S6 
FIRST CLASS OF THE VEIITEBRATED ANIMALS. 
has been formed in the bottom of the jaws, ultimately finds itself on a level with their 
exterior margin. This protrusion of the crow'n appears to be owing, at first, to the 
growth of that part of the capsule which is about to give birth to the roots — a growth 
which does not make its appearance until after the formation of the crown. Subse- 
quently to this, it must be attributed to some special impulse of the circulation, 
which maintains itself in a high degree as long as the dentary capsule preserves its 
secreting power. Further, at this particnlar period of its growth, the gum no longer 
opposes any resistance to the growth of the teeth. 
There is seen in the molars of the Horse a second kind of movement, which they 
probably possess in common with all other herbivorous animals as well as the 
Ruminantia. It consists in the continued obtrusion of their teeth, even when 
completely formed, and opposed by others in the opposite jaw, against which 
thev act during mastication. This moveraet.t was fully demonstrated by Tenon 
(Acatlemie des Sciences, an. 6), who, however, did not investigate its cause. It 
may be regarded as a continuous ossification of the jaws, — on operation which only 
erases with the life of the animal. In fact, a third movement of these teeth exhibits 
this ossification tending continually to expel the teeth from their sockets; and this 
occurs when a tooth, not being opposed by others, is pushed out of the jaws. As no 
force then opposes the continued secretion of bone, the sockets become tilled, and 
the teeth are driven from the place which they occupied, as if tliey were foreign 
bodies. This movement, which is prejudicial to most animals, has one advantage 
for those which are obliged to wear out their teeth in grinding their food ; for, al- 
though the wealing out of the teclli in these Mammalia is nften very unequal, the 
duntarv organs do not on that accoant remam uneven at their summits ; and the con- 
sequence is, that the grinding of the food may be continued to the most advanced 
periods of life. 
There is a secondary movement in the incisors or front teeth of the Rodentia 
wholly opposite to the preceding, and still more difiicult to explain. Thai part of 
the tooth which corresponds in sitmition with the root, is placed much less towards 
the front of the bones containing them, among young animals, than among the old. 
^hesc teeth continually fall back at their extremity where the bulb is placed, in pro- 
portion as the animal grows, while they advance forward by the other extremity. 
M. Frederic Cuvier, who observed this singular phenomenon in the Rabbit and 
Guinea-Pig, supposes that the bulb continues to grow by its binder part, being in- 
(luenced by the nerves and vessels which thence derive their life ; and this pbeno- 
nicnon appears to be common to all teeth approaching to tusks in their general cha- 
racter. 
Another problem connected with the incisors of the Rodentia is much less diili- 
eidt to solve, we mean their curvature, and the peculiar curves which they affect. 
To produce an arched tooth, it is sufficient that the capsule he arched; but if the 
curvature of the capsule remain always the same, these teeth, which can grow in- 
definitely when no obstacle arrests their course, would present in this case the form 
of a re'-ular circle, of which frequent examples are found. Instead, however, of this 
curve, the incisors of the Rodentia exhibit one nearly approaching to a spiral, where 
the first portions of the teeth are inclosed in those that follow. It is necessary, 
therefore, that the capsule producing the teeth should change iU curvature, and that 
it should approach continually towards the right lino, as these animals advance in 
a»e, up to a certain point, perhaps, when it ceases to be further modified. We 
may also remark, that these changes of curvature arc tlic same in the incisors of 
both jaws ; for these teeth, at all periods of life, preserve among themselves the 
same relations. 
The appearance of the teeth beyond the gums usually commences, among the Mam- 
malia with tlie period when the milk begins to he insufficient for the nourishment of 
the voung animal ; but it very rarely happens that they are all developed at the same 
time” In this respect, great differences are found among I hem ; and Nature, in most 
cases fails to impart at one time all the teeth necessary for the use of each animal. 
