) Side view of skull of an idiot.

THE TEETH OF PRIMATES. 413

surface is marked by that finely wrinkled pattern which is
common to all the unworn teeth of the orang. One is
struck by the great backward elongation of the jaws, by
their squareness, by the parallelism of the two sidea
which converge slightly at the back, and by the large size
of the teeth in proportion to the bulk of the whole animal.

The large size of the canines being in a measure a sexual
character, is, as is so often the case, not very noticeable in
the young animal : the two acompanying illustrations of
a, young and an adult male orang may serve to show this, as
well as some other differences developed by age.

The differences which serve to distinguish the dentition
of the most anthropomorphic apes from that of man are
mainly these. Relatively to the size of the cranium, and of
the whole creature, the teeth and jaws are very much larger
in all their dimensions ; hence the creatures are progna-
thous, and the facial angle small, even when compared
with the jaws and cranium of an idiot. As might be ex-
pected this difference is not so great in the young as in the
adult animal.

In place of the teeth being arranged in a sweeping curve,
the jaws are squarish, the incisors being arranged in some-
thing approaching to a straight line between the two great
outstanding canines, behind which the premolar and molar
series run in straight lines, converging somewhat as they go
backward. There is a " diastema " (J) or interval in front of
the upper canine, into which the point of the lower canine
passes, when the mouth is closed. Both the greater square-
ness of the jaws, and the existence of a diastema, are direct
results of the great size of the canines, and are consequently
not marked in young specimens.

The upper premolars are implanted by three roots, the

(*) Fomething approaching to a diastema is said to have been observed
by Vogt and Broca in early European skulls.

414

A MANUAL OF DENTAL ANATOMY.

lower by two roots, just like the true molars, and the pre-

molars when unworn partake more of the pointed character
than they do in man.

(1) Upper teeth of a Caffir. The oblique ridge of the upper molar is
distinct, not only upon the first and second, but also upon the third molar
or wisdom tooth, which in this skull has the normal three roots well
marked.

(2) Lower jaw of a Caffir, in which the quinquicuspid form of the first
and third molars is well seen, it being somewhat less strongly indicated in
the second molars.

THE TEETH OF PRIMATES. 415

The wisdom teeth present the same pattern of grinding
surface, are larger than the other molars in the gorilla
and the orang, and there is abundant space for them, so
that they play an important part in mastication. The
molar teeth of these apes are also squarer, their cusps
sharper and longer, and the characteristic patterns more
strongly pronounced, than in man.

Anthropidae. — In passing from the highest of the apes
to the lowest of mankind, there is a sudden change in the
character of the dentition ; but while it cannot but be
admitted that there is a gap, yet the differences are rather
of degree than of kind.

Even in the lowest of human races the facial angle is
greater, that is to say, they are much less " prognathous "
than the apes, and the upper and lower incisors are more
nearly vertical in position, not meeting one another at such
an angle as in the apes. Mr. Perrin (Monthly Review Dent.
Surgery, 1872) states that in a gorilla skull there is an inch
of bone in front of the anterior palatine foramen : in a negro
half an inch, and in a Greek skull it was close behind the
incisors.

Ifc is generally said that in man the molars decrease in
size from before backwards ; that is to say, that the first
molar is the largest, while in anthropoid apes the contrary
is the case. Though this is on the whole true, it requires
some qualification : thus in certain lower races, such as the
Australian blacks, the second and third molars are not
smaller than the first, and of the chimpanzee the same
thing may be said.

There is no diastema ; no sexual difference in dentition ;
no tooth projecting beyond its fellows, and the teeth are
arranged in an unbroken arch. The premaxillary bones
become fused with the superior maxillary early in- life,
whereas^ in the Quadrumana they remain distinct.
\, In general terms it may be said that the dentition of th

416 A MANUAL OF DENTAL ANATOMY.

lower races of mankind differs from that of the higher in the
following particulars : the arch is not so rounded, but is
squarer in front ; the teeth are larger, and are disposed
with greater regularity ; the wisdom tooth has ample space
to range with the other teeth, and is a characteristic upper
or lower molar, the pattern of its grinding surface (quadri-
cuspid if it be an upper, quinquicuspid if it be a lower
tooth) and the disposition of its roots corresponding with the
first and second molars, which do not greatly exceed it in
size. Specimens of negro skulls may be found in which
there is scanty room for the wisdom tooth, and in which
consequently it is a little stunted in its development : 011
the other hand, plenty of well formed and well placed
wisdom teeth may be picked out of European mouths,
though as a rule the wisdom tooth is much smaller than
the other molars, does not bear the characteristic pattern
of cusps and grooves, has its roots connate, and it is not
very infrequently a mere rudimentary peg. The stunted
development of the wisdom tooth would seem to be a
consequence of want of space during its formative period ;
the upper wisdom tooth is especially apt to be cramped in
this way.

There is some little evidence that the wisdom tooth is in
process of disappearance from the jaws of civilized races :
in anthropoid apes the wisdom tooth is nearly or quite as
large as the other molars, and shows no variability, whilst
it comes into place almost simultaneously with the canine :
in the lowest races of mankind the wisdom tooth appears to
vary but little, is of large size, and is seldom misplaced ; in
highly civilised races it is very variable in size, form, and in
the date of its appearance, is often misplaced, and is not
uncommonly quite rudimentary. It seems to be a legitimate
inference that a further modification of the race in the same
direction will result in the disappearance of the wisdom
tooth altogether.

THE TEETH OF PRIMATES. 417

Some exception must however be taken to such general
statements : thus the Esquimaux not uncommonly have the
wisdom teeth small and sometimes crowded out of place ;
and amongst the African races instances on the one hand of
the wisdom teeth being small, and on the other, of fourth
true molars existing, are to be met with.

Nevertheless, for the present, a case in which the wisdom
teeth are very small can hardly be called a typical well-de-
veloped European mouth.

In many low races (Bosjesman, Negro, Australian, New
Caledonian, Caffir) the second lower molar has five cusps,
just like the first : this is so in the anthropoid apes, but in
European races the fifth cusp is generally wanting in the
second lower molar.

It is not a little interesting thus to find that the differ-
ences which serve to distinguish the teeth of the lowest
savage from those of an European, are to a certain extent the
same with those that mark the step from a Quadrumanal to
a human dentition, though of course the divergence of the
dentition of the savage from that of the ape is far greater
than is that of the European from the lowest savage.

It is very possible that the larger development of the
jaws of the savage may be simply due to the harder work to
which they are put while he is growing up. And after the
attainment of adult proportions, the teeth of such a man
become greatly worn down by reason of the hard and often
gritty nature of his food.

It was pointed out by Mr. Mummery, in a very instructive
paper (" Transactions of the Odontological Society," vol. iL^
new series, 1869), that destructive wearing down of the
teeth was of very common occurrence amongst rude (!) races,
while the contrary is true of highly Civilised races ; this was

(*) To those races mentioned by Mr. Mummery may be added the
mound builders of North America, whose teeth were always worn down to
an excessive extent.

E E

418 A MAXUAL 01 DENTAL ANATOMY.

very likely due to the admixture of earth and other foreign
matter with the food. And, furthermore, that the occurrence
of dental irregularities, due to an insufficient size of the
arches, was comparatively speaking unknown among the
ruder races, whilst it has been common amongst peoples of
more luxurious habits for a long period of time.

The range of variation in the size of the jaws of healthy,
otherwise well-developed adults is great : thus the smallest
jaw (occurring in a man of stout build, above middle height)
with which I am acquainted measures in width only 1 J inch,
and in length from back to front '1J inch; whilst the largest
(occurring in a gentleman of lesser stature ; of Basque ex-
traction, moreover, which makes it the more striking) (J)
measures no less than 2J inches in width and 2£ inches in
length : and even larger dimensions are recorded in the
"Dental Cosmos " of September, 1876 ; the width being taken
between the centre of the alveolar borders at the position of
the wisdom teeth, and the length being measured 011 a line
drawn from the incisors to another line joining the two
wisdom teeth.

On the whole, it must be said that there are fewer
constant differences between the teeth of the various races
of mankind than might have been a priori expected ; in fact,
we may almost saj7 that the teeth of savage man are pretty
much what we should look upon as an exceedingly perfectly
formed set of teeth if we were to see them in the mouth of
an European.

