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masses of cells attached to the labial side of the flat common dental lamina " like
swallows' nests built against a board " (fig. 57, 3). The common dental germ
extends backwards in the substance of the jaw a short distance behind the last
of these special dental germs for the milk-teeth. This backward extension of
the dental lamina is not directly connected with the buccal epithelium. At
about seventeen weeks (embryo of 18 centimeters long) it shows another

Fig. 59. GlNGIVAL SURFACE OP THE
MODEL WHICH IS SHOWN FROM
ABOVE IN FIG. 57, 4.

P., palate ; L., upper lip ; l.d.f.,
labio-dental furrow ; d.f., dental fur-
row ; d.l., prominence caused by dental
lamina with its enlargements.

enlargement, which is the special
germ of the first permanent
molar, and, in connection with
this enlargement, the corre-
sponding papilla soon makes its
appearance (fig. 57, 4, w 1 ). Behind
this again the dental lamina is
continued backwards into the gum
as a thin flat band of epithelium.
About four months after birth an
enlargement for the second per-
manent molar appears, and the
corresponding papilla at six
months, and about the third year
the enlargement for the third
permanent molar, or wisdom-
tooth, begins to be visible in a
still further backward extension
of the dental lamina, and its
papilla is seen about the fifth year.

Meanwhile, important changes have been occurring in the dental lamina, in the
attachments of the special germs to it, and in the special germs themselves.

Changes in the dental lamina. The changes in the common dental lamina
consist in the formation of numerous apertures of irregular size and form, with the
result that from a complete flat band of cells it becomes partly atrophied and changed
into a cribriform tract (fig. 03), so that in transverse sections of the jaw it appears to
be broken up into separate portions (fig. G4). This is, however, not the case,
although the lamina is pierced with apertures so as to be almost reticular in
character. This atrophic change begins in front about the seventeenth week, and
gradually extends backwards, but the most posterior part is still complete (non-
cribriform) at the time of birth, and even for some time after. Moreover, at
certain points the reticulation is absent, viz., on the lingual side of and slightly
lateral to the special germ of each milk-tooth, and at these points the dental lamina




THE TEETH.



47.



again undergoes a thickening. These thickenings become the special enamel germs
for the teeth of succession of the second dentition. In connection with each one a
papilla becomes formed in the same way as for the milk teeth, but by no means-



Mr.




JilL



Fig. 60. SAGITTAL SECTION THROUGH THE FIRST LOWER MILK MOLAR OF A HUMAN EMBRYO
30 MM. LONG. (Rose.) iaa.

L.E.L., labio-dental lamina, here separated from and well in advance of the dental lamina ;
Z.L., placed over the shallow dental furrow, points to the dental lamina, which is spread out below to
form the enamel germ of the future tooth ; P.p.. bicuspidate papilla, capped by the enamel germ ;
Z.S. , condensed tissue forming dental sac ; M.E., mouth-epithelium.

simultaneously, for the germs of the permanent incisors and canines are formed,
along with their papillae, at about the twenty-fourth week (embryo of 30 c.m.),
whereas the enlargements of the dental lamina, which are eventually to form the
enamel organs of the first and second premolars, are not visible until the twenty-




Z.S'.



Fig. 61. -SIMILAR SECTION THROUGH THE CANINE TOOTH OP AN EMBRYO 40 MM. LONG.

(Rose. ) J4p.

L.F., labio-dental farrow. The other lettering as in Fig. 60.

ninth and thirty-third weeks respectively, and the corresponding papilla? are not
formed until the tenth and eighteenth months after birth.

Changes in the connection between the special dental germs and the
common dental germ. The special dental germs are at first simply enlargements
of the common germ which grow out on its lingual side. They soon become
globular, and rapidly increase in size, and are then connected by a broad tract of
cells with the common germ. This connecting strand gradually gets thinner and



ORGANS OF DIGESTION.



flatter, and, like the common dental lamina itself, becomes cribriform, so that in
sections there appear to be breaches of its continuity. Its connection, however,

Fig. 62. SECTION THROUGH

THE GERM OF THK FFRST
MILK MOLAR OF A COW'S
FO3TUS 47 MM. LONG.

(Rose.) -^p.