There are very few, we may almost say, no Mammalia, where some of these organs are 
not renewed ; that is to say, that certain kinds of teeth fall, and are reproduced, or 
rather replaced, once or oftener, by the successive growth of other teeth beneath, 
behind, or before the former. 
These first teeth, which give place to new ones, are distinguished by the term 
■milk teeth, or teeth of the first dentition ; those which succeed are termed the second 
teeth. But these terms, founded upon what has been observed in the human species, 
ought not to be taken in the strict sense when applied to other Mammalia ; for among 
these, it will be seen that the milk teeth may fall before birth, or a long time after 
the adult age. To avoid the mistakes to which the ambiguity of these terms might 
give rise, it will he proper to employ the terms first, second, and third teeth, to de- 
note the order of their appearance. 
This department of Natural History, which shows the succession of the teeth, their 
mutual iiifluenec, the coincidence of their appearance with other parts, and with the 
new Wiints of each animal, as well as the relations of form and number between the 
teeth of different dentitions, has, unformnately, been much neglected, and it is only 
now beginning to receive that attention winch it merits. In a Zoological point of 
view, a knowledge of the dentary system in different ages is almost indispensable; hat 
we arc still without a series of drawings, showing the teeth of young Mammalia, cor- 
responding to the valuable lithographic sketches of M. Frederic Cuvier (Sur les DeuU 
des Mamraiferes), made from adult specimens. 
Before explaining the few particulars that have hitherto been nseertuinod on this 
subject, it will be proper to premise a few words respecting the different kinds of teeth, 
as well’as to explain the system of notation which we intend to use in describing 
ihcm. 
The teeth of the IMammalia emerge solely from the inter-maxillary and maxillary 
bones. The incisors, or front teeth, make their appearance first ; .and these may be 
followed either bv the canines or molars. The last are subdivided into false molars, 
camassiermolars.and tuborculons molars: while the tuberculous molars themselves may 
be further distinguished bv their having simple, compound, nr proper tubercles. 
(1.) The Incisor, or Cutting Teeth (Ineisores or Primures), are somewhat broad 
ami long, with their margins often parallel, and cut away obliquely at their free ex- 
tremity. Wc see them so, for example, in the front teeth of Man ; but this form is 
still more strongly marked in the Rodentia or Gnawers. This term Incisor ought 
properly to be applied only to those teeth which have a form especially fitted for 
cutting ; but it has been extended not only to all such as arc found in the incisive 
or inter-maxillary bone, but even to those opposed to them in the lower jaw, although 
the latter often have neithiT the form nor use of true Incisors. 
(2.) The Canines (Laniarii), or Tearing Teeth, have the genersal form of those 
teeth which appear most prominently in the Dogs. They arc longer than all the 
others, and always hare a single root and a single point to the crown. i\s these 
teeth are usually ]>]aced in the upper and lower jaw, immediately behind the Incisors, 
the term Canine has been extended to all teeth which appear to occupy this place. 
They are likewise called Corner teeth ; .ind, from the chief use to which they are 
applied, have obtained the name of Laniariii from laniure, to tear. 
(3.) As the names of most parts of animals are derived from the corre.sponding 
parts in Man and the Ruminantia, which were principally dissected by the ancient 
Anatomists, ll»e term Moltr (^Molares) is correctly applied only to those which act, 
as we may remark in the Ruminantia, almost like a mill-stone — (in Latin wio/«, a 
mill). IJencc, we understand Molar teeth to be compound, semi-compound, or even 
simple teeth, having the crown broad and flat, with broken projections and small 
eminences corresponding to each other in both jaws. Afterwards, however, the 
term has come to be applied indiscriminately to all teeth situate behind the Canines, 
and occupying the entire inner extremities of the dentary lines, although they some- 
times possess no other character of molars than the place they occupy. Thus, in the 
Cats, whetc these teeth are trenchant, and correspond in each jaw, so as, in fact, to 
act in the same manner as true Incisors, they are not on that account deprived of the 
common appellation of Molars. 
