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orifice. In the incisor teeth the cavity is pro-
longed above into two tapering canals/which proceed
one to each corner of the crown ; in the bicuspid
and molar teeth it advances a short distance into
each cusp. In the case of a root formed by the

blending of two or more fangs, each division has a separate canal prolonged to its
apex.

Pulp of the teeth. The central cavity of a tooth is called \hQpulp-cavity,
because it is occupied by a soft, highly vascular, and sensitive substance, called the
dental pulp. This pulp (fig. 45, F) consists of jelly-like connective tissue containing
cells, blood-vessels and nerves, and fine fibres. The fibres appear to be formed from
processes of the cells : according to Rose they are not collagenous, although a few




THE TEETH.



31



bundles of ordinary collagenous connective tissue fibres may accompany the blood-
vessels and nerves. The cells are partly disseminated in the matrix, and partly form
a stratum at the surface of the pulp, where, during the formation of dentine, they are
elongated, somewhat like the cells of columnar epithelium (see fig. 05, c, p. 44),
but after the dentine is completely formed they become flattened like the osteoblasts
under the periosteum of bone. These superficial cells (odontoblasts, fig. 45, Od, Od l )
send processes into tubules in the dentine, to be afterwards noticed, of which more
than one may come from the same cell. The filaments within the tubules were first
noticed by J. Tomes, and are known as Tomes' fibres. The arteries and nerves,
which are derived from the internal maxillary and fifth pair respectively, enter by




Fig. 45. SECTION AOHOSS THE ROOT OP A YOUNG TOOTH SHOWING ITS PULP in situ. (Rose.) s fJ.

P, pulp ; V, V, veins ; A, A, arteries ; N, nerve-bundles ; Od, columnar odontoblasts still
depositing dentine ; Od t , flattened odontoblasts, which have ceased to form dentine.

the aperture at the point of each fang. The vessels form a capillary network
beneath the superficial cells ; the nerves end in fine non-medullated fibrils, which
are distributed abundantly at the surface of the pulp and run up between the
superficial cells, but they have not been traced into the dentinal tubules.

Weil has described a clear layer (basal layer) under the odontoblasts. pervaded with
fibres, which he regards as derived from the odontoblasts. It is. however, somewhat
uncertain whether the layer described is not an artefact, due to the shrinking of the main
mass of the pulp from the layer of odontoblasts. It is only seen in the crown of the
teeth. No lymphatics have been seen in the pulp.

Hard tissues of the teeth. The hard part of a tooth is composed of three
distinct substances, viz., the proper dental substance, ivory or dentine, the enamel,



ORGANS OF DIGESTION.



and the cement or crusta petrosa. The dentine constitutes by far the larger portion :
the enamel is found only upon the exposed part or crown ; and the cement covers
with a thin layer the surface of the fang.

The dentine (Owen) resembles bone in its general aspect and chemical
constitution, but is not identical with it in structure.

The dentine of human teeth is composed of 28 parts per cent, of animal, and 72 of earthy
matter. The former is resolved into gelatin by boiling. The composition of the latter.




Fig. 46. SECTION OP A TOOTH ACROSS THE CROWN. (Rauber. ) f

ffl, dentine, the tubules cut longitudinally ; a', the same cut obliquely ; a", the same cut across ;
c, enamel, showing contour lines ^5) ; d, d, portions of the pulp cavity extending into the cusps, with
the dentinal tubes converging horizontally towards them (1).

according to Bibra, is as follows, viz., phosphate of lime 66'7 per cent., carbonate of lime 3'3,
phosphate of magnesia and other salts, including a trace of fluoride of calcium. T8. Berzelius
found 5'3 carbonate of lime.

