THE MAMMALIA—IVIAN AND BEASTS. 
85 
havs Uio appearance of brilliant ncedlos, porpondiculai* to tbo surface of the ivory. 
I’hc ivory an I onaniel do not form one body, although they aro united to;;cthcr very 
C'losi'ly, for the enamel can be detached from the ivory without injuring the latter, 
and reciproeally. But the essential distinction between them consists in the circum- 
stance that the enamel does not possess gelatine for its base ; for, although it contains 
some traces of that, substance, they are always very minute in quantity. The enamel, 
on the other hand, is essentially composed of Huato of lime, which contributes its 
stony character, and imparts a degree of hardness superior to that of any other por- 
tion of the teeth, and indeed of any animal substance. 
The Cortex, like the Enamel, is deposited from within, outwards; but it cannot 
bo discovered upon teeth possessed of enamel until the latter is entirely formed. 
M. F. Cuvier is of opinion that, in tcetl> composed of ivory and cortex alone, it is 
deposited over the? ivory like the enamel. Tlu* intimate nature of cortex is absolutely 
the same as that of ivory, on which account it might with propriety bo termed the 
^‘Xiernal ivory. Gelatine forms its principal base, ami phosphate of lime is deposited 
between the meshes of that substance. The cortex is found in layers more or less 
thick. It is of an extreme thinness on tho projecting surfaces of the molars in the 
Uuminanlia, but is much thicker in the liollows found on the summits of their crowns. 
It is observed, however, to possess a still greater thickness in the crowns of tho teeth 
belonging to Cachalots. In this place it equals the ivory in quantity and thickness ; 
for (he whiter substance, which surrounds the centra! part of these teeth, is not enamel, 
as some Naturalists have supposed, but a true external ivory. 
It commonly happens tliat the cortex contains nothing but gelatine and phosphate 
of lime. In some cases, however, it contains some colouring matter in addition to 
these, as may be seen in the teeth of several Ruminantia, and in the incisors of the 
Beavers. I’acas, Agoutis, Porcupines, and some others. The colour of the anterior 
part of these teeth depends upon a very delicate layer of true cortex, as M. F. Cuvier 
ascertained by many careful experiments. Tlie colour becomes brown only on that 
part of the tooth which projects from the gums, while the portion within them is of 
a dark green. It has been said that this colour is owing to the presence of iron, 
and that the change which it undergoes from the contact of the air is a true oxi- 
dation. 
The above details regarding the structure of the dentary capsule, which produces 
the teeth, as well as the composition and structure of tho teeth themselves, have 
been hitherto demonstrated upon a very small number of Mammalia, and they are 
applied only by analogy to the remainder. In fact, the teeth of Man, of some Car- 
tiassiers, Hodentia, and Ruminantia, with the Solipeda and the Indian Elephant, 
have alone been studied in respect to their dentary capsules, and the substances of 
which the teeth are composed. It is probable that a special investigation of teeth 
belonging to other Mammalia may lead to the restriction or extension of some of the 
preceding observations. 
The above remarks explain to a certain extent the manner in which the crown of 
the tooth is formed. As the dentary bulb is the mould of the crown, and as the 
matter which it secretes is deposited upon its surface, tho crown cannot fail to exhibit 
the same projections, hollows, and angles — in a word, to have the same identical 
Hguro ; but there is nothing in the structure of this bulb which can explain the form 
of the roots. 
By the terra root is commonly understood, that part of the tooth contained within 
the gums ; but it is essential, as has already been explained, to distinguish those in- 
»ortions which differ from the crown neither in structure nor form, from the roots 
properly so called, which begin from the neck of the tooth, and diminish gradually, 
'^ntil they terminate in a point more or less obtuse, and more or less irregular. The 
first are not real roots, but arc formed merely by the prolongation of the crown 
''■'ithin the gums. 