0) Magitot (Bullet, de la Soc. Anthropol. de Paris, 1869) says : — " Les
liasques, par exemple, reinarquables par la petitesse extreme de leurs
dents. "

CHAPTER XIV.

THE TEETH OF MARSUPIALIA.

The great sub-class of Marsupials, consisting- of animals very
sharply marked off from placental Mammals by many striking
peculiarities and amongst others, by the very helpless condition in
which the foetus is born, was once very widely distributed over the
globe. Now, however, Marsupials are numerous only in Australia,
where they are almost the sole representatives of the Mammalian
class ; there are a few Marsupials elsewhere, as in America
(Opossums) and New Guinea ; but there are no Marsupials in
Europe, most parts of Asia, and Africa.

The Marsupials of America are all Opossums {JDidelphidai), and
this family is not represented in Australia. There is evidence to
indicate that the Marsupials of America have nothing at all to do
with the Australian Marsupials, but were derived from a different
source, at the time when Marsupials abounded all over Europe.

The Marsupials of Australia almost monopolise that country ;
thus Mr. Wallace says of it : " The Australian region is broadly
distinguished from all the rest of the globe by the entire absence
of all the orders of non-aquatic mammalia that abound in the old
world, except two — the Winged Bats (C/iiroptera), and the equally
cosmopolite Rodents. Of these latter, however, only one family is
represented — the Muridae— (comprising the Rats and Mice), and
the Australian representatives of these are all of small or moderate
size — a suggestive fact in appreciating the true character of the
Australian fauna.

" In place of the Quadrumana, Carnivora, and Ungulates, which
abound in endless variety in all the other zoological regions under
equally favourable conditions, Australia possesses two new orders
or sub-classes, Marsupialia and Monotremata, found nowhere else
in the globe, except a single family of the former in America.

" The Marsupials are wonderfully developed in Australia, where
they exist in the most diversified forms, adapted to different modes
of life. Some are carnivorous, some herbivorous, some arboreal,
others terrestrial. There are insect-eaters, root-gnawers, fruit-
eaters, honey-eaters, leaf or grass-feeders.

E E 2

420 A MANUAL OF DENTAL ANATOMY.

" Some resemble wolves, others marmots, weasels, squirrels, flying-
squirrels, dormice, or jerboas.

" They are classed in six distinct families, comprising about thirty
genera, and subserve most of the purposes in the economy of nature
fulfilled in other parts of the world by very different groups ; yet
they all possess the common peculiarities of structure and habits
which show that they are members of one stock, and have no real
affinity with the old-world forms, which they often outwardly
resemble." — " Geographical Distribution of Animals." p. 391.

WHAT Mr. Wallace says, speaking of the creatures in their
entirety, is equally applicable to their teeth.

In Australia, the present home of the Marsupials, repre-
sentative species abound ; that is to say, widely different
though the animals really are, there are many genera and
species which have the habits of, and, as it were, fill the
place of such creatures as the Carnivora and Insectivora and
Rodents amongst the placeutal mammalia. And not only
do they possess something of their habits and external con-
figuration, but in those characteristic structures which are
subservient to the creature's immediate wants, the marsupial
representatives closely mimic the more highly organised
placental mammals. Thus the teeth of an insect-eating
marsupial very closely resemble those of a true Insectivore,
though retaining certain eminently marsupial characters ;
in the same way the dentition of the marsupial Thylacine
mimics that of a dog (compare Figs. 187 and 188).

And although marsupial dentitions do vary very much,
yet there are many transitional forms by which we are some-
times able to trace the successive modifications through
which extreme divergence has been ultimately attained.

Just as we ascribe to placental mammals the formula —

as the typical or parent dental formula, so to the Marsupials
we must ascribe the following —

.3134

THE TEETH OF MARSUPIAL1A. 421

That is to say, though the total number of teeth is the
same, the marsupial has only three premolars and has four
true molars. The premolars (false molars) differ from the
true molars in the greater simplicity of their crowns, just as
in most placental mammals ; but, although, looking at the
complete adult dentitions, no hesitation would be felt in
classing the teeth under the heads of premolars and true
molars, yet there is a curious anomaly in the succession of
the teeth which applies to the whole of the sub-class Mar-
supialia, and to some extent invalidates the definition of
" premolar " as applied to their teeth. Only one of the
premolars (the hindmost) has vertically displaced a milk
tooth ; indeed, the whole milk dentition of Marsupials con-
sists of four milk molars (one on each side of each jaw),
there being no milk incisors nor canines in any known
marsupial. It is further pointed out by Professor Flower,
who was the first to fully describe these peculiarities in the
succession of marsupial teeth (" Phil. Trans.," 1867), that the
extent to which the solitary milk molar is developed varies
much in the different families j no trace of any succession
has been observed in the Wombat ; in the Thylacine (a dog-
like creature) the small milk molar is calcified, but is
absorbed or shed prior to any other teeth being erupted,
whilst in the Kangaroos it is retained till a much later
period (see page 430), and in the Kangaroo Pvat (Hyp-
siprymnus) the milk molar has not yet given place to its
successor at the time when the last permanent molar has
come into place, so that it for a long time ranges with the
other teeth and does work. It is difficult to obtain very
young Marsupials, and material for the complete elucidation
of the subject is wanting ; but I have had the opportunity
of making sections of the jaws of several young specimens
( Perameles and Halmaturus), taken from the pouch by my
Iriend Prof. Moselcy, and I have not so for succeeded in

422

A MANUAL OF DENTAL ANATOMY

(*) Enamel and dentine of a Kangaroo (Macvopus major).
The dentinal tubes in the dentine (A) are furnished with numerous short
branches at the line of juncture with the enamel ; they are dilated, and a
little bent out of their course, while beyond the dilatation they pass on
through about two-thirds of the thickness of the enamel in a straight
course and without branches. Only a part of the whole thickness of the
enamel is shown in the figure. B. Enamel penetrated by the tubes.
C. Individual dentinal tube.

THE TEETH OF MARSUPIALIA.

423

finding any uncalcified germs or other indications of any of
the missing teeth of the milk dentition.

A further peculiarity of the Marsupials is the structure of

FIG. 187

jim

their enamel, which is penetrated by the dentinal tubes.

(*) Upper and lower teeth of the Thylacine. The rudimentary milk
molar, which is absorbed before birth, has been placed over the third or
last of the preinolars, which succeeds to it vertically.

(-) Upper and lower teeth of a Dog, which are placed side by side with
those of the Thylacine to show the many points of resemblance between
the two dentitions.

424 A MANUAL OF DENTAL ANATOMY.

My father, some years ago, described and figured the teeth
of a large number of Marsupial genera (" Philos. Transac.,"
1850), and found that although in the different families the
tube system of the enamel varied in its richness and in the
depth to which the tubes penetrated, yet it was con-
spicuously present in the whole class, with the sole exception
of the Wombats, in whom nothing of the kind is to be found.
Prof. Moseley's specimens have afforded to me the opportu-
nity of study ing the development of this tubular enamel, and
the result of my investigations will be detailed elsewhere ;
but it may be mentioned that the formation of the enamel
tube appears to be precisely analogous to that of a dentine
tube, and at a certain period the enamel cells have appended
to them long processes like the dentinal fibres. The dila-
tation noticeable at the boundary line of the enamel and
the dentine (see Fig. 186) is a kind of clumsy joint brought
about by the coalescence at this point of the tube-forming cells
— on the one side odontoblasts, on the other enamel cells.

There exists one genus of flesh-eating marsupials whose
ferocity is such as to have gained for them the names of
wolf and tiger, while the resemblance of the head to that of
a dog has given origin to the popular name of " dog-headed
opossums." (*)

The resemblance to the dog is in dentition even more
close than in external form : whilst retaining certain mar-
supial attributes, the teeth of the Thylacine are, so far as
their working capabilities go, almost exactly like those of the
dog. The dental formula is —

.4 1 3 4
1 3 ° 1 P 3 m 4

The incisors are small, close set, and sharp edged, the

(!) It has of course no real relationship to the true opossums, which are
not found in Australia.

THE TEETH OF MARSUP1ALIA. 425

outermost being somewhat cauiniform. The canines are
stout, pointed teeth, not quite so long relatively as those
of tlio clog. The premolars are conical teeth, implanted by
two roots, and very similar to those of the dog ; they are
followed in the upper jaw by four molars, increasing in size
from the first to the third, but the last true molar is again
a smaller tooth.