The dentine is penetrated throughout by fine tubes (dentinal tubes), which being
nearly parallel, give it a striated aspect (tigs. 46, 47). When a thin section of a

Fig. 47. SECTION OF THE SAME TOOTH
ACROSS THE FANG. (Rauber.) a

a, pulp cavity ; b, dentine ; c, cement ;
] , incremental lines ; 2, granular layer.

macerated tooth, prepared by
grinding, is viewed under the
microscope by transmitted light,
the solid substance, or matrix, is
transparent and apparently homo-
geneous, while the tubes, being (in
a dried specimen) filled with air,

are dark ; but when seen with reflected light on a dark ground, the latter appear
white ; in these respects they resemble lacunae and canaliculi of bone.

The dentinal tubules open at their inner ends into the pulp-cavity, which has
accordingly very numerous minute orifices over the whole surface. Thence they
pass in a radiated manner through every part of the ivory towards its periphery.
In the upper part of the crown they have a vertical direction ; but towards the
sides, and in the neck and root, they become gradually oblique, then horizontal, and




THE TEETH.



33



are finally even inclined downwards towards the point of the fang. The tubules
describe in their course two or three gentle curves (primary curvatures, fig. 35),
and each is besides twisted throughout its whole length into numerous" fine spiral
turns, which follow more closely one upon another ; these are the secondary curva-
tures (fig. 48). In form a tubule may accordingly be likened to the thread of a
corkscrew, stretched so that the turns are drawn far apart, and their breadth
proportionally diminished (Welcker).

The tubes are only slightly divergent as they pass towards the surface ; and, as
they occasionally divide dichotomously, and at first without being much diminished
in size, they continue to occupy the substance of the dentine at almost equal
distances, and their nearly parallel primary curvatures produce, by the manner in
which they reflect the light, an appearance of concentric undulations in the dentine,



Fig. 48. SECTION OF FANG, PARALLEL TO THE DENTINAL
TI-BULES (HUMAN CANINE). Magnified 300 diameters.
(Waldeyer.)

1, cement, with large bone lacunae and indications of
lamellae ; 2, granular layer of Purkinje (interglobular
3, dentinal tubules.



which may be well seen with a low magnifying
power (Schreger's lines}. The average diameter
of the tubules at their inner and larger end is
0'0055 mm., and the distance between adjacent
tubules is commonly about two or three times
their width. From their sides, numerous im-
measurably fine branches are given off, which
penetrate the hard intertubular substance, where
they either anastomose or terminate blindly.
These lateral ramuscules are more abundant in
the fang. Near the periphery of the ivory the
tubules, which by division and subdivision have
become very fine, terminate imperceptibly by
free ends.

The tubules have each a proper wall (dentinal
sheath of Neumann) independent of the inter-
tubular matrix, but intimately adhering to it.
By steeping sections of decalcified dentine in
strong hydrochloric acid, the matrix is destroyed,
and the membranous tubes, which consist of a
more resisting material (probably elastic substance), remain
that these sheaths, which are formed of dentinal matrix which




behind,
calcifies



Rose
either



states
late or



Fig. 49. SECTIONS OP DENTINAL TUBULES. (After Fraenkel.)
a, cut across ; b, cut obliquely. (About 300 diameters.)

not at all, anastomose freely, and that it is their anastomoses which
have been often taken for that of the tubules themselves.

In the temporary, and sometimes even in the permanent teeth,
the tubules are constricted at short intervals, so as to present a
moniliform character. The terminal branches of tubules are
occasionally seen to pass on into the cement which^covers the fang, and to commu-
nicate with canaliculi proceeding from the characteristic lacunse found in that
osseous layer. Tubules have likewise been observed by Tomes and others passing
into the enamel in the teeth of marsupial animals, and in a less marked degree 5 ,

VOL. III., PT. 4.




ORGANS OF DIGESTION.



in human teeth. In this case they pass, not into the enamel prisms, but into the
inter-prismatic substance.

