84 
FIRST CLASS OF THE VERTEBRATED ANIMALS. 
with which alone we are at present concerned, may furtlier be restricted by the cir- 
cumstance already alluded to, that they are only found upon the margins of the maxil- 
lary bones. 
It was for a long time supposed that the teeth were bones, that they were produced 
in the same manner, and had a similar structure. This view of the subject has been 
wholly abandoned, since the publication of the admirable treatise of the Baron Cuvier 
upon the grinders of the Elephants, in the Annales du Museum dTiiatolre Naturelle, 
tome viiij in the year 180(i. Although the diSerence.^ between teeth and bones appear 
to be very numerous and essential, there seems, however, to be a cou.siderable ana- 
logy between them, especially when considered in a point of view purely anatomical. 
When physiologically considered, they possess many peculiarities in common with the 
horns, nails, and hair. 
At first, the constituent matters of the teeth and bones are precisely the same ; 
and if we revert to the first formation of these bodies, it appears that they are 
equally secreted and deposited by proper vessels. Under this point of view, the 
teeth may be considered as bones, the vessels of which are united in a single mass, 
and deposit the osseous matter around them ; while the bones may, on the other hand, 
be viewed as teeth, within which the minute subdivisions of the vessels cause this 
matter to circulate in every direction. 
At their origin, and during the greater part of their existence, the teeth arc com- 
posed of a secreting organ and a secreted substance. The former, or secreting organ, 
is always concealed in the lower part of the tooth, or in the interior; and when en- 
tirely formed, consists of three, or at least two other organs. It is essentially com- 
posed of vessels and nerves, which communicate directly with the remainder of the 
organization. The latter, or secreted substance, is merely deposited outside tbo first. 
It is composed of a greater or less number of different substances, and being deprived 
of all vessels and nerves, bears no necessary or immediate connexion with the other 
organs. 
The secreted substance is of a calcareous appearance, and composed of two parts ; 
the one e.KternaI, called the croini (fust or conronne) of the tooth; the other being 
more or less concealed, is termed the root (racinef The intermediate point is dis- 
tinguished by the appellation neck (collet). 
^ The crown of the tooth may be composed of different hinds of matter, deposited 
one over the other. In the most complicated kinds of teeth, three of these may be 
obtained by a mechanical analysis. The central part is termed the ivory; the second 
the enamel, and the most external part the cortex. These three substances are found 
combined in four different ways. Some teeth are composed of ivory, enamel, and 
cortex; others only of ivory and enamel. Some, again, are formed of ivory and 
cortex, the enamel being wanting ; others of ivory alone, this last being never observed 
to be deficient except in those Mammalia which are wholly destitute of teeth. 
The root may bo real or apparent, in the first case, it is formed of ivory alone, as 
in Man, the Caruassiers, and the Ruminantia; or of ivory and cortex, as in the Cacha- 
lots. In the second case, the root is merely a continuation of the crowm, and has all 
the characters of the latter. Such are the roots of all tusks properly so called, the 
incisive teeth of all the Rodentia ; the molars of Hares, of Guinea-pigs, and of the 
Cabiais {Hydrockfcrus). 
The secreting organ of the tooth or dentary capsule, according to M. Frederic 
Cuvier, appears to be dependant on, or produced by, the nerves and maxillary vessels. 
It is not, however, without relation to the contiguous parts, being even united to 
the gums ; but rnu<;h leas than some authors have imagined. It is certain that the 
secreting organ of the second teeth, for a long time after its formation, is altogether 
independent of these parts, aud it is only subsequently that it becomes united to the gums. 
The dentary capsule corresponds, both in its structure and functions, with the 
substances or materials of which the teeth are composed, in such a way, that it is 
more simple in teeth formed of one substance alone, than in those composed of two 
or three. It is the same with its forms, as well as its growth, in relation to tho 
forms and growth of tho teeth, ihe one always being the eonsequenco of the others. 