The upper molars are all of the " carnassial " pattern ;
there is a " blade " elevated into subsidiary cusps, and
internally to this a " tubercle," supported by a third root.

The lower molars also bear some resemblance to the car-
nassial teeth of the dog, consisting of a strong, sharp-edged
blade, with anterior and posterior subsidiary cusps, the latter
being somewhat broad and tubercular.

An allied animal (Dasyurus ursinus), though smaller than
the Thylacine, and having teeth of a less sectorial character,
is so destructive to sheep, and so fierce and untamable, that
it has earned the name of " Tasinanian Devil."

Within the limits of the same genus, a species (Dasyurus
viverriiius) is to be found in which the molar teeth are
studded over with long sharp cusps, like the teeth of
Insectivora, a group which it resembles both in its habits
and food.

A number of smaller Marsupials approximate in their
dentition more or less to the Insectivorous type, whilst a
tolerably complete chain of existing forms serves to bridge
over the gap between the rapacious Dasyuridee and the
herbivorous Kangaroos and Wombats.

Amongst the Opossums the larger species have large
canines, and a dentition in its general features approxi-
mating to the Dasyuridee ; they feed upon birds and small
mammals, as well as upon reptiles and insects, while the
smaller species are more purely insectivorous.

Myrmecobius, a small Australian Marsupial of insectivo-
rous habits and dentition, is remarkable as having teeth in

426 A MAN.UAL OF DENTAL ANATOMY.

excess of the number of the typical mammalian dentition,
having

.4 1 3 6

1 3 c r p 3 m o

In the Phalangers, nocturnal arboreal animals found in
Australia and a part of the Malay Archipelago, the canines,
though present, are feeble ; an interspace also separates the
incisors from the molar series.

The lower incisors, reduced to a single pair, are procum-
bent, and grow from persistent pulps ; and a slight exagge-
ration of the peculiarities of the dentition of the Phalangers
brings us to that of the Kangaroo Rats.

The name " Kangaroo Rats " (Hypsiprymnus) is applied
to a genus consisting of about a dozen species ; they are all
small creatures, not much larger than rabbits, but having
the general proportions of kangaroos. They are quiet,
gentle little creatures, of strictly herbivorous habits, and
they are interesting to the odontologist as possessing a
dentition which throws some light upon several anomalous
extinct forms, whose habits and affinities have been the
subject of much controversy.

The dental formula is

i I c 1 p 1 m t
1 0 J 1 4

The first pair of upper incisors are sharply pointed, are
directed nearly vertically downwards, and grow from per-
sistent pulps. The second and third do not grow from
persistent pulps, and their worn crowns do not attain to the
same level as those of the first pair.

All three pairs are antagonised by the single pair of large
procumbent lower incisors, of which the sharp points meet
the first pair of upper incisors, while the obliquely-worn
surface behind the cutting edges impinges against the second
and third upper incisors.

The arrangement of the incisors, and the sharpness of

THE TEETH OF MARSUPIALIA.

427

the'r cutting edges, are calculated to effect the same objects
as those attained by the incisors of a rodent ; a still closer
resemblance would be brought about by the dwindling
(which occurs in other genera) and final disappearance of
the second and third upper incisors, and a compensating
extra development of the first pair.

The canines are not large ; yet they are not so small as

to be termed rudimentary ; in the lower jaw they are
absent.

Only one premolar exists in the adult, and this is a very
peculiar tooth ; its crown is very long from back to front
(at least twice as long as any of the molars, and in some
species as long as three of the molars), and consists of a
finely furrowed blade with a sharp edge ; the blades of the
upper and lower teeth slide over one another. Behind this
there are four true molars, with square quadricuspid crowns,
which become much worn down by use.

The third premolar, the tooth to which attention has

(J) Upper and lower teeth of Hypsiprymnus (Bettongia) (Graii ?). The
dentition represented is that of the adult animal, the permanent premolar
(pm 3) being already in place.

£
428 A MANUAL OF DENTAL ANATOMY.

already been drawn on account of its size and other pecu-
liarities, by virtue of its great size displaces not only the
milk molar, to which it is the legitimate successor, but also
turns out the second premolar, a tooth belonging to the
" permanent " series.

In this particular the succession of the teeth in the
Hypsiprymnus is the same as that of the true kangaroos,
which may be understood by a reference to fig. 190.

There are two extinct Marsupials, known only by their
jaws, which have been the subject of much controversy.
Professor Owen, basing his arguments largely upon the
presence of premolars which possessed elongated and sharp-
edged blades, held that Plagiaulax and Thylacoleo were
carnivorous, saying of the latter that it possessed the simplest
and most effective dental machinery for predatory life known
among Mammalia ; Dr. Falconer, in the case of the first, and
Professor Flower in the case of the Thylacoleo, having shown
this view to be untenable, or at least unsupported by adequate
evidence.

The clue to the nature of the great blade-shaped teeth of
these two extinct creatures is afforded by the form of the
premolar of the herbivorous Hypsiprymnus (see fig. 189).
The incisors were reduced in number and were large ; the
teeth between them and the large premolar were stunted ;
but both these points are true of the herbivorous kangaroos.
The Thylacoleo differs, however, from all known animals by
the immense size of the thin-edged premolar (worn flat in
aged animals), and by the rudimentary condition of its true
molars. But its incisors, lying forwards and closely ap-
proximated in the middle line, are particularly unsuitable
for catching and holding anything alive and struggling,
whilst the nearest resemblance to the blade-shaped tooth is
to be found in harmless herbivorous creatures, so that the
balance of evidence is much against Professor Owen's view.
It has been cited here merely as an instance of how the
evidence afforded by teeth alone may be misleading.

THE TEETH OF MARSUPIALIA.

429

The Kangaroos, comprising many species of very varying
size, are all docile creatures (with the exception of a few old
males), of herbivorous habits; they in some particulars
recall the ruminants.

Their dental formula is

. 3 0

i _ c ~
1 0

1 4
p . m -.

The three pairs of upper incisors are more equal in size

ji nv

than in the Hypsiprymnus, and the central pair do not
;grow from persistent pulps. The lower incisors are very
peculiar teeth : they grow from persistent pulps, are pro-
cumbent, projecting forwards almost horizontally, and are
very much flattened from side to side, their outer surfaces
being but slightly convex, and their inner surfaces flat, with
a median ridge. Their margins are almost sharp. There is
an unusual amount of mobility between the two halves of the

0) Upper and lower teeth of Halmaturus ualabatus. The permanent
premolar is not yet erupted, and is shown in its crypt : when it comes
into its place it will displace the milk molar, and one of the anterior pre-
molars as well. In the upper jaAv a rudimentary canine is shown. The
point of the lower incisor would fit, in closure of the mouth, behind the
long anterior upper incisor, but the width of the page did not admit of the
teeth being placed in their true relative positions without reduction in
size.

430 A MANUAL OF DENTAL ANATOMY.

lower jaw, so that these two teeth can be to a slight extent
separated from one another.

The upper canine is often present as a very minute
rudiment, but in no kangaroo does it attain to a greater
size.

The dentition of the Kangaroo is somewhat perplexing
to, the student, for two reasons : the one, that the last or
third permanent premolar not only displaces the solitary
milk molar, but also, as in Hypsiprymnus, on account of its
greater size, the second permanent premolar, which was in
front of the milk molar ; and, besides this, in animals past
adult age, teeth are shed off from the front of the molar
series till at last only the last two true molars on each side
remain.

Thus the dentition of the kangaroo at successive ages
may be thus represented :

.301 14

1 r ' o p T dm I m ?

or, in all, six molar teeth. Then the third premolar dis-
places the second true permanent premolar as well as the
milk molar, and we have

i j e - p - (a new one) m -,

or, in all, only five molar teeth.

Then, one after another, teeth are shed off from the front
of the molar series, just as in the Phacochserus (see page
328), till all that is left is

3002
i i c - p - m -.

The milk molar of the kangaroo is a fully-developed
tooth, which takes its place with the other teeth, and is not
distinguished from them by any special characters, so that
mere inspection of the jaw of a young Kangaroo having
it in place, at the same time with a premolar in front of it

THE TEETH OF MARSUPIALIA.

431

and four true molars behind it, would not lead an observer
to suspect its real nature.

No existing creature serves to connect the Kangaroos
closely with the wombat, but the extinct Diprotodon appears
to have in a measure bridged across the gap.