The interiubular substance is translucent. The animal matter which remains
after the earth has been removed by an acid, may be torn into laminae (Sharpey),

Fig. 50. VERTICAL SECTION OP THE UPPER PART OF AN
INCISOR TOOTH. (From Kblliker. ) MAGNIFIED 7 DIA-
METERS.

a, the pulp-cavity ; 6, dentine ; c, arched incremental
lines ; d, cement ; e, enamel, with bands indicating the direc-
tion of the ranges of fibres ; /, coloured lines of the enamel.

parallel with the internal surface of the pulp-cavity,
and therefore across the direction of the tubules.
It has been shown by Ebner and Mummery that
the matrix contains fine fibrils like those of the
matrix of bone. These fibrils are not themselves
calcified but are enveloped in the calcined inter-
fibrillar substance, and, according to Mummery,
are continuous with fibrils of the dental pulp.

The laminated structure is an indication of the
deposition of dentinal substance in successive strata
in the process of formation of the tooth the
lamina? corresponding with the shape of the pulp-
surface at successive stages of the process. Not
unfrequsntly lines, varying in number and breadth,
are seen in sections of the dry tooth, conforming in
direction with the lamination just spoken of (mere-
mental lines, Suiter, fig. 50, c). They are caused
by the drying of imperfectly calcified dentine,

which shows little cavities bounded by, and therefore receiving their figure from,
minute nodules or globules of dentine, and hence named interglolular spaces (fig.
51, c). The interglobular spaces, and the globules surrounding them, vary in size






WITH

350



Fig. 51. A SMALL PORTION OF THE DENTINE

INTERGLOBULAR SPACES. (From Kblliker.)

DIAMETERS.

C, portion of incremental line formed by the inter-
globular spaces, which are here filled up by the trans-
parent material used in mounting the specimen.

within wide limits. A layer, in which they
are very fine granular layer (fig. 48, 2) is
not uncommonly found towards the outer
surface of the dentine.

The enamel is that hard white covering
which encrusts and protects the exposed
portion or crown of a tooth. It is the

hardest of all the dental tissues, but is gradually worn down by protracted use. It
is thickest on the grinding surface and cutting edge of the teeth, and becomes
gradually thinner towards the neck, where it ceases.

According to Bibra, it contains of earthy constituents 96'o per cent., viz., phosphate of
lime with traces of fluoride of calcium 8'J\S, carbonate of lime 4'4, phosphate of magnesia and
other salts 1 - I5. and of animal matter only 3*5 per cent. Berzelius, however, gave the
proportion of carbonate of lime as 8, and of animal matter as only 2 per cent.



A



THE TEETH.



35





The enamel is made up entirely of very hard and dense microscopic columns or
prisms, arranged closely together, side by side, and set by one extremity upon the

Fig. 52. SECTION OF PART OP THE CROWN OF A TOOTH,

PARALLEL WITH THE GENERAL SET OF THE ENAMEL

PRISMS. (Rauber. ) -^^

, pointed projection of dentine ; b, tubules extending
from the dentine into the enamel ; c, enamel prisms ; d, prisms
cut across ; e, cuticle of the enamel.

subjacent surface of the dentine (fig. 52). The

columns are disposed in ranges, which, on the

grinding surface, are set vertically, but on the sides

of the crown get more horizontal. Near the

dentine the prisms cross one another in the alternate

ranges, but become parallel as they approach the

surface of the tooth. An effect of radial alternate

light and dark stripes is obtained (as in fig. 50)

(A. Eetzius). A series of concentric lines is

likewise to be seen crossing the enamel fibres :

these are termed coloured lines from their brown

appearance. According to Ebner, these are

produced by some of the inter-prismatic spaces

becoming in the dried tooth filled with air.