Hence it becomes necessary further to distinguish the diiTerent kinds of Molars 
from each otlier. In many IMammalia, the IMolars difier greatly both in size and 
form, and have on that account been divided into False and True Molars. false 
Molars, being most commonly the anterior ones, we understand such as are small and 
pointed. True Molars are considered to be tliicker and larger, with ilicir crowns 
studded with several points, or altogether flat. In the Carnivorous animals, there is 
found a very large Molar, which more especially fulfils the tearing purposes of these 
animals ; and this tootli is further distinguished by the terms carnussier Molar. 
The importance of possessing a good system of Nomenclature for the teeth becomes 
sufiiciently obvious, as soon as the necessity of defining clearly the different kinds and 
combinations of teeth, both in respect to their forms and relative position, has been 
made apparent. 
In stating the dentary systems of animals, two methods of notation have hitherto 
been employed ; for example, the adult teeth of the human species, being eight inci- 
sors, four canines, eight false molars, and twelve tubcrculou-S molars, have long been 
represented thus : 
Incisors^; Canines ; Molars = 32. 
In his work on the Teeth of the Mammalia, M. Frederic Cuvier expresses the 
same thing, under the following form : 
32 Teeth. 
! 
IG upper. 
IG under. 
1 
4 Incisors. 
2 Canines. 
10 Molars. 
4 Incisors. 
2 Canines. 
10 IHolars. 
The former expression, besides its inconvenience from the smallness of the figures, 
does not represent the nature of the Molars. Tl»c latter is not compact. In bolh> 
the teeth on each side are confounded together unnecessarily. 
As a new system of notation, which will combine the advantages of brevity and 
clearness, yet remains to be proposed, wc venture to suggest the following, which 
possesses, in our opinion, some of the most essential requisites. Let M represent 
anv Molar tooth ; C a Canine ; F a false Molar j and C' a carnassicr Molar. Let a 
number annexed to an explanatory letter denote that there arc as many teeth of the kind, 
represented by the letter, as there are units in tlie number. Further, let a number 
without an explanatory letter denote an Incisor, or front tooth, and a Molar, unless 
otherwise expressed, be always understood to be tuberculous. Then, adopting the 
ordinary signification of the Algebraical symbols, the deiitary system of the AduU 
Man will be conveniently icpresented as follows; 
^2 + C + (2 F + 3) M _ _ 
I'i + C + (2 1' -f- 3) M 16 
where the numerator denotes the number, nature, and relative position of the teeth 
on one side of the upper jaw, and the denominator of those on one side of the lower* 
The small figure in the corner indicates that each expression must be doubled 
represent both sides of each jaw, and the vertical line on the left hand shows the 
medial axis of the body, passing in the middle of the front teeth. 
To avoid repetitious, only one side of either jaw is alluded to in referring to p^'*' 
ticular teeth ; and what is said of one side must be understood of the other, which 
precisely resembles the first in all its relations. It is always customary to count 
from the anterior extremity of all the parts which bear these organs. Thus, the 
first Incisor among the Mammalia is tltat tooth found nearest to the suture, by 
the inter-maxillary bones are united. This suture is represented m the formula* 
the vertical line. All extraordinary cases are excepted, such as the appearauc® 
teeth before birth or in extreme old age, while the ordinary and most natural proc^- 
of development is always understood to be meant, unless otherwise expressed. 
In the human species, the first dentition generally takes place from the 
or eighteenth month to the age of two years or two years and a half, and it u^o 
commences with the lower jaw. The first Incisors precede all the others, and tie_^ 
are followed by the second ; so that, towards the end of the first year, all the lU ^ 
sors are developed. The first tooth which pierces the gums after tho incisors ^ 
molar ; and it is subsequent to this that the Canine, though placed before it» 
iiake* 