The Wombats (Phascolomys) are heavily-built, inoffensive
creatures, which burrow in the ground and subsist largely
upon roots. In their dentition they closely simulate the

Fro. 191 (').

Rodents, as they possess but a single pair of chisel-edged
incisors in either jaw, growing from persistent pulps, and
embedded in very deep and curved sockets. These differ from
the corresponding " dentes scalprarii " of true Rodents in that
there is a complete investment of cement, which passes over
the enamel in front of the tooth as well as covering its back
and sides. They are unlike the teeth of other Marsupials
in their structure, as the dentinal tubes do not penetrate
the enamel, which is therefore, probably, harder and denser
and so less readily worn away.

The molar teeth also grow from persistent pulps, and

432 A MANUAL OF DENTAL ANATOMY.

are very deeply grooved upon their sides, so that their
grinding surfaces are uneven.

Their dental formula is,

.101 4
1 1 C0 P 1 m I'

The first tooth of the molar series is a single column,
whereas the deep grooving of the others divides them into
two columns, so that its simpler appearance, as well as
analogy, would indicate that it is a premolar. But no
succession whatever has been observed in the wombats.

The adaptive resemblance to the dentition of the true
Rodents is exceedingly close, though the Wombat is an
undoubted Marsupial ; and the very closeness of the imita-
tion is an exemplification of the fact that adaptive charac-
ters are very apt to mislead, if used for the purposes of
classification.

Extinct wombats, of very much larger size than the
recent species, are found in the later tertiary deposits of
Australia.

Amongst the Marsupials there is a pretty little arboreal
creature (Tarsipes), not larger than a small rat, which sub-
sists upon insects and the nectar of flowers, which it reaches
by means of a long protrusible tongue. Its molar teeth
are rudimentary, variable in number, and are soon shed ;
the lower incisors, which are procumbent, are however re-
tained, as are a few small teeth which are opposed to them
above.

The wonderful diversity of the forms into which the
Marsupials have branched out in Australia seems to prove
that they have been established in that region, and have
been without the competition of more highly organised
placental Mammals, for a prodigious length of time j and
one cannot better conclude the very brief survey of the
teeth of Mammalia which has been attempted in this volume

0 Upper and lower teeth of Wombat (Phascolomys wombat).

THE TEETH OF MARSUPIALIA. 433

than by calling the reader's attention again to the character of
the Marsupial fauna : this microcosm, in which every place
is filled by a Marsupial that mimics the placental Mammal
which it represents, for nowhere can we more plainly see
the workings of natural selection than in areas thus isolated
and deprived of immigrant creatures for countless ages.

In the foregoing pages much stress has been laid upon
the variability of animals, and the agencies by means of
which the variations have been preserved and intensified,
so to speak, so that ultimately permanent hereditary modi-
fications have been the result ; and it is possible that in
laying this aspect of the matter prominently before the
reader an impression of too great instability may have been
conveyed ; and thus the forms of creatures made to appear
more plastic and more shifting than they really are, for it
is hardly possible to realize the enormous lengths of time
during which the agencies have been at work, and without
which they would have been powerless to produce profound
alterations.

The process which we term inheritance is constantly
reproducing animals which are minute copies of their
parents ; copies which are even more exact than we can at
first sight realise.

Thus., even amongst different species of the same genus,
whose teeth are apparently quite similar so far as their
number and pattern goes, differences exist which are con-
stant for the species, and which may be brought into
prominence by any method of investigation which is
sufficiently accurate.

A plan of representing in the form of diagrams certain of
the characteristics of an animal's dentition, by means of
which differences of proportion so slight as to be barely
recognisable by an inspection of the teeth are brought con-
spicuously into view, has been devised by Mr. Busk ("Proc.
Roy. Soc. 1870 ").

P F

434

A MANUAL OF DENTAL ANATOMY.

If " odontograms " of various Felidse be constructed, differ-
ences between them will be apparent at a glance, although
the forms of the several teeth in this family are so very
closely similar, that nothing but the very closest observa-
tion would have detected the smallest difference between
them.

I have not practically tested the applicability of this
method of comparison, but it is said that these diagrams,
embodying as they do only one set of facts about a dentition,
have proved less useful than might have been anticipated,
and occasionally may even prove misleading.

INDEX.

A.

PAGE

Absorption of teeth . . .197
Acrodus, teeth of . . . . 222
Adaptive modification, meaning

of term 267

Alveolar processes . . . . 24
„ process, development of 192
Alveoli, attachment by means of ib.
Alveolo-dental membrane 110, 212
Anarrhicas, teeth of . . . 229
Anchylosis of teeth . . . 207
Angler, teeth of . . . . 228
Anoplotherium, teeth of . . . 332
Anteaters, teeth of ... 305
Anthropoid apes, teeth of . . 409
„ „ order of erup-

tion of teeth in ib.
„ „ comparison of

with man . 410

Antrum 27

„ teeth in relation with . 29

Archacopteryx, teeth of . . . 261

Armadillo, teeth of . . .305

Articulation of the lower jaw . 32

„ „ teeth with one

another . 5
Articular process, share of in

growth of jaw . . . .188

Artiodactyle ungulata . . . 325

Attachment of teeth . . .202

„ „ by membrane ib.

,, „ by hinges . 203

„ „ by anchylosis 207

„ „ by sockets . 212

Aye-aye, teeth of. . .271,404

B.

Babirussa, teeth of . . .329
Baboon, teeth of . . . . 408
Balaenidee, whalebone of . . 312

PAGE

Barracuda pike, teeth of . . 235
Basal ridge, or cingulum . . 10
Basement membrane of tooth-
genii 171

Bats, teeth of . . . . 401
„ milk teeth of ... 402
Bear, teeth of . ... 387
Beaver, teeth of . .45, 372
Bettongia, teeth of . . . 427

Bicuspids, human . . .14
Bilophodout, meaning of term . 317
Birds, teeth of . . . . 260
Boll, on dentine! fibrils . 66-68
Bone, primary . . . . 168
„ development of. . . ib.
Brown stria) of Eetzius . . 52

Buccinator, attachment of . .30
Bunodont, meaning of term . . 328

Calcified teeth . . . . 3
Calcification, dates of in the seve-
ral teeth. . . 146
process of . . 148
of enamel . . . 151
of dentine . . 158
of vaso-dentine . .164
ofosteo- „ . 165
ofcementum . . 166
Calcoglobulin . . . .150
Calcospherites . . . . 149
Camel, teeth of . . . .337
Canaliculi of cementum . . . 97
Canida?, teeth of . . . .379
Canine teeth of ruminants . . 282
of lemurs . . 283
of oreodon . . ib.
of mole . . .284
of insectivora . . ib.
Canine definition of . . .280

436

INDEX.

Canine, sexual development of . 273

„ true signification of term 281

Canines, human . . . . 12

Capybara, teeth of . . 2G8-370

,, molar teeth of . . 296
Carcharias, teeth of . . .219
Carnassial tooth . . . . 376
Caniivora, milk dentition of . 377

„ teeth of . ... 374
Carnivorous dentition, general

character of . . . 376
Cement organ . . . .144
,, doubtful existence

of ... 145
Cement, over crown of tooth . 100
Cementum 95

,, rudimentary . . . 96

,, structure of . . ib.

,, distribution of 42

,, calcification of . .166
Ceratodus, teeth of. . . . 238
Cestracion, teeth of ... 221
Cetacea, teeth of . . . . 807
Chaetodonts, teeth of . . .235
Cheiromys, milk teeth of . . 406
teeth of . . . 271
Chelonia, teeth of ... 241

Chiroptera, teeth of ... 401
Cingulum, definition of . . 11

„ developed into addi-
tional cusps . . 292
Coatimundi, teeth of . . 286

Cobra, teeth of . . . .247
Complex teeth, manner of forma-
tion of . . . ISSetscq.
Con tour lines of Owen . .61
Coronoid process, use of in growth 189
Correlations of growth . , . 275
Crocodiles, teeth of . . 256

„ implantation of teeth

of . . . . 213

„ succession of teeth in 258
Crypts of developing teeth . .180
Curvatures of dentinal tubes . 60
Cusps, formation of . . . 292
Cuticula dentis . . . .99

Cynodraco 2o9

Cystophora, teeth of . . .391
Czermak, interglobular spaces of 74

I).