P., bicuspidate papilla ;
Z.S. , dental san ; M.K. ,
mouth-epithelium ; Z. W., its
thickening over the dental
germ, characteristic of Ruini-
nantia ; 8.E. , enamel epi-
thelium; S. P., enamel pulp.

with the common dental
germ, and through this
with the buccal epithe-
lium on the one hand
and the germ of the
corresponding tooth of
the secondary dentition
on the other hand, long
persists. As with the
common lamina, this
atrophic process begins
in connection with the
frontteeth,andgradually
extends backwards, so
that at birth the con-
necting bands of the milk incisors are almost completely broken up, whilst that of
the second milk molar is still uninterrupted. The common dental lamina and the

Fig. 63. SECTION OF SECOND MILK INCISOR OF

AN EMBRYO 30 CENTIMETERS LONG. THK
EPITHELIAL STRUCTURES ONLY ARE REPRE-
SENTED. (From a model by C. Rose.)
d.f. , dental furrow in the buccal epithelium

d.L, dental lamina now become cribriform ; p.

space occupied by the papilla ; d. , dentine

en., enamel of trie developing tooth ; en. or.

enamel organ, its surface cribriform ; a.d.

adamantoblasts ; r., reticular tissue ; and o.c.

outer epithelium of the enamel organ.

bands connecting the special dental
germs with it thus become ultimately
broken up into separate fragments or
islands of dental epithelium of varying
size and form. Such "islands" are
frequently seen in the infant near the
surface of the gum, as pearl-like masses
or nests, the so-called " glands of
Serres." Normally they have no
functional importance, and gradually
entirely disappear ; but, abnormally,

they may give rise to cysts and other new formations, and in some cases fragments of
dentine, and even more or less complete teeth, may become developed from them.








THE TEETH. 43

Changes in connection with the special dental germs. The special
dental germs are at first masses of rounded or polyhedral epithelial cells, but the
outermost layer early shows a tendency to be columnar. This becomes pronounced
as soon as the papilla begins to make its appearance, and now while the cells which
immediately rest upon the papilla become long, regular prismatic columns, the central
cells of the germ develop processes, fluid being at the same time secreted between
them. The result is the formation of a reticular tissue, which to the naked eye has
the appearance of a jelly, to which the name of enamel-pulp has been applied
(fig. 64, S.P.). The more peripheral cells do not participate in this change, but
remain polyhedral or become cubical and flattened (outer enamel epithelium) ; they



-ME



it M : Vm^




?y! > 't&&- -

.. . .!%$& ...

j&&"ff>l/, - '-'.;&'

KteS&imm,



, .... , s ~-^.x-5:'W?W&mm#v

Fig. 64. SECTION OF THE FIRST MILK INCISOR OF A HUMAN EMBRYO 30 CENTIMETERS LONG.
FRONTAL SECTION THROUGH LOWER JAW. (Rose.)

D.K., papilla; Od., odontoblasts ; K., bone of the alveolar process of the jaw ; S.E.a., S.E.i.,
outer and inner layers of the enamel organ ; S.P., enamel pulp ; Z.F. , dental furrow ; M.E., mouth-
epithelium ; Z.L., remains of dental lamina ; V. B., cell bridge, connecting this with tooth germ;
E.L., reserve germ for the permanent tooth ; J.l.n., germ of second milk incisor cut obliquely across.

pass gradually into the long columnar prisms which invest the papilla. These
prisms are the cells which form the enamel fibres, and which determine by their
presence the production of dentine by the superficial cells of the papilla. They may
be termed the enamel-cells or adamanloblasts ; they form the membrana adamantines
of Purkinje. The whole epithelial dental germ thus transformed is known as the
enamel organ (organon adamantine of Purkinje). These changes begin in the
milk incisors at about 14 weeks. At about 20 weeks (embryo of 24 c.m.) the first
traces of calcification are visible in the form of a simultaneous deposit of enamel
and of dentine upon the crown of the central incisors. The outer enamel epithelium
now begins to grow out into the surrounding connective tissue in the form of
epithelial sprouts, and before long there seem to be breaches of continuity between



44 ORGANS OF DIGESTION.

these sprouts ; but, according to Rose, the enamel-pulp is never invaded by vascular
connective tissue, as has been sometimes described.

Changes in the meantime have been occurring in the dental papillae. These are
composed at first of undifferentiated mesoblasb ; but their more superficial cells
those which are immediately in contact with the columnar epithelium of the special
dental germs early become elongated, and abut by their distal end against that
epithelium, whilst the other end is tapered, and may be branched like the other cells
of the embryonic connective tissue. It is from this superficial layer of cells which
in sections have a palisade-like appearance that the dentine becomes gradually
formed, and they have accordingly received the name of odontollasts. There is
nothing of the nature of a membrane the so-called membrana preformativa
between the adamantoblasts and the odontoblasts, but the two layers abut at first the
one against the other.

