ON THE TEETH OF MAMMALIA. 
133 
in the lower jaw. In form and number they are subject to considerable variety. 
I shall say nothing with respect to them in Man, as I presume every one to be 
sufficiently acquainted with the form and uses of his own incisors. In the Roden- 
tia (as the Mouse, Squirrel, Dormouse, Lemming, Guinea-pig, &c.) these teeth 
have a sharp, chisel-like cutting edge, and penetrate the jaw-bone to a great 
depth. In the-Rat, Beaver, &c., the lower ones will be found to possess very 
long roots. In the Marmot the upper ones are the longest. In this class there 
is a very remarkable provision for keeping a cutting edge constantly on the 
incisors; the enamel is found to exist, to any thickness at least, only on the 
anterior surface, and as it is very much harder than the bone, it does not wear 
out so rapidly, and consequently the tooth is always kept sharp; indeed the 
more the animal uses them the sharper they will become. The tusks of the 
Hippopotamus are similarly formed. There is also in this class a wonderful 
provision made for the very great wear and tear to which these teeth are subject 
in gnawing nuts, trees, &c. They are, as I before stated, very deeply embedded 
in the jaw; the posterior part is hollow, and is filled with a vascular pulp, con¬ 
stantly secreting new bone and enamel, which pushes the tooth forwards, and is 
thus always providing a new surface instead of that which has been worn away 
by the act of gnawing. This process is continued during the life of the animal, 
and if one of these teeth be removed, the corresponding one in the other jaw will 
continue growing, and will in time form more or less of a circle. This occasionally 
happens to animals in the wild state, as in Rabbits, Hares, &c., so as sometimes 
even to destroy the animal in consequence of its being unable to use its other 
teeth.* 
These teeth have no true root, but will be found to be of the same form 
throughout. These teeth in the lower jaw extend into the bone as far as the 
last, but in the upper jaw only as far as the first molar tooth. In some of the 
Rodentia (as the Beaver, Marmot, Squirrel, Dormouse) the incisors are of a 
brownish colour on their anterior surface. 
The Ruminantia (as the Cow, Sheep, &c.) have no incisors in the upper jaw, 
but the gum is formed into a hard and very insensible pad, which answers the 
* The teeth of the Babbit mentioned by Mr. Alington, Yol. II., p. 325, seem to me to have 
grown in the manner described, in consequence of an original malformation in the lower jaw, it 
appearing from the plate to be bent downwards towards its anterior extremity, which would give 
the teeth that direction also, and therefore take them out of the reach of the upper incisors, which 
must still continue growing, and would eventually assume the form there shewn. The most 
curious circumstance about it is, that the other upper incisor did not grow in the same way: 
possibly it may have either been broken off, or else the corresponding tooth in the lower jaw may 
have been so situated as to allow of some friction taking place between them. We might, I think, 
infer, from the lower teeth not being of so great a length as the upper, that their growth is slower > 
and therefore that the chief part of the labour is performed by the upper incisors. 
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