Dasyurus, teeth of . . . . 425
Deciduous dentition . . . 297

PAGE

Decussation of enamel prisms . . 50
Deer, teeth of .... 335

Deficiencies of teeth in hairy men 276

„ „ in Turkish

dogs. . 275

Dentine, calcification of . . 158

composition of . . . 58

fibrils of , 64

germ . . 113, 132

globular . . .163

granular layer of . . 72

interglobuiar spaces of . ib.
matrix of . . 58

modifications of in la-

byrinthodoii 79

in lepidosteus 78

in manatee 83, 347

in megathe-

rium. . 84

in myliobates 81

in varairus . 76

osteo- . . . .88

plici- . . . . 76

(secondary . . .93

sensitiveness of . . 71

sheaths of Neumann in 63

i ermination of tubes of 72

theories as to forma-

tion of . . . . 162

tubes . . . .61

unvascular . . 56

varieties of . . .91

vascular . . . . 82

vaso- . . . . ib.

Dents en velours, en brosse, en

cardes 235

Dermal spines of fish . . 2,219
Dcsmodus, teeth of ... 401
Development of the teeth 113 et seq.
„ commencement of

113, 128
. . 121
. 115
. . 125
. 128
. . 122
. 126
138

in eel
in fishes
in li/ards .
in mammalia .
in reptiles .
in snakes
of the true molars
of the jaws . . 176
of the alveolar pro-
cesses . . . 198

Diastema 413

Dicynodon, teeth of ... 243
Dinoceras, teeth of . 341

Dinotherium, teeth of . 249, 360
Diodon, teeth of .... 231
Diprotodon, teeth of . . .431

INDEX.

437

PAGE PAGE

Dog, teeth of . . . . 378 Follicle, dental . . .133, 142

,, variation in teeth of . . ib.

Frog, teeth of . . . . 239

Dog-fish, teeth of . . .2, 217

Dryptodon, teeth of . . .343

G.

Dugong, teeth of . . . . 345

,, tusks of male . . ib.

Galeopithecus, teeth of . . 401

Germ, tooth 113

Glenoid cavity, form of in cami-

.h.

vora . . 34

Edentata, teeth of . . .304

„ „ form of in herbi-

Eel, development of teeth of . 121
„ enamel-tipped teeth of . 40, 210
Elasmobranch fish, teeth of 115, 215
Elephant, milk teeth of . . 354
„ molars of . 356, 361
,, succession of teeth in . 354
tusks of . . 350

vora . . . ib.
Globular dentine . . .163
Goodsir, special views of . . 129
Grampus, teeth of ... 308
Granular layer of dentine . . 72
Growth of the jaws . . . 177
Gubernaculum . . . . 145

Enamel . . . 41 et seq.
,, absence of . . .41

Gum, the 108
Gymnodonts, teeth of . . . 230

„ cavities in . . . 52

„ calcification of . . 151

H.

„ cells 153
,, „ calcification of . ib.
„ entry of dentinal tubes

Haddock, teeth of . . .211
Hair and teeth, correlation be-

tween 275

into , . . , 53
„ fracture of . . .44

Hairy men 276

Hairless dogs, teeth of . . . 275

,, germ . . . . 114
„ of sargus . . .55
„ of marsupials . . . 53
„ organ, development of . 134
„ ,, externarepithelium
nf 1^91 ^

Hake, dentine of ... 87
„ hinged teeth of . 203, 228
Halicore 345
Halmaturus, teeth of . . 429
Hare, teeth of . . . . 367

(Jl . . JO-i, lOo

„ „ internal epithelium
of . . 132, 153

Tjoplr n^P 1 QO

Hatteria, teeth of . . 239, 243
Hedgehog, teeth of ... 394
Helodermis, teeth of . . . 242

?j 1) 11LCK OI . . . JLuZ

„ pigment in . . .52
„ prisms of . . . 44
„ rudimentary . .41
„ organ, stellate reticu-

Hesperornis, teeth of . . 263
Hinged teeth .... 202
Hipparion, teeth of ... 321
Hippopotamus, teeth of . .331
Homnlodontotherium . . . 286

in in oi . . * 13o
,, „ striation of . 50,
„ theories of formation of . 154

Homologiesof the teeth . .278
Horny teeth 3
Horse, teeth of . . . .321

Eruption of teeth, date of . .193
„ „ mechanism of 190
External pterygoid muscle, action
of 34

Human teeth, forms of . . . 414
Huxley, Prof., special views on
development . . . 152, 157
Hyaena, teeth of . . . . 381

Hyoenodon, teeth of . . . 385

Hydrophis, teeth of . . . 247

F.

Hydromys, teeth of . . . 366

Hypsiprymnus, teeth of . . . 427

Felida), teeth of . . . . 375

Hyrax, teeth of . . .348

Fibrils of dentine . . .64

Fishes, teeth of . . . . 214

I.

,, classification of . . ib.

„ structure of teeth of . . 236
Foetus (nine months), teeth of .179

Ichthyosaurus . . . . 258
Ichthyornis, teeth of . . .261

433

INDEX.

Iguanodon, teeth of
Incisors, definition of .

,, human, description of .
Insectivora, teeth of .

,, characteristic molars

of

Internal pterygoid muscle
Interglobular spaces .

Kangaroos, teeth of
Kangaroo, enamel of .

L.

Labyrinthodon, teeth of

,, dentine of

Lacunae, of cementum .

,, development of

,, encapsuled

„ in pits of enamel

„ of Howship
Lamna, teeth of .

„ dentine of .
Lemurs, teeth of

,, canines of
Lepidosiren, teeth of .
Lepidosteus, dentine of .
Leptothrix, in interglobulai

spaces

Lines of Schreger .
Lizards, teeth of
Lophius, teeth of .
Lumen, appearance of

M.

Machairodus, teeth of .
Mammalia, teeth of . 266

„ typical dentition of . 27

Mammoth, tusks of .
Man, teeth of .

,, teeth of different races of . 415
Manatee, teeth of

„ enamel of

,, dentine of .
Mandible

Mark of horses' incisors
Marsupialia, teeth of

milk teeth of .

,, peculiar enamel of 53,423

Masseter muscle .
Mastication, muscles of .

PAGE PAGE

. 243 Mastodon, teeth of . . .357

. 280 „ molars of . . .

360

9 „ milk teeth of .

359

. 394
I

Maxilltt, description of . .
,, development and growth

24

. 400

of ....

176

. 34

„ V-shaped . ...

201

2,74

Meckel's cartilage

177

Megatherium, dentine of . .

84

Membrana eboris . . 158,

140

„ preformativa

171

Mental foramen, position of .

183

. 430

Milk dentition, character of

297

. 53

„ rudimentary

301

Molars, definition of . .18,

280

Mole, teeth of . . . .

397

„ milk teeth of ...

399

Monkeys, teeth of . . . .

407

240

Monodon, teeth of ...

309

80

Monotremata, teeth of . . .

304

96

Muscles of mastication

33

168

Musk deer, teeth of . . .

336

169

Mustelida?, teeth of

385

54
197

Myliobatcs, dentine of . . .
„ teeth of .

82
223

216

Myrmecobius, teeth of . . .

425

90

Myxine, teeth of ...

215

403

283

237

N.

76

1

Narwal

309

74

„ teeth of .

ib.

60

Nasmy th's membrane, nature of

99

241

Neck of enamel organ .

132

228

„ of tooth

7

63

Nerves of dentine

71

„ of the pulp . . 71,

106

„ of the teeth

37

Neumann, sheaths of

63

Newt, teeth of ...

240

. 384

5< scq.
. 279

0.

. 350

. 1

Oblique ridge of human molars

19

. 415

Odontoblast cells . . 69

158

. 347
. 45

Odontopteryx, teeth of .
Odontorniths, teeth of

260
261

5, 347
. 30

Ophidia, development of teeth o
Opossum, teeth of

125
425

. 289

Orang, teeth of ...

410

. 419

Oreodon, teeth of

332

. 421

„ canines of . ,

ib.

>3,423
. 34

Orycteropus, dentine of
Osseous fish, teeth of

82
224

. 33

Osteoblasts

166

INDEX.

439

l-AGE

Osteoclasts

197

Osteodentine

88

,, in teeth of rodents

371

Iiachiod.0

,, in teeth of sperm

Rattlesna

whales . 92
Ostracion, dentine of

309
86

Rhinocer
Rlivncocc

Otaria, erosion of teeth of .
„ teeth of .