Minute fissures not unfrequently exist in the

deep part of the enamel, which run between clusters of the prisms down to the

surface of the dentine (fig. 53, c) ; and other much larger and more evident fissures

Fig. 53. THIN SECTION OF THE ENAMEL AND A PART OF THE DENTINE.
(From Kblliker.) 350 DIAMETERS.

a, cuticle of the enamel ; b, enamel-fibres or columns with, fissures
between them and cross striae ; c, clefts in the enamel communicating
with the extremities of some of the dentinal tubules (d).

are often observed leading down from the depressions or
crevices between the cusps of the molar and premolar teeth.
The unworn surface of the enamel is finely striated.

The enamel-columns (fig. 52) have the form of solid six-
sided prisms. Their diameter is ordinarily about O'OOo mm.
They are marked by frequent dark transverse shadings,
which are usually ascribed to the existence of shallow con-
strictions along the fibres. Although this may be in part
the cause, it is not improbable that the transverse markings
are largely the result of the manner in which the prisms are
built up in successive stages by the cells which produce them,
each marking representing the termination of a stage. The
inner ends of the prisms are implanted in minute hexagonal
depressions on the surface of the dentine ; whilst the outer
ends are free, and present, when examined with a high
magnifying power, a tesselated appearance (fig. 54, B). The
prisms are united by a small amount of a substance which

appears similar to the intercellular substance of an epithelium, but is perhaps
calcified. In marsupials and some rodents there are regular canaliculi in this
interprismatic substance.

D 2




36



ORGANS OF DIGESTION.



When submitted to the action of dilute acids, the enamel is almost entirely
dissolved, and leaves scarcely any discernible traces of animal matter. The centre





Fig. 54. ENAMEL-PRISMS. (From Kolliker.) 350 DIAMETERS.

A, fragments and single columns of the enamel, isolated by the action of hydrochloric acid.

B, surface of a small fragment of enamel, showing the hexagonal ends of the prisms.

of the prisms is first dissolved, showing this part to be less firmly calcified than the
periphery. After exposure for a short time to the action of an acid, the enamel of

Fig. 55. SECTION OF THE FANG OF A TOOTH

SHOWING DENTINE AND OEMKNT TOGETHER
WITH THE DENTAL PERIOSTEUM. (Rose. )

2Q<>
1

Ep., nests of epithelial cells within the
dental peiiosteurn which are the remains of the
epithelial sheath of Hertwig ; 0., osteoblasts
which have formed the cement : If., lacuna of
the cement ; /., granular layer of the dentine ;
J), dentine.

newly formed or still growing teeth
may be broken up, and its structural
elements more easily distinguished. In
broken enamel prisms thus treated a
longitudinally striated structure has
been described (Annell).

It is further foutd, on treatment
with acid, that a very thin membrane
(enamel cuticle, Nasmyth's membrane)
entirely covers the enamel of unworn
teeth upon its outer surface (figs. 52,
53). This membrane forms a protective
covering to the enamel. It is of an
epithelial and horny nature, and with-
stands prolonged boiling as well as the action of acids and other re-agents. It is
formed of short flattened prisms which are the remains of the last formed portions
of the enamel prisms, and which have remained uncalcified. After the action of
nitrate of silver, it exhibits markings like those seen in a pavement epithelium.

The crusta petrosa or cement is the third substance which enters into the
formation of the teeth. This is a layer of true bone, slightly modified in structure,




THE TEETH.



37



and investing that part of the dentine which is not protected by the enamel. It
covers the whole fang, towards the lower end of which it becomes gradually thicker,
and is specially developed at the apex, and along the grooves of the compound
fangs. As life advances, the cement generally grows thicker, especially near the
point of the fang, where it sometimes blocks up the orifice leading to the pulp-
cavity.

The crusta petrosa is lamellar in structure, and contains lacunas and canaliculi
resembling those of bone but larger and more irregular (fig. 55, K). Where the
cement is very chick it may contain Haversian canals. On the milk teeth the
cement is thinner, and contains fewer cells. Perforating and decussating fibres,
similar to those of ordinary bone, occur in the cement. It is covered externally
by the dental periosteum, by means of which it is firmly fixed into its bony socket.