Meanwhile the whole tooth-germ papilla and enamel organ has become in-
cluded within a vascular membrane of connective tissue which is known as the
dental sac.

Formation of the dentine. The odontoblasts, either by secretion or, as some
think, by direct transformation of the peripheral end of each cell, form a layer of
dentinal matrix immediately at the surface of the papilla at its apex, or if it have
more than one cusp, then at the apex of each cusp. This layer is at first uncalcified,

Fig. 65. SECTION OF DEVELOPING DENTINE FROM THE

INCISOR TOOTH OF A YOUNG RAT. (E. A. S.)

a, outer layer of fully formed dentine ; b, uncalci-
fied matrix, with one or two nodules of calcareous
matter near the calcified part ; c, odoutoblasts send-
ing processes into the dentine ; d, pulp. The section
is stained with carmine, which colours the uncalcified
matrix, but not the calcified part.

and is probably the structure which used to
be described as membrana preformativa.

Globules of calcareous matter soon begin to appear in it. They are at first isolated,
but by further deposition of lime salts they become more or less blended into a con-
tinuous calcification, which thus forms the first cap of dentine. In the meanwhile
the odontoblasts have formed a second layer of uncalcified matrix within the first
one, and calcification proceeds in this as in that. In like manner a succession of
layers become formed, each one extending laterally rather further than its pre-
decessor, and thus in teeth where there are at first separate deposits for different
cusps these become ultimately blended, or as each successive layer is calcified its
calcareous deposits blend with that of the preceding and more superficial layers. In
places this blending remains incomplete, portions of the dentinal matrix remaining
uncalcified between the successive layers ; and in a macerated tooth these portions
get destroyed, and cleft-like spaces arise. Since these are bounded by calcified
deposit which has been originally laid down in globules, they present a knobbed out-
line, and are known as interglobular spaces.

As the odontoblasts form the successive layers of dentine in the manner above
described, they retire gradually towards the centre. But whilst thus retiring they
leave in situ, in the layers of forming dentine, filamentous processes of cell-proto-
plasm, themselves provided wit'i finer side-processes, and the dentinal matrix
becomes formed and moulded around these processes, leaving them within tubes
which become dentinal tubules, whilst the processes of the odontoblasts become the
fibres of Tomes. The same cell continues to spin out such a filament in this manner
as long as the formation of dentine continues, each tubule being thus completed in




THE TEETH.



45



its whole length from a single odontoblast. In many cases two or more processes
are connected with each cell at first, and these coalesce as the cell recedes, the main
branches of the dentinal tubules being thus formed.

The other cells of the dental papilla which are not immediately concerned in the
formation of dentine become, as the tooth approaches completion, the cells of the
dental pulp.

Formation of the enamel. The prismatic fibres which compose the enamel
of the teeth appear to be formed by the direct agency of the ends of the adamanto-
blasts which abut against the dental papilla. In connection with each of these cells
at the end in question a finely globular deposit occurs (Annell), which stains with
osmic acid and resembles keratin in its extreme resistance to the action of mineral



If - str.int.



\ . ' i




Fig. 66. PART OF THE ENAMEL ORGAN OP THE CANINE OF A YOUNG KITTEN. (Rose.) ^f- 5 -.

d., superficial layer of dentine ; e., newly forming enamel stained black by osmic acid ; T., Tomes'
processes from the adamantoblasts, ad. ; str.int., stratum intermedium of the enamel organ; p., branched
cells of the enamel pulp.

acids (enamel droplets, v. Spee). The layer which is thereby formed, and which is
not yet calcified, is outside the main body of the adamantoblasts although a process
from each adamantoblast extends into it as a tapering fibre (process of Tomes) and
it is usually produced simultaneously with the first layer of uncalcified dentine
against which it is applied. Before long it undergoes calcification, and the first
layer of enamel is then complete. After a time the adamantoblasts yield a second
layer of keratin-like material, and from this after calcification has invaded it, another
stratum of enamel is formed, and so on. As with the dentine, the formation of enamel
appears first at the apex of each cusp, so that there are at first as many caps as
cusps. Whilst these changes are being effected, the adamantoblasts gradually retire
as the successive layers of the enamel are being produced by them, and this gradual
shifting goes on as long as the formation of enamel continues that is to say, until
the crown of the tooth is completed. By this time the enamel pulp has greatly



46



GROANS OP DIGESTION.



diminished, and in fact almost disappeared, and the remainder of the enamel organ
forms a thin epithelial cap over the crown, which soon disappears on the emergence
of the tooth beyond the gum. But besides this epithelial cap, and underneath it,
there is a very thin membrane, which is more persistent, and covers the crowns of
the teeth for some little while after their emergence (fig. 52, e, e). This is Nasmyttts
membrane, or the enamel cuticle, and, according to v. Brunn, it is the last formed
keratinous layer of enamel which has remained uncalcified.



od.




cp.scL. - >,- J.