389
ib.

Ehytina,
Eidsre for

Papilla, formative . . . . 139
Parrot fish, teeth of ... 231
Periosteum, alveolo-dental . .109
Perissodactyle ungulata, teeth of 315
Permanent teeth, eruption of .198
,, „ development of 137

Persistent dental capsule . , 99
Phacochan-us, teeth of . . . 328
Phalanger, teeth of ... 426
Pharyngeal teeth . . . .

„ ,, of carp . . 234

pike . . 227

„ ,, rachiodon . 247

,, „ scarus . . 233

Phocidte, teeth of . . . . 388

Pigment in enamel . . 52, 401

Pig, teeth of . . . . 325

Pike, teeth of ... 206, 225

Plagiostomi, teeth of . 215

Plagiaulax, teeth of ... 428

Plicidentine 76

Poison fang, development of . 156
„ mechanism of . . 248

,, structure of . . 252

„ succession of . . 254

Poison gland .... 256
Porcupine, enamel of . . 47

Premblars, human . . .12
„ definition of . . . 280
Primates, teeth of ... 403
Pristis, teeth of . . . 222

Proboscidea, teeth of . . . 349
„ affinities with ro-

dents . . .363
Procybnidrc, teeth of . . . 385
Proteles, teeth of ... 382

Pseudoscarus 230

Pterodactyls, teeth of . . . 259

Pterosauria . . . . ib.

Pulp, the . ' . . . .104

,, degeneration of . . 107

„ nerves of . . .106

,, vessels of . . . . ib.

Python, teeth of . . . .246

E.

PAGE

th of . . . 247
seth of . . . 248
Jthof . . .316
is, teeth of . . 243
of . . .347
Eidge formula), of proboscidea . 362
Eodentia, teeth of ... 364
„ milk teeth of . . . 366
,, enamel of . . 45, 371
Eoot membrane . . . . 109
Eostral teeth of saw fish . . 222
Eudimentary teeth, examples of

267,271,277,311,347,382,430
Eudimentary milk teeth, ex-
amples of . 301, 399, 402, 423
Euminants, teeth of . . . 334
„ absence of upper in-
cisors in . . . ib.

S.

Salivary glands .

Salmon, sexual weapons of .

Sargus, enamel of

„ teeth of ...
Saurians, teeth of
Saw-fish, teeth of .
Scalpriform incisors of rodents
Scarus, teeth of .
Schreger, lines of .
Seals, teeth of .
Second dentition
Secondary dentine
Selenodont .
Serres, glands of .
Sexual weapons, teeth used as
Sexual differences in teeth

37
236
55
235
241
222
365
231
60
388
194
92
338
108
273
of

Babirussa 330
in teeth of

boar
in teeth

deer
in teeth

325

of

. 274
of

dugong . 345
in teeth of

elephant . 350
in teeth of

horse . 320
in teeth of
monkeys

409, 413
in teeth of
narwal . 309

440

INDEX.

PAGE

, i

AGE

Sharks, development of teeth of . 117
„ teeth of . . . 215

Tillodonts, teeth of .
Tillotherium, teeth of .

342
343

Sharpey, fibres of . . .98

Tomes' fibrils . . . . 64

,68

Sheep's head fish, teeth of . . 235
Shrews, teeth of . . . .396

Tomes' processes of enamel cells .
Tooth, definition of . .

153

1

Simiina, teeth of . . . . 407

Tooth sac . '.•••'. . .

142

Sirenia, teeth of ... 344

Tooth germ .

113

Sloths, teeth of . . . . 306

Tortoises, teeth of . . . .

241

Snakes, development of teeth of . 125

Toxodon, teeth of ...

339

„ non-Arenomous, teeth of . 244

Trichecfrus, teeth of

391

„ colubrine, poisonous . 247

Tusks of wild boar .

325

,, viperine, poisonous . . 248

„ of elephant, foreign bodies

Socketed teeth . . . .212

in . . . .

352

Sphenodon, teeth of ... 243

Typical tooth i

278

Sphyraena, teeth of ... 235

Stellate reticulum of enamel organ

131, 135, 156

U.

Stratum intermedium of enamel

organ t . .135

Ungulata, teeth of . . . .

314

Stratum Malpighi, the . . . 108
Striae of enamel prisms . .50

,, molar patterns of

319

„ ofRetzins . . . . 51

Succession of teeth in armadillo . 306

v.

„ „ in lizards . 239

„ „ in mammals . 297

Vampire, teeth of ...

401

„ in marsupials . 421

Varanus, teeth of . . . .

242

,, in osseous fish 121

„ dentine of ...

76

,, in proboscidea 354

Vaso-dentine . 82 et

sea.

,, in reptiles . 122

Viper, teeth of ....

248

„ in sharks . 115

,, succession of teeth in .

253

„ in snakes . 125

Viverridae, teeth of . . .

380

,, in poisonous

snakes . 253

Supernumerary teeth in dogs . 380

W.

Sus babirussa, teeth of . . . 330

Sus scrofa, teeth of :. . . 325

Walrus, teeth of .

391

Wart-hog, teeth of ...

328

Whalebone . . . 311,

312

T.

Whale, rudimentary teeth of

311

Wisdom teeth .

22

Tapir, teeth of . . . . 316

„ of lower races of

Tarsipes, teeth of . . .432
Teething 182

man . . .
„ ofmonkevs . 410,

416
415

Teeth, equivalent to dermal spines 2
Teleostei, teeth of ... 224
„ development of teeth of 120

Wolf-fish, teeth of . ".
Wombat, teeth of . '.' .
„ enamel of '. . .

229
432
53

Temporal muscle, action of . .32

Wrasse, teeth of . . . • .; .,

235

Temporary teeth, eruption of . . 193

Tetrodon, teeth of ... 231

Theriodonts, teeth of . . 259

Z.

Thylaoimis, teeth of . . . 422

Thylacoleo, teeth of ... 428

Ziphoid cetacea, teeth of . .

311

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FLINT. — Clinical Medicine : a Systematic Trea-
tise on the Diagnosis and Treatment of Disease. By AUSTIN FLINT,
M.D., Professor of the Principles and Practice of Medicine, &c., in
Bellevue Hospital Medical College. 8vo, 20s.

HALL. — Synopsis of the Diseases of the Larynx,

Lungs, and Heart : comprising Dr. Edwards' Tables on the Examination
of the Chest. With Alterations and Additions. By F. Da HAVILLAND
HALL, M.D., F.R.C.P., Assistant-Physician to the Westminster Hos-
pital. Royal 8vo, 2s. 6d.

SANSOM. — Manual of the Physical Diagnosis

of Diseases of the Heart, including the use of the Sphygmograph
and Cardiograph. By A. E. SANSOM, M.D., F.R.C.P., Assistant-
Physician to the London Hospital. Third Edition. With 47 Woodcuts.
Fcap. 8vo, 7s. 6d.

WARNER.— Student's Guide to Clinical Medi-
cine and Case-Taking. By FRANCIS WARNER, M.D., F.R.C.P., As-
sistant-Physician to the London Hospital. Second Edition. Fcap.
8vo, 5s.

WEST. — How to Examine the Chest : being a

Practical Guide for the Use of Students. By SAMUEL WEST, M.D.,
M.R.C.P., Physician to the City of London Hospital for Diseases of
the Chest, &c. With 42 Engravings. Fcap. 8vo, 5s.

11, NEW BURLINGTON STREET.

J. Sf A. Churchill's Medical Class Books.

MEDICINE— continued.
WHITTAKER.— Student's Primer on the Urine.

By J. TRAVIS WHITTAKER, M.D., Clinical Demonstrator at the Royal
Infirmary, Glasgow. With Illustrations, and 16 Plates etched on
Copper. Post 8vo, 4s. 6d.

MIDWIFERY.

BARNES. — Lectures on Obstetric Operations,

including the Treatment of Haemorrhage, and forming a Guide to the
Management of Difficult Labour. By ROBERT BARNES, M.D., F.R.C.P.,
Obstetric Physician to, and Lecturer on Diseases of Women, &c., at, St.
George's Hospital. Third Edition. With 124 Engravings. 8vo, 18s.