The most complicated kind of dentary capsule, being that observed whenever tho 
teeth arc composed of three substances, is itself formed of three very distinct secret- 
ing organs. The central one, called the bnlhi produces the ivory ; the second, 
under the form of a membrane, secretes the enamel, and may thence be termed the 
enamellaiing membrane ; and tho third, which sunounds ail the other parts, produces 
the cortex or external ivory. The last may be termed the exlcnial membrane. 
The bulb which secretes the ivory by its external surface, appears to be entirely 
composed of nerves and vessels. Several arterial trunks, which extend from the 
one extremity to the other, are ramified infinitely before arriving at its extremities, 
where their divisions sometimes form tufts and fringes of an almost imperceptible 
degree of fineness. This part of the teeth may be studied with tho greatest facility 
when they first begin to form; it is then found to be naturally injected, and is net 
exposed to injury during the abstraction of those bony portions in which the teeth 
are enclosed, while a very slight degree of maceration is sufficient to extract tlie 
bulb from the coating of ivory by which it is surrounded. It seems to be homo- 
geneous throughout, and always has the same shape as the tooth will ultimately have. 
In fact, it is the mould upon which the tooth is modoUed. 
The enamellating raembratie produces the enamel by its internal surface. It sur- 
round? the bulb entirely, and follows all its circuitous outlines, thus possessing the 
same form, except at tho h&se of tho bulb, corresponding to the neck of the tooth 
where it abuts and terminates. IM. F. Cuvier was unable to detect any vessels in 
this membrane. It is transparent and brittle when thick and about to deposit 
<’«amel; but it soon softens, becomes of a milky whiteness and great elasticity. 
Finally, it cutis by disappearing altogether, when it has no longer any function to 
discharge, that U, when the external membrane, by depositing the cortex, resumes 
its place. The transparency of this membrane, its extreme thinness thereafter, aud 
its final obliteration, in those teeth where the ivory is formed, have been tUc cause 
that many Naturalists have failed to observe it. But it may be seen very easily 
upon tho parts contiguous to tho molars of the Ruminantia, aud especially on the 
hinder ones, at the moment when those animals are born ; and, if once remarked 
here, it becomes easy to detect it upon all teeth possessed of enamel. 
The external membrane, like the bulb, is of a nature essentially vascular, and 
may be considered as an external bulb. It is homogeneous in respect to its intimate 
structure ; but its two faces have not always the same forms, nor do they perform 
tho same functions. It deposits the cortex by its internal surface, and follows all 
the contortions of the tooth. In the compound teeth it juts outwards, whenever 
they present any hollows. The parts which line these cavities are not merely mem- 
branes, at least when the cnrtical matter is about to be deposited, for they then have 
the same Ihickuess as these cavities, and this gives them all the appearance of bulbs. 
Before the above period, it is sufficiently thin upon the surface of the compound 
teeth, and this observation U applicable to most teeth. But it may be presuin€*d 
that tho external membrane is always of a great degree of thickness in the capsules 
of those teeth where the ivory has to be covered with a great thickness of cortex, 
as may probably happen in the molar teeth of the Cachalots. Its external surface 
is always simple, being merely the protecting aud uniform envelope of the entire den- 
tary organ, and its form when complete is always more or less spherical. It is 
pierced at its summit by the evolution of the tooth ; but its margins arc attached to 
the gums, and become in some measure a continuation of them. 
These three pares, comjmsing the dentary capsule, are intimately united, and be- 
come confounded together towards the inferior part of this organ, at the point where 
the vessels and principal nerves are introduced, at least from the time when the 
roots begin to develop themselves, and to become distinguished from the crown. It 
appears that all the three parts originate from this point, and likewise all the cssen* 
tial vessels which traverse and nourish them pass from thence. Their other 
portions are from the very commencement entirely independent of each other. The 
external membrane may be raised without occasioning the slightest injury to the 
enamellating membrane, which detaches itself without effort from the layers of enamel 
just deposited ; and the bulb may be separated from its cones of ivory' like a blade 
from its scabbard ; or if the cones be broken, it may be disengaged and displayed 
without being destroyed, or in any way injured. 