VARIETIES OF DENTINE.

Certain varieties of hard tissue are liable to be formed in the pulp-cavity of a tooth after
the regular production of the dentine is completed. The two chief kinds hitherto described
are the following :

1. Osteodentine (Owen). This is a hard substance which sometimes becomes deposited
within the pulp-cavity, somewhat resembling bone in structure. It is traversed by canals,

d

Fig. 56. LONGITUDINAL SECTION OF INCISOR TOOTH SHOWING DENTINE OF
REPAIR. SLIGHTLY MAGNIFIED. (Reduced from Salter.)

d, d', denuded surfaces of dentine ; r, r', corresponding deposits of
secondary dentine. Two or three incremental lines are observed in the
dentine.

which contain blood-vessels and pulp-tissue, and may be surrounded
by concentric lamellae like the Haversian canals of bone. From these
canals numerous tubules radiate, larger than the canaliculi of bone,
resembling, in this respect, and also in their mode of ramification, the
tubes of the dentine. It may or may not coalesce with the previously
formed dentine.

2. Secondary dentine, Dentine of repair (Salter). When the
outer surface of the dentine becomes denuded at any place, so that the
peripheral ends of the tubules are there exposed, as may happen
in the crown from injury or wear of the enamel, or at the cervix
from continued friction and abrasion of the cement, a deposition of
dentinal matter occurs on the inner surface of the dentine exactly
corresponding in position and extent with the area occupied by
the central ends of the exposed tubules. Many of the affected
tubules become subsequently filled up by a deposit of hard matter
within them, so that on section both the secondary dentine and
the corresponding part of the primary dentine appear clearer
and more transparent than the remainder of the dentinal substance
(see fig. 56).

When the surf ace- in jury has been considerable, the dentine
of repair is largely in excess, and may in such cases completely
cavity.'

DEVELOPMENT OF THE TEETH. 2

The first trace of the teeth appears during the sixth week of intra-uterine life (in
embryos of 11 mm. to 12 mm. long) in the form of a longitudinal thickening of the
epithelium of the mouth along the line of the future jaw. The thickening in
question is produced by multiplication of the deeper-lying cells of the epithelium,
and in some animals, e.g., ruminants, is marked by a prominence raised above the
general level of the epithelial surface. A prominence is found in the human

1 In some animals other kinds of dentine arc found ; for a description of these, and other details
regarding the structure and development of the teeth of* vertebrates, the student is referred to the
" Manual of Dental Anatomy," by C. S. Tomes, F.R.S.

2 The following account of the development of the teeth is mainly based upon the description given
by Rose.




fill up the pulp-



38



ORGANS OF DIGESTION.



embryo for a very short time, and at one point only of the jaw. Nevertheless
it is considered by Rose of great morphological importance, as representing a



L f .




Fig. 57. FIGURES (FROM C. ROSE'S MODELS) SHOWING FOUR SUCCESSIVE STAGES IN THE DEVELOPMENT

OF THE DENTAL LAMINA AND TOOTH GERMS OF THE MILK TEETH OF THE UPPER JAW. ONLY THE
BUCCAL EPITHELIUM AND THE EPITHELIAL STRUCTURES OF THE TOOTH GERMS ARE REPRESENTED,
AND EVERYTHING IS SHOWN AS SKEN FROM ABOVE.

1. From an embryo 25 mm. long. d.l., dental lamina ; M., aperture of mouth.

2. From an embryo 40 mm. long. M., mouth; l.f., reverse of labio-dental furrow; d.l., dental
lamina ; p. , mould for papilla of milk canine ; p 1 , mould for papilla of second milk molar.

3. From an embryo 115 mm. long. Z/., epithelial layer of upper lip ; l.f., reverse of labio-dental
furrow ; d.l., dental lamina ; m.i. 1 , epithelial rudimentof first milk incisor ; m.i.' 2 , m.c., m.m. 1 , m.m.' 2 ,
epithelial rudiments of second milk incisor, and of milk canine, and first and second molars respectively.