CLAY. — The Complete Handbook of Obstetric

Surgery ; or, Short Rules of Practice in every Emergency, from the
Simplest to the most formidable Operations connected with the Science
of Obstetricy. By CHARLES CLAY, M.D., late Senior Surgeon to, and
Lecturer on Midwifery at, St. Mary's Hospital, Manchester. Third
Edition. With 91 Engravings. Fcap. 8vo, 6s. 6d.

RAMSBOTHAM. — The Principles and Practice

of Obstetric Medicine and Surgery. By FRANCIS H. RAMSBOTHAM, M. D. ,
formerly Obstetric Physician to the London Hospital. Fifth Edition.
With 120 Plates, forming one thick handsome volume. 8vo, 22s.

REYNOLDS. — Notes on Midwifery: specially

designed to assist the Student in preparing for Examination. By J. J.
REYNOLDS, L.R.C.P., M.R.C.S. Fcap. 8vo, 4s.

ROBERTS.— The Student's Guide to the Practice

of Midwifery. By D. LLOYD ROBERTS, M.D., F.R.C.P., Physician to
St. Mary's Hospital, Manchester. Third Edition. With 2 Coloured
Plates and 127 Engravings. Fcap. 8ro, 7s. 6d.

SCHROEDER.—A Manual of Midwifery; includ-

ing the Pathology of Pregnancy and the Puerperal State. By KARL
SCHROEDER, M.D., Professor of Midwifery in the University of Erlan-
gen. Translated by CHARLES H. CARTER, M.D. With Engravings.
8vo, 12s. 6d.

SWAYNE.— Obstetric Aphorisms for the Use of

Students commencing Midwifery Practice. By JOSEPH G. SWAYNE,
M.D., Lecturer on Midwifery at the Bristol School of Medicine.
Eighth Edition. With Engravings. Fcap. 8vo, 3s. 6d.

11, NEW BURLINGTON STREET.

10

J. fy A. Churchill's Medical Class Books.

MICROSCOPY.

CARPENTER. — The Microscope and its Revela-
tions. By WILLIAM B. CARPENTER, C.B., M.D., F.R.S. Sixth Edition.
With 26 Plates, a Coloured Frontispiece, and more than 500 Engravings.
Crown 8vo, 16s.

MARSH. — Microscopical Section-Cutting : a

Practical Guide to the Preparation and Mounting of Sections for the
Microscope, special prominence being given to the subject of Animal
Sections. By Dr. SYLVESTER MARSH. Second Edition. With 17
Engravings. Fcap. 8vo, 3s. 6d.

MARTIN. — A Manual of Microscopic Mounting.

By JOHN H. MARTIN, Member of the Society of Public Analysis, &c.
Second Edition. With several Plates and 144 Engravings. 8vo, 7s. 6d.

OPHTHALMOLOGY.
HIGGENS.— Hints on Ophthalmic Out-Patient

Practice. By CHARLES HIGGENS, F.R.C.S., Ophthalmic Surgeon to,
and Lecturer on Ophthalmology at, Guy's Hospital. Second Edition.
Fcap. 8vo, 3s.

JONES. — A Manual of the Principles and

Practice of Ophthalmic Medicine and Surgery. By T. WHARTON JONES,
F.R.C.S., F.Il.S., late Ophthalmic Surgeon and Prof essor of Ophthalmo-
logy to University College Hospital. Third Edition. With 9 Coloured
Plates and 173 Engravings. Fcap. 8vo, 12s. 6d.

NETTLESHIP.—The Student's Guide to Diseases

of the Eye. By EDWARD NETTLESHIP, F.R.C.S., Ophthalmic Surgeon
to, and Lecturer on Ophthalmic Surgery at, St. Thomas's Hospital.
Third Edition. With 157 Engravings, and a Set of Coloured Papers
illustrating Colour-blindness. Fcap. 8vo, 7s. 6d.

TOSSWILL. — Diseases and Injuries of the Eye

and Eyelids. By Louis H. TOSSWILL, B.A., M.B. Cantab., M.R.C.S.,
Surgeon to the West of England Eye Infirmary, Exeter. Fcap. 8vo,
2s. 6d.

WOLFE. — On Diseases and Injuries of the Eye :

a Course of Systematic and Clinical Lectures to Students and Medical
Practitioners. By J. R. WOLFE, M.D., F.R.C.S.E., Senior Surgeon to
the Glasgow Ophthalmic Institution, Lecturer on Ophthalmic Medicine
and Surgery in Anderson's College. With 10 Coloured Plates, and 120
Wood Engravings, 8vo, 21s.

11, NEW BURLINGTON STREET.

J. fy A. Churchill's Medical Clam Books.

PATHOLOGY.

JONES AND SIEVEKING.—A Manual of Patho-
logical Anatomy. By C. HANDFIELD JONES, M.B., F.R.S., and ED WAR])
H. SIEVEKING, M.D., F.R.C.P. Second Editioji. Edited, with consider-
able enlargement, by J. F. PAYNE, M.B., Assistant-Physician and
Lecturer on General Pathology at St. Thomas's Hospital. With 195
Engravings. Crown 8vo, 16s.

LAN CERE AUX.— Atlas of Pathological Ana-
tomy. By Dr. LANCEREAUX. Translated by W. S. GREENFIELD, M.D.,
Professor of Pathology in the University of Edinburgh. With
70 Coloured Plates. Imperial 8vo, £5 5s.

VIRCHOW. — Post-Mortem Examinations: a

Description and Explanation of the Method of Performing them,
with especial reference to Medico-Legal Practice. By Professor
RUDOLPH VIRCHOW, Berlin Charite Hospital. Translated by Dr. T. B.
SMITH. Second Edition, with 4 Plates. Fcap. 8vo, 3s. 6d.

WILKS AND M OXON.— Lectures on Pathologi-
cal Anatomy. By SAMUEL WILKS, M.D., F.R.S., Physician to, and late
Lecturer on Medicine at, Guy's Hospital ; and WALTER MOXON, M.D.,
F.R.C.P., Physician to, and Lecturer on the Practice of Medicine at,
Guy's Hospital. Second Edition. With 7 Steel Plates. 8vo, 18s.

PSYCHOLOGY.

BUCKNILL AND TUKE.—A Manual of Psycho-

logical Medicine : containing the Lunacy Laws, Nosology, ^Etiology.
Statistics, Description, Diagnosis, Pathology, and Treatment of Insanity,
with an Appendix of Cases. By JOHN C. BUCKNILL, M.D., F.R.S.,
and D. HACK TUKE, M.D., F.R.C.P. Fourth Edition, with 12 Plates
(30 Figures). 8vo, 25s.

CLOUSTON. — Clinical Lectures on Mental

Diseases. By THOMAS S. CLOUSTON, M.D., and F.R.C.P. Edin.; Lec-
turer on Mental Diseases in the University of Edinburgh. With
S.Plates (6 Coloured). Crown 8vo, 12s. 6d.

MANN. — A Manual of Psychological Medicine

and Allied Nervous Disorders. By EDWARD C. MANN, M.D., Member
of the New York Medico-Legal Society. With Plates. 8vo, 24s.

11, NEW BURLINGTON STREET.

12

J. fy A. Churchill's Medical Class Books.

PHYSIOLOGY.

CARPENTER.— Principles of Human Physio-
logy. By WILLIAM B. CARPENTER, C.B., M.D., F.R.S. Ninth Edition.
Edited by Henry Power, M.B., F.R.C.S. With 3 Steel Plates and
377 Wood Engravings. 8vo, 31s. 6d.

DALTON. — A Treatise on Human Physiology :

designed for the use of Students and Practitioners of Medicine. By
JOHN C. DALTON, M.D., Professor of Physiology and Hygiene in the
College of Physicians and Surgeons, New York. Seventh Edition.
With 252 Engravings. Royal 8vo, 20s.

FREY.— The Histology and Histo-Chemistry of

Man. A Treatise on the Elements of Composition and Structure of the
Human Body. By HEINRICH FREY, Professor of Medicine in Zurich.
Translated by ARTHUR E. BARKER, Assistant-Surgeon to the University
College Hospital. With 608 Engravings. 8vo, 21s.

SANDERSON.— Handbook for the Physiological

Laboratory : containing an Exposition of the fundamental facts of the
Science, with explicit Directions for their demonstration. By J.
BDRDON SANDERSON, M.D., F.R.S.; E. KLEIN, M.D., F.R.S.; MICHAEL
FOSTER, M.D., F.R.S.; and T. LAUDER BRUNTON, M.D., F.R.S. 2 Vols.,
with 123 Plates. 8vo, 24s.