This capsule, however, is not entirely formed before the teeth are secreted, in 
those at least which have roots. It develops itself successively, and in proportion 
as the different parts are formed, beginning from the summit of tbo crown, and finish- 
ing by the extremity of the root. 
The bulb and enamellating membrane seem to deposit simultaneously the matters 
which they respectively secrete ; and tho first molecule of ivory receives the first mole- 
cule of enamel. It is ouly at a later period, that tho external membrane deposits the 
cortex, being at the time when the crown is already formed, and when the bulb, as 
well as the enamellating membrane, cease to deposit matter in this part of tho tooth, 
for these secreting organs have still to give birth to tho roots. 
The above detailed analysis of the most complicated kind of dentary capsule, cn* 
ables us to pass rapidly over those destined solely to secrete the ivory and enamel, 
or the ivory and cortex, or the ivory alone, aud being consequently of a more simple 
structure. 
Those capsules intended to form tooth composed of ivory and enamel alone, arc 
not, on that account, depriveti of the external membrane, but this body always ap- 
pears to be thinner, instead of being thick, as in the preceding kind of teeth, when 
about to deposit the cortex. It is raised with difficulty and by shreds, aud seems 
only to be intended to protect the function of dentition ; it accordingly envelopes tin* 
organ in every part. Tho enamellating membrane presents itself in these capsule's 
with all the general characters which have been assigned to it. Tho bulb does nnt 
differ from that belonging to teeth composed of three substances. 
With respect to those teoth which are composed of ivory and cortex, such as the 
molars of Cachalots, we also find tho external membrane in them to be of a cer- 
tain thickness, in addition to the bulb, which is never svauting. 
Having thus shown that the dentary capsule of the most coinpUcat<‘d kind of teetln 
produces three distinct and different substances which can be accurately separated 
from each other, it now' remains for us to consider tho secreted bodies themselves, 
composing, as they do, the proper substance of tho teeth. 
The ivory forms the essential and fundamental part of the tooth. -As it covers 
the organ by which it is secreted, it is deposited from without, inwards, aud 
not appear to be absolutely identical in all kinds of teeth. In some, as tlic tusks of 
the Elephants for example, it is deposited by concentric beds, in such a manner 
that they are composed of cones, the one encasing another, and being numerous m 
proportion to the length of the tusk. This conical appearance is especially show'U 
in fossil tusks, as the cones themselves do not appear to have been separated artifi- 
cially. Other teeth have a more homogeneous kind of ivory, but tho differences of 
texture which this substance presents are very numerous. These tusks of the Blo- 
phants show on their transverse sections a iiurabor of segments of circles regularly 
disposed, which intersect each other, and form a waving mark, bv which the true 
ivory may always be recognised. ITie teeth of Man, the Quadrumana, and Carna^- 
siera, possess an ivory of a silky appearance, appaienlly composed of fibres. ThoaU 
of the Cetacea, the tusks of the Hippopotamus and otliers, have their ivory simph' 
and of the most uniform texture; tliose of the Rat-!Moles (Bathyeiyui;) 
formed of longitudinal and parallel liiires, like those of a rush. 'I'hese ebarac 
ters arise doubtless from the peculiar structure of the bulbs which secrete these d* 
ferent kinds of ivory ; yet their essential differences have not been determined b) 
experiment, but will probably be ascertained hereafter, when these bulbs are su 
mitted to a more minute investigation. 
This central part, being the most Important and considerable portion of the crow 
of the teeth, is chiefly formed of a very compact gelatinous substance. Fhe 
reous matter which gives it the external appearance, is merely deposited in the mes 
of this substance, and composes only the smallest portion. It may be abstrac! 
,lcd by 
and of the 
means of a small quantity of dilute acid, and the gelatine remains pure, 
same form as the ivory. This calcareous matter, the only part of the tooth rea J 
destitute of life, is a phosphate of lime. 
The Enamel is deposited in a manner contrary to tho ivory, being from ' > 
outwards, and always immediately over the latter; this it appears to do by a km 
crvstallizing process. On being examined upon a section of a tooth, it is tmi 