4. From an embryo 180mm. long. m.i. 1 , m.i.", m.c., m.m. 1 , and m.m.-, as before; m. 1 , rudi-
ment of first permanent molar.

vestige of the primitive larval epithelial teeth which occur in fishes and amphibians.
The longitudinal epiblastic thickening grows into the mesoblasc, as a strand
of cells which is at first semicircular in section, and the mesoblast encloses



THE TEETH.



39



it in a continuous groove ; for there is at first no indication of the formation of
separate teeth. The strand in question has been called the "dental lamina"
(Zahnleiste) : it is also known as the " common enamel germ," because some of its
cells are concerned in the production of the enamel of the teeth. But it is not
merely a dental germ, for before long (embryo of 17 mm., or seventh week) it is
found that the originally simple strand of cells is separating longitudinally into
two. One of these, the outer, or labial, which dips vertically into the embryonic
jaw, becomes hollowed out from the surface to form the labio-dental furrow,
and may accordingly be termed the labio-dental strand, whilst the other, lingual or
inner part, takes at first a vertical and then an inward (lingual) direction, and is the
actual tract of cells in connection with which the teeth of both dentitions subsequently



all



Fig. 58. PALATINE SURFACE OF THE MODEL

WHICH IS SHOWN FROM ABOVE IN FIG. 57. 3.

P, palate ; L, upper lip ; d. l.f., labio-dental
furrow ; d.f., dental furrow.

become developed. The name dental
lamina, or common dental yerm, 1 should
therefore be retained for this portion
of the original strand of epiblasfc cells
alone. The separation of the two
strands begins in front and extends
gradually backwards : it is not com-
plete until the eleventh or twelfth week.
The dental lamina, when thus separated
from the labio-dental strand, forms a
flat band of cells (fig. 57, 1, d.l.} con-
nected by one edge with the epithelium
lining the mouth, whilst the other, or
free edge, projects almost horizontally
inwards (i.e., tongue-wards) in the
substance of the embryonic jaw. Sub-
sequently, however, as the milk teeth
develop, it takes a vertical direction.
Over the line of its attachment to the
epithelium of the mouth there is a shal-
low furrow, the dental furroiv (fig. 58,
d.f.}, which is at first rather outside (or

on the labial side of) the most prominent part of the jaw, but gradually comes to lie
further inwards. At about nine weeks (embryo of 25 mm.) the free border of the
dental lamina begins to exhibit ten enlargements in each jaw corresponding in situa-
tion to the ten milk teeth. At ten weeks (embryo of 32 mm.) these enlargements
show a moulding on one of their surfaces (upper in the upper jaw and lower in the
lower) (fig. 57, 2,/?.,^. 1 ), and the adjacent mesoblast fits against this moulded surface
and becomes differentiated into the form of a papilla, which thus at once begins to
have the shape of the crown of the future toor.h, being simple in the incisors and
multiple in the molars. 2 The papillas have all appeared by eleven and a half weeks,
and the enlargements of the dental lamina, which are now very evident, grow around

" Dental germ " is used instead of the former expression "enamel germ," because the cells in
question not only form enamel, but also appear to determine the formation of dentine (by the mesoblast
cells in contact with them) (see p. 46).

2 In the canines, however, the papilla is at first double, not single. It has been suggested that this
is probably an indication of the originally premolar character of these teeth.




40



ORGANS OF DIGESTION.



and gradually invest the papillse at their sides also. The dentine and pulp of the
milk teeth are formed from these papillae, whilst the enlargements of the dental
lamina which invest them form special dental germs for those teeth, to which also
they furnish the enamel.

In the meantime the dental lamina has grown further inwards (tongue-wards)
beyond these prominent special dental germs, which appear now as ten rounded



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