When the time at which the true roots have to be formed has arrived, the ena- 
^^'ellating membrane ceases to maintain its activity, and even bccotnes wholly obli- 
terated. The bulb and the external mcmbniue alone continue to grow and to pro- 
duce roots, which usually correspond, in number and situation, with the principal 
tubercles of the crown, and appear to be numerous in proportion to the number of 
I^'ading branches which the maxillary arteries transmit into the bulb. It seems 
probable that these vessels and their branches form an inferior prolongation of the 
I>ulb, as soon as the crown has been deposited ; or, in other words, that the bulb 
continues to develop itself under their influence, which is restricted to the points 
*niruodiately surrounding them. Under this point of view, the roots of the teeth 
be regarded as the evanescent crowns of tho same teeth, reduced to a rudi- 
*tieatary state; for we can easily see how they might bo continued, if the vascular 
system did not become obliterated. In fact, those teeth, where the capsule never 
leases to reproduce the crown as fast as it wcais away, and which are consequently 
destitute of true roots, only become such in consequence of the undiminished vitality 
their bulb, wliich continually maintains its vigour and activity as at the com- 
‘’fienceinent. Thus we sec that teeth possessed of roots, obtain them at periods of 
Ificir existence more or less advanced. Among the herbivorous animals, the Horse 
example, tho vitality of the bulb continues for several years, while it ceases in a 
short time with the Carnassiers. In this respect, the Mammalia offer a great 
^i^riety of examples. 
There are several circumstances which serve to confirm the accuracy of those 
As long as the dentary capsule is wholly occupied in depositing the crown. 
See, at the precise point where tho membranes composing it reunite and become 
Unfounded, a uniform disc, supplied with au immense number of vessels, which 
distinguish it readily from all the adjacent parts. It is from this surface that the 
'^^psule continues to grow uniformly, until the crown has acquired its entire height. 
*^he latter period, however, it undergoes a total change ; the isolated portions of 
vessels disappear, and those which remain compose little circles, more or less 
*"j^tueruu3, and distinct from each other. From these circles the roots grow ; during 
ich operation, the external membraue detaches itself from the bulb on all the in- 
^■‘“ediato points of tho partial circles. The crown is then terminated by the dc- 
P*^sition of ivory between tho roots and beneath both the crown and the bulb ; further, 
* deposition takes place from different points of the circumference of the tooth, 
22 
it is at the internal surface of the roots that it reunites. The little circles continue 
to diminish ; suinetimes they divide after a certain grow th has taken place in the 
root, causing them to appear more or less forked ; and they end in disappearing gra- 
dually, so as to occasion all the roots to terminate in a point or thin layer. By 
this growth, the bulb, now reduced within very narrow dimensions, remains inclosed 
within the crown, and the roots are found to be pierced through their entire length 
by those vessels and nerves which formed them; thus connecting them with the bulb 
on the one ham), and on the other with tho maxillary vesseds and arteries. 
The first traces of the dentary capsule can bo discovered In the fmtus, it is said, 
during the earliest days of its life. There can be no doubt, however, that the teeth 
are in a great measure formed at the period of birth in a large portion of the Mam- 
malia, and the young animals are even compelled to use them before the perioil of 
their lactation has entirely terminated. Physiologists are not, however, agreed as 
to the nature of the process carried on within the jaws, in those parts which are 
traversed by the teeth, before leaving the gums. Some have supposed, that there 
exists a natural passage, leading from the capsule, out of the gums; and it is imagined, 
that this cavity is enlarged by the expansive force of the tooth, aided by the elasticity 
of the adjacent parts. Others have conceived, that the tooth tears everything 
which opposes its passage; and they have even attributed to this c.ause, some of the 
accidents which occasionally accompany the dentition of young animals. 
The former of these views is opposed by the observed plieiiomuna of the sccono 
dentition, where another set of teeth is developed immediately beneath tho first, in 
such a way, that the second cannot appear before the fir.st have fallen. No such na- 
tural passage has been observed; and it ought not to be presumed before adequate 
proof, that Nature has employed two different methods of evolving these organs. It 
should rather be inferred, that if the second teeth are able to surmount the obstacles 
presented to their growth by the first teeth immediately above them, these will also 
be able to overcome the resistance of the membranes and cjirtilages, when they aro 
required to leave the jaws, to satisfy the new wants of the young animal. It further 
appears, that teeth of tho most complicated form, having their crowns terminated by 
many tubercles, and having bctw'een them many intervals of considerable depth, ob^ 
trude themselves, by the summits of their tubercles, on several points at the same 
time, beyond the gums; yet tho gums still continue to occupy the intervals which se- 
parate their tubercles. 