Fig. 67. LONGITUDINAL SECTION OF THE LOWER PART OP A GROWING TOOTH, THE CHOWN OF WHICH

IS FORMED, SHOWING THE KXTKNSION OF THE LAYER OF ADAMANTOBLASTS BEYOND THE CROWN TO
MARK OFF THE LIMIT OF FORMATION OF THE DENTINE OF THE ROOT. (Rose. )

p., pulp; od., odontoblasts ; d., dentine ; en., enamel; ad., adamantoblasts, continuous belo\\
\\ith, ep.sch., the epithelial sheath of Hertwig.

Fig. 68. SECTION SIMILAR TO THAT SHOWN IN FIG. 67, BUT FROM A TOOTH MORE ADVANCED

IN GROWTH.
cp. sh., epithelial sheath ; ep.' sh'. t remnants of this, overlying dentine of fang.

It has usually been considered (Tomes, Waldeyer and others) that the enamel prisms are
formed by direct calcification in Kit it of the inner ends of the adamantoblasts, the outer
nucleated end growing pari paxxu with the extent of the inner end which has thus been
converted into enamel. But a different view has been taken by some authorities, viz. that
the layer of adamantoblasts sheds out the substance within which calcareous matter is sub-
sequently deposited ; the enamel prisms being formed therefore rather as a secretion from the
adamantoblasts than by transformation in xitii. This is supported by the readiness with
which the adamantoblasts separate from the enamel, but on the other hand it is more difficult
if it is accepted to account for the fact that the enamel prisms take the exact diameter and
shape of the adamantoblasts. The view seems to me to be corroborated by appearances seen
in sections which have been placed at my disposal by Mr. J. L. Williams, which show a
well-marked layer of highly refractive globules imbedded in a fibrinous-looking matrix lying
between the adamantoblasts and the already formed strata of enamel.

Formation of the cement. The roots of the teeth are gradually formed
shortly before the time for the emergence of the crowns beyond the gum, but they



THE TEETH.



47



are not completed until long after the crowns have come through. They are deter-
mined in their form moulded, as it were by a growth of the epithelium of the
dental germ, which extends in the form of a fold, the so-called epithelial sheath,
(fig. 67) towards the future apex of each fang (v. Brunn). On the inner or papilla
surface of this sheath odoutoblasts form dentine, as in the crown, and thus the
root is gradually produced. The epithelial sheath becomes gradually atrophied and
ultimately broken up into isolated portions, which may be seen occasionally, even in
the adult, as epithelial islands, in the connective tissue of the dental periosteum
(fig. 55;.

C. Tomes has shown that an epithelial sheath is formed in the same manner, even in the
teeth of animals (e.g., Taturia) in which the dental germ produces no enamel at all.

After the formation of the dentine of the root has begun, the vascular tissue of
the dental sac begins to break through the epithelial sheath near the crown, and
forms a layer of bone-forming tissue at the surface of the newly-deposited dentine.
The osteoblasts of this tissue deposit layers of true bone, with osteogenic fibres,
lacunaB, and canaliculi, upon the surface of the dentine of the root, and these layers
form the cement of the fang. The very apex of each fang, which is the last part ta
be produced, is formed wholly of cement, for the epithelial sheath which determines
the formation of dentine, never extends quite as far as the permanent apex.

In some animals the cement of the teeth is preceded by the formation of cartilage, which
becomes ossified as in the endochondral formation of bone (Magitot. v. Brunn). According to
Magitot. in animals such as ruminants in which the cement covers the crown, a special
cartilaginous " cement organ " is developed for the production of this cement. It is question-
able, however, whether the cartilaginous tissue which gives origin to this cement is sufficiently
specialized to deserve a distinct name.

The dental sacs are well seen in the jaw of an infant a few mouths old, before
the eruption of the teeth. They are represented at this state in fig. 69. They

Fig. 69. THE DENTAL SACS EXPOSED IN

THE JAW OP A CHILD AT BIKTH.