YEO.—A Manual of Physiology for the Use of

Junior Students of Medicine. By GERALD F. YEO, M.D., F.R.C.S.,
Professor of Physiology in King's College, London. With 301 Engrav-
ings. Crown 8vo, 14s.

SURGERY.
BELLAMY.— The Student's Guide to Surgical

Anatomy ; a Description of the more important Surgical Regions o
the Human Body, and an Introduction to Operative Surgery. By
EDWARD BELLAMY, F.R.C.S., and Member of the Board of Examiners ;
Surgeon to, and Lecturer on Anatomy at, Charing Cross Hospital.
Second Edition. With 76 Engravings. Fcap. 8vo, 7s.

BRYANT.— A Manual for the Practice of

Surgery. By THOMAS BRYANT, F.R.C.S., Surgeon to, and Lecturer on
Surgery at, Guy's Hospital. Fourth Edition. With about 750 Illus-
trations (many being coloured), and including 6 Chromo-Lithographic
Plates. 2 Vols. Crown 8vo, 32s.

11, NEW BURLINGTON STREET.

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J. 8$ A. Churchill's Medical Class Books.

SURGERY — continued.
CLARK AND WAGSTAFFE. -- Outlines of

Surgery and Surgical Pathology. By F. LE GROS CLARK, F.R.C.S.,
F.R.S., Consulting Surgeon to St. Thomas's Hospital. Second Edition.
Revised and expanded by the Author, assisted by W. W. WAGSTAFFE,
F.R.C.S., Assistant Surgeon to St. Thomas's Hospital. 8vo, 10s. 6d.

DRUITT.—The Surgeon's Vade-Mecum ; a

Manual of Modern Surgery. By ROBERT DRTJITT, F.R.C.S. Eleventh
Edition. With 369 Engravings. Fcap. 8vo, 14s.

FERGUSSON.—A System of Practical Surgery.

By Sir WILLIAM FERGUSSON, Bart., F.R.C.S., F.R.S., late Surgeon and
Professor of Clinical Surgery to King's College Hospital. With 463
Engravings. Fifth Edition. 8vo, 21s.

HEATH.— A Manual of Minor Surgery and

Bandaging, for the use of House-Surgeons, Dressers, and Junior Practi-
tioners. By CHRISTOPHER HEATH, F.R.C.S., Holme Professor of
Clinical Surgery in University College and Surgeon to the Hospital.
Seventh Edition. With 129 Engravings. Fcap. 8vo, 6s.

By the same Author.

A Course of Operative Surgery : with

Twenty Plates (containing many figures) drawn from Nature by
M. LEVEILLE, and Coloured. Second Edition. Large 8vo, 30s.

ALSO,

The Student's Guide to Surgical Diag-
nosis. Second Edition. Fcap. 8vo, 6s. 6d.

MAUNDER. — Operative Surgery. By Charles

F. MAUNDER, F.R.C.S., late Surgeon to, and Lecturer on Surgery at,
the London Hospital. Second Edition. With 164 Engravings. Post
8vo, 6s.

SOUTH AM. — Regional Surgery : including Sur-
gical Diagnosis. A Manual for the use of Students. BY FREDERICK
A. SOUTHAM, M.A., M.B. Oxon, F.R.C.S., Assistant-Surgeon to the
Royal Infirmary, and Assistant-Lecturer on Surgery in the Owen's
College School of Medicine, Manchester.
Part I. The Head and Neck. Crown 8vo, 6s. 6d.
,, II. The Upper Extremity and Thorax. Crown 8vo, 7s. 6d.

11, NEW BURLINGTON STREET.

-/. 4* -£• Churchill's Medical Class Books.

TERMINOLOGY.

DUNGLISON. — Medical Lexicon : a Dictionary

of Medical Science, containing a concise Explanation of its various
Subjects and Terms, with Accentuation, Etymology, Synonyms, &c.
By ROBERT DUNGLISON, M.D. New Edition, thoroughly revised by
RICHARD J. DUNGLISON, M.D. Royal 8vo, 28s.

MAYNE. — A Medical Vocabulary : being an

Explanation of all Terms and Phrases used in the various Departments
of Medical Science and Practice, giving their Derivation, Meaning,
Application, and Pronunciation. By ROBERT G. MAYNE, M.D., LL.D.,
and JOHN MAYNE, M.D., L.R.C.S.E. Fifth Edition. Crown 8vo,
10s. 6d.

WOMEN, DISEASES OP.
BARNES.— A Clinical History of the Medical

and Surgical Diseases of Women. By ROBERT BARNES, M.D., F.R.C.P.,
Obstetric Physician to, and Lecturer on Diseases of Women, &c., at, St.
George's Hospital. Second Edition. With 181 Engravings. 8vo, 28s.

COURTY. — Practical Treatise on Diseases of

the Uterus, Ovaries, and Fallopian Tubes. By Professor COURTY,
Montpellier. Translated from the Third Edition by his Pupil, AGNES
M'LAREN, M.D., M.K.Q.C.P. With Preface by Dr. MATTHEWS DUNCAN.
With 424 Engravings. 8vo, 24s.

DUNCAN. — Clinical Lectures on the Diseases

of Women. By J. MATTHEWS DUNCAN, M.D., F.R.C.P., F.R.S.E.,
Obstetric Physician to St. Bartholomew's Hospital. Second Edition,
with Appendices. 8vo, 14s.

EMMET. — The Principles and Practice of

Gynaecology. By THOMAS ADDIS EMMET, M.D., Surgeon to the
Woman's Hospital of the State of New York. With 130 Engravings.
Royal 8vo, 24s. *

GALABIN.— The Student's Guide to the Dis-
eases of women. By ALFRED L. GALABIN, M.D., F.R.C.P., Obstetric
Physician to, and Lecturer on Obstetric Medicine at, Guy's Hospital.
Third Edition. With 78 Engravings. Fcap. 8vo, 7s. 6d.

11, NEW BURLINGTON STREET.

15

J. & A. Churchill's Medical Class Books.

WOMEN, DISEASES OP — continued.
REYNOLDS.— Notes on Diseases of Women.

Specially designed to assist the Student in preparing for Examination.
By J. J. REYNOLDS, L.R.C.P., M.R.C.S. Second Edition. Fcap. 8vo,
2s. 6d.

SAVAGE.^-The Surgery of the Female Pelvic

Organs. By HENRY SAVAGE, M.D., Lond., F.R.C.S., one of the Con-
sulting Medical Officers of the Samaritan Hospital for Women. Fifth
Edition, with 17 Lithographic Plates (15 Coloured), and 52 Woodcuts.
Royal 4to, 35s.

SMITH.— Practical Gynaecology : a Handbook

of the Diseases of Women. By HEYWOOD SMITH, M.D., Physician to
the Hospital for Women and to the British Lying-in Hospital. With
Engravings. Second Edition. Crown 8vo. [In preparation.

WEST AND DUNCAN.— 'Lectures on the Dis-
eases of Women. By CHARLES WEST, M.D., F.R.C.P. Fourth
Edition. Revised and in part re-written by the Author, with numerous
additions by J. MATTHEWS DUNCAN, M.D., F.R.C.P., F.R.S.E.,
Obstetric Physician to St. Bartholomew's Hospital. 8vo, 16s.

ZOOLOGY.

CHAUVEAU AND FLEMING.— The Compara-
tive Anatomy of the Domesticated Animals. By A. CHAUVEAU,
Professor at the Lyons Veterinary School; and GEORGE FLEMING
Veterinary Surgeon, Royal Engineers. With 450 Engravings. 8vo,
31s. 6d.

HUXLEY. — Manual of the Anatomy of Inverte-

brated Animals. By THOMAS H. HUXLEY, LL.D., F.R.S. With 15G
Engravings. Post 8vo, 16s.

By the same Author.

Manual of the Anatomy of Vertebrated

Annuals. With 110 Engravings. Post 8vo, 12s.

WILSON.— The Student's Guide to Zoology:

a Manual of the Principles of Zoological Science. By ANDREW WILSON,
Lecturer on Natural History, Edinburgh. With Engravings. Fcap.
. 8vo, 6s. 6d.

11, NEW BURLINGTON STREET.

16

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