The liypothesis of a violent tearing is still less admissible than that of a natur.il 
passage. During the time that the teeth are growing, not the slightest trace of such 
a phenomenon can be observed; and analogy does not appear to justify this second 
supposition. ■ Nature appears to have a surer and more effectual means than tiiese 
mechanical hypotlieses would lead us to infer; for the present is, in reality, only a, 
particular case of a very general law, of which it forms one of the most exact appli- 
cations. 
There is no truth in Physiology better established than this, that the nutritive 
power of any organic part is enfeebled, when it receives the continued mechanical 
action of any foreign body whatever, and the nutrition of the part may even be wholly 
mtcrnipted, if this action acquire a certain degree of intensity. It seems, that in the 
perpetual interchange of particles which constitutes life, the new molecules become 
incapable of replacing the former, whenever a foreign body compresses the parts from 
which the others have escaped. It may be said, either that the place of the first bodies 
has ceased to be occupied, or that the assimilating force, which ought to have sup- 
plied new molecules, has ceased to act. The consequence is, that the part becomes 
obliterated; and the molecules, which should have nourished it, are dissipated, or go 
to supply the adjacent parts. 
There can be little doubt that the development of tho teeth is a phenomenon of 
this description. When the crown of a lootli begins to be formed, and still more, 
before this period, all that part of the gum, which is intended to he opened for its pas- 
sage, is tliick and filled with vessels. As the tootii grows, this part becomes smaller, 
and the time at length arrives, when it consists of nothing more than a compact and 
dry skin, which soon disappears in ortler to allow a free passage to tlie tooth. This 
view of the subject is, however, incapable of explaining bow' it happens that the 
pressure of the teeth is exerted contrary to the gums, rather than in the opposite 
direction. Although the tooth begins at first to form only on the side next to the crown, 
this circumstance does not completely account for the fact that the tooth tends exclu- 
sively to emerge on this side. The reaction of a tooth growing in the direction of its 
root, is equal to its action in the direction of the crown ; and if the degree of firm- 
ness possessed by the adjacent parts be regarded in this question, instead of piercing 
the guirts, the teeth ought to descend on the side where the roots arc afterwards 
found ; for the inferior pans of the capsule and Its bulb would offer much less resist- 
ance than the denser structure of the gums. It is therefore probable that wc ought 
to attribute the natural direction of the teeth to some special impulse which the cir- 
culation impresses upon the dentary organ, as well as to the mere growth of tho 
capsule by its interior part. The addition of matter to the inner extremity of the 
crown is far from being sufficient of itself to explain this phenomenon. The pressure 
of the gum upon tho teetli would even bo sufficient wholly to arrest it, and it is, on 
the contr.ary, the life of the gum which would thou have to bo suspended. While 
the teeth are growing, the vital action of their capsules is raised to an intense degree, 
the blood is directed towards them with great force, their irritation becomes extreme, 
and hence probably result the fatal consequences which frequently occur to young 
animals during the period of their dentition. 
Tiie protrusion of the teeth from the sockets, in consequence of their secretion and 
growth, is not the sole movement which these organs present. Other changes suc- 
ceed, the object of which being the mastication of the food, is rather more obvious 
than the causes which produce them. 
Among these may bo ronsidered, in the first place, the secondary movement of the 
crown in teeth with distinct roots, after tliey have emerged from (he sockets, The 
capsules of all these teeth being entirely inclosed within the jaws, have their lower 
parts, which correspond with the neck of the tooth, much below the deniary margin 
of these bones ; but when tho teeth are entirely formed, the neck is on a level witn 
this same border that is to say, the inner part of the crown, which in some manner 