, the left half seen from the inner side ;
b, the right half shown from the outer side ;
part of the bone has been removed so as to
expose the dental sacs as they lie below the
gum ; the lower figure shows the sues of the
milk teeth and the first permanent molar,
exposed by removing the bone from the
outside ; the upper figure shows the same
from the inside, together with the sacs of
the permanent incisor and canine teeth
adhering to the gum.

consist of an outer fibro-vascular
coat connected with the periosteum,
and an inner highly vascular layer
with a little jelly-like tissue inter-
posed between the two. The inner
coat is lined with the epithelium of

the enamel organ to be hereafter described. Their blood-vessels are derived partly
from the dental arteries which course along the base of the sacs, and partly from
those of the gums. Their extreme vascularity doubtless has relation to the nutrition
of the enamel organ.

At birth the crowns of all the milk incisors and canines are fairly advanced in
calcification. The separated cusps of the milk molars have also blended, and the
calcification of the first permanent molar is just beginning in the form of separate





48



ORGANS OF DIGESTION.



caps for each cusp, one of which has usually appeared at birth, and the others follow
shortly after. These, however, do not run together until six months after birth.

The germs of the permanent incisors and canines are visible to the naked eye at
birth, lying behind and slightly lateral to the corresponding milk-teeth ; but there
is no trace as yet either of the premolars or of the second permanent molar. The
last-mentioned makes its appearance between four and six months after birth, the
papilla of the first pre molar about the tenth month, and that of the second premolar
about the eighteenth month. At two years, when the second milk molars are
just coming through the gum, the crown of the first permanent molar is finished,
but there are still only isolated cusps on the second permanent molar of the upper
jaw, and none on the second permanent molar of the lower jaw. In the premolars
also the (two) cusps are still separate at this time.

The various phases in the formation of the teeth occur almost simultaneously in
the corresponding teeth of both jaws.

6 7




Pig. 70. DIFFERENT STAGES IN THE FORMATION OF A MOLAR TOOTH WITH TWO FANGS. (From Blake.

1, the distinct caps of dentine for five cusps in the earliest stage of formation ; in 2, and the
remaining figures, the crown is downwards ; in 2 and 3, the formation of the crown having proceeded
as far as the neck, a bridge of dentine stretches across the base of the tooth -pulp ; and in 4, the
division of the fangs is thus completed ; in 5, 6, and 7, the extension takes place in the fangs.

Formation of the alveoli. All the tooth germs are at first included in a
common trough or groove, which encloses the whole dental lamina and the adjacent
-connective tissue. This begins to be formed at about 14 weeks (embryo of
lit c.m.). Bony septa subsequently become formed and subdivide the trough into
loculi, but even at birth these septa are incomplete, and up to this time, and even
later, both the milk tooth and the corresponding permanent tooth germ are enclosed
in the same loculus. As the fangs become developed the loculi deepen and also
become subdivided to form separate cavities for the teeth of both first and second
-dentition. Around the milk-teeth they become narrowed to form alveoli which

Fig. 71. DIAGRAM (AFTER WELCKER)
SHOWING ON THE LKFT SIDE THE
ORDER, AND ON THE RIGHT SIDK
THE TIME (IN MONTHS) OF APPEAR-
ANCE OF THE MILK TEETH OF THE

UPPER JAW. (Rauber.)

closely invest the roots ; but
although the whole of the de-
veloping tooth is at one time
embedded in the cavity of the
alveolus, the bone never com-
pletely closes over it, an aperture
being always left over the crown,
through which the dental sac is
connected by soft tissues with

the surface of the gum. In the same way, when the teeth of the second dentition
become invested within alveoli, these always have a narrow opening through which
the so-called gub&rnaculum dentis, a band of connective tissue containing remains
of the common dental lamina, passes.




THE TEETH.



49



Eruption of the milk-teeth. The eruption of the teeth does not occur in
regular succession from behind forwards, and by a gradual and continuous process,
but in batches, with intervals of repose between the successive batches. The follow-
ing shows the most usual time of eruption (C. S. Tomes). The first to appear are
the lower central incisors, at six to nine months. Their eruption is rapid, and is
completed in about' ten days ; then follows a resting period of two or three months,
after which the upper incisors appear, both central and lateral. Then, after a rest
of a few months, come the lower lateral incisors and the first molars ; then, after
four or five months, the canines, and finally, about the second year, the second
molars.

TABLE OF ERUPTION OF THE MILK-TEETH.

Lower central incisors . . . ' . . 6 to 9 months.

Upper incisors . . . . . . 8 to 10 months.

Lower lateral incisors and first molars. . . 15 to 21 months.

Canines . . . 1 . . . 16 to 20 months.



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