COMPARATIVE ANATOMY. 
tlon of its condyle. The articulation of 
these processes varies according to the 
structure of the masticating, organs. They 
are both situated in the same straight hori- 
Eontal line in the ferae ; their form is cylin- 
drical ; and they are completely locked in 
an elongated glenoid cavity, whose margins 
are so extended, before and behind the con- 
dyle, that all rotatory motions are rendered 
impossible, and hinge-like movements only 
allowed. This structure is most strikingly 
exemplified in the badger, where the cylin- 
drical condyles are so closely embraced by 
the margins of the ai’ticular cavity, that the 
lower jaw (at least in the adult animal) is 
still retained in its situation, after the soft 
parts have been entirely removed by mace- 
ration. In many herbivorous animals (in 
the most extensive sense of the term) these 
condyles are really rounded eminences ; uiz. 
in the elephant and beaver. Their surface 
is flattened in the pecora, which have also 
the lower Jaw narrower than the upper, so 
that the two sets of teeth do not meet to- 
gether when the mouth is shut, but are 
brought into contact by the free lateral 
motion which takes place in rumination. 
As the motions of tlie lower jaw must be 
materially influenced by the form of its 
condyles, and by the manner in which those 
processes are connected to the articular ca- 
vity of the temporal bone ; we shall find, as 
might have been expected, a close relation 
between these circumstances, and the kind 
of food by which an animal is nourished. 
Thus, the lower jaw of the carnivora can 
only move upwards and downwards, and is 
completely incapable of that horizontal 
motion which constitutes genuine mastica- 
tion. Hence these animals cut and tear 
their food in a rude and coarse manner, and 
swallow it in large portions, which are after- 
wards reduced by the solvent properties of 
the gastric juice. Such mammalia, on the 
contrary, as live on vegetables, have, in 
addition to this motion, a power of moving 
the lower jaw backwards and forwards, and 
to either side, so as to produce a grinding 
effect, which is necessary for bruising and 
triturating grass, and for pulverising and 
comminuting grains. In all these, there- 
fore, the form of the condyle, and of its 
articular cavity, allows of free motion in 
almost every direction. The teeth may be 
compared, in the former case, to scissars ; 
in the latter, to the stones of a mill. 
THE teeth. 
The jaws of the mammalia, with a very 
few exceptions, contain teeth. The proper 
whales (balmna), the pangolin (manis), and 
the American ant-eaters are the only genera 
entirely destitute of these organs. 
• Animals of the genus balsena (the proper 
whales) have, instead of teeth, the peculiar 
substance called whalebone, covering the 
palatine surface of the upper jaw: this re- 
sembles jn its composition hair, horn, and 
such matters. 
The lower surface of the upper jaw forms 
two inclined planes, which may be compared 
to the roof of a house reversed ; but the 
two surfaces are concave. Both tliese are 
covered with plates of the whalebone, plac- 
ed across the jaws, and descending vertical- 
ly into the mouth. They are parallel to 
each other, and exist to the number of two 
or three hundred on each of the surfaces. 
They are connected to the bone by the in- 
tervention of a white ligamentous substance, 
from which they grow ; but their opposite 
edge, which is turned towards the cavity of 
the mouth, has its texture loosened into a 
kind of fringe, composed of long and slender 
fibres of the horny substance, which there- 
fore covers the whole surface of the jaw. 
This structure probably serves the animal 
in retaining and confining the mollusca, 
which constitute its food. 
The teeth of the oraithorhyncims para- 
doxus and hystrix deviate very considerably 
from those of other mammalia. In the for- 
mer animal there is one on each side of the 
two jaws : it is oblong, flattened on its sur- 
face, and consists of a horny substance ad- 
hering to the gum. There are likewise two 
horny processes on the back of the tongue : 
these point forwards, and are supposed by 
Mr. Home to prevent the food from passing 
into the fauces before it has been sufficiently 
masticated. In the ornithorhynchus hys- 
trix tliere are six transverse rows of pointed 
horny processes at the back of the palate, 
and about twenty similar horny teetli on 
the corresponding part of the tongue. 
The teetli of the human subject seem to 
be designed for the single purpose of masti- 
cation, and hence an erroneous conclusion 
might be drawn, that they serve the same 
office in other animals. Many exceptions 
must, however, be made to this general 
rule. Some mammalia, which have teeth 
for the office of mastication, have others, 
which can only be considered as weapons 
of offence and defence; viz, the tusks of 
the elephant, hippopotamus, walrus, and 
manati. The large and long canine teeth 
of the carnivora, as the lion, tiger, dog, cat, 
y 2 
