COMPARATIVE ANATOMY. 
maxillary bones, they can hardly be said to 
possess a fang or root ; for tlie form of the 
partis the same throughout; the covering 
of enamel is likewise continued ; and that 
part, which at one period is contained in 
the jaw, and would form the fang, is after* 
wards protruded to constitute the body of 
the tooth. 
The constant growtli of these teeth there- 
fore proceeds in the same manner, and is 
effected on the same principles as the origi- 
nal formation of any tooth ; and can by no 
means furnish an argument for the existence 
of vessels in the substance of the part. 
We cannot help being struck with the 
great size of these teeth, compared with the 
others of the same animal, or even with the 
bulk of the animal. Their length in the 
lower jaw nearly equals that of the jaw it- 
self, although a small proportion only of this 
length appears through the gum. They 
represent the segment of a circle ; and are 
contained in a canal of the bone, which de- 
scends under the sockets of the grinders, 
and then mounts up, in some instances, to 
the root of the coronoid process ; hence, al- 
though their anterior cutting edge is in the 
front of the mouth, the posterior extremity 
is behind all the grinding teeth. No animal 
exhibits this structure better than the rat. 
The beaver also affords a good specimen of 
it on a larger scale. It has been drawn in 
this animal by Blake, (“Essay on the 
structure, &c. of the teeth.”) The tooth 
does not extend so far in the upper jaw ; 
it is there implanted in the intermaxil- 
lary bone, and terminates over the first 
grinder. 
The observations which have been made 
respecting the constant growth of the inci- 
sor teeth of the glires, will apply also to the 
tusks of the elephant. These are hollow in- 
ternally, tlirough the greater part of their 
length, and the cavity contains a vascular 
pulp, which makes constant additions of 
successive layers, as the tusk is worn down. 
One of the elephants at Exeter Change is 
said to have nearly bled to death from a 
fracture of the tusk, and consequent lace- 
ration of the vessels of the pulp. The tusks 
of the hippopotamus, and probably all other 
teeth of this description grow in the same 
manner. Farther and more accurate obser- 
vation may hereafter shew, that the same 
mode of growth obtains also in other classes 
of teeth, when tliey are exposed to great 
friction. Something similar may certainly 
be observed in the grinders of the horse. 
The tooth is not finished when it cuts the 
gum : the lower part of its body is com- 
pleted, while the upper part is worn away 
in mastication; and the proper fang is 
not added till long after. Hence we can 
never get one of these teeth in a perfect 
state, for if the part out of the gum is com- 
plete, the rest of the body is imperfect; 
and there are no fangs : on the contrary, 
when the fangs are formed, much of the 
body has been worn away in mastica- 
tion. Blake also asserts, that this struc- 
ture is found in the grinders of the beaver, 
(p. 99.) 
The narwhal is particularly distinguished 
by its long and spiral tusk. This animal is 
found so constantly with only one tusk, that 
it has been called in common language, the 
sea-unicorn ; and Linnaeus has even given it 
a similar appellation, that of monodon. Yet 
there can be no doubt that it possesses ori- 
ginally two of these; one in either jaw 
bone : and that which is wanting must have 
been lost by some accidental circumstance, 
as we can easily suppose, (“ Shaw’s Zoolo- 
gy”, vol. ii. p. 473.) These tusks often 
equal in length that of the animal’s body; 
which may be 18 feet or more : yet they are 
always slender; 
In many baboons, and most particularly 
in the larger predacious mammalia, the 
canine teeth are of a terrific size; in the 
latter animals, the whole profile of the ante- 
rior part of the cranium, forms a continuous 
line with these teeth ; which is very visible 
in the tiger. Tlie canine tusks of the babi- 
roussa, which are very long, and recurved 
so as nearly to describe a complete circle, 
present the most cm-ious structure. Their 
utility to the animal appears quite obscure, 
when their length, direction, and smallness 
are considered. 
The distribution of the enamel and bony 
substance varies in the teeth of different 
animals, and even in the different orders of 
teeth in the same animal. 
All the teeth of the carnivora, and the in- 
cisors of the ruminating animals, have the 
crown only covered with enamel, as in the 
human subject. The immense fossil grind- 
ers of the animal incognitum, or mammoth, 
have a similar distribution of this substance. 
The grinders of graminivorous quadru- 
peds, and the incisors also of the horse have 
processes of enamel, descending into the 
substance of the tooth. These organs have 
also in the last-mentioned animals a third 
component part, differing in appearance 
from both the others, but resembling the 
bone more than the enamel. Blake has 
