COMPARATIVE ANATOMY, 
bird to overcome, which being accom- 
plished, it slips in with some force, like the 
end of a dislocated bone, 
SKELETON OF THE AMPHIBIA. 
Turtles and tortoises have no teeth : but 
their jaws are covered with a horny sub- 
stance somewhat resembling the horse’s 
hoof in the mode of its connection with the 
bone. The cavity containing the brain is 
extremely small in comparison with the 
size of the skull. This circumstance is still 
more remarkable in the crocodile, where, in 
an individual measuring 13 or 14 feet, this 
cavity will hardly admit the thumb. The 
vast muscles of the jaw fill up the sides of 
the cranium. 
The body of the turtle and tortoise is 
provided with two broad and flattened 
bony shells, to which the ti'unk of their 
skeleton is consolidated. 
Frogs and toads have no teeth. In no 
other animal are the jaws of such immense 
size, in comparison with the extremely 
small cavity of the cranium, as in the cro- 
codile. The anterior part of the upper 
jaw, consists of a large intermaxillaiy bone ; 
and the lateral portions of the lower 
maxilla are formed of several pieces joined 
together. The lower jaw is articulated in a 
peculiar manner in these animals : it has an 
articular cavity, in which a condyle of tlie 
upper jaw is received. 
The condyle resembles, in some mea- 
sure, the pulley at the inferior extremity of 
the humerus (the trochlea, or rotula of Al- 
binos) ; this, at least, is the case in tlie 
skull of the alligator. 
The old error, of supposing that the upper 
jaw of the crocodile is moveable, and the 
lower, on the contrary, incapable of mo- 
tion, which has been adopted even by such 
anatomists as Vesalius and Columbus, has 
perhaps arisen from this pecuhar mode of 
articulation. An examination of the cra- 
nium shows, that if the lower jaw remains 
unmoved, the whole remainder of the skull 
may be carried backwards and forwards 
by means of this joint. And such a motion 
is proportionally easier in tlie present in- 
stance, than in any other animal, both on 
account of the very great relative size of 
the lower jaw, as well as from its anoma- 
lous mode of articulation. There is, how- 
evei', no motion of the upper jaw-bone 
only, upon the bones of the cranium, si- 
milar to that which occurs in most birds, 
serpents, and fishes. 
The most surprising singidarity in the 
skeleton of the crocodile consists in an ab- 
dominal sternum, which is quite different 
from the thoracic sternum, and extends 
from the ensiform cartilage to the pubis, ap- 
parently for the purpose of supporting the 
abdominal viscera. 
The serpents have an upper jaw, un- 
connected with the rest of the • skull, and 
more or less moveable of itself. 
We find in their teeth the important and 
clearly defined dilference, which distin- 
guishes the poisonous species of serpents 
from the much more numerous innoxious 
tribes. 
The latter have, in the upper jaw, four 
maxillary bones, beset with small teeth, 
which form two rows, separated by a con- 
siderable interval from each other. One of 
these is placed along the front edge of the 
jaw ; the other is found more internally, 
and is situated longitudinally on either side 
of the palate. 
The external row is w^anting in the poi- 
sonous species ; which have, in their stead, 
much larger tubular fangs, connected with 
the poison bladder, and constituting, in 
reality, bony excretory ducts, which con- 
vey the venom into the wound inflicted by 
the bite of the animal. 
It appears, in general, that the number of 
vertebrae in red-blooded animals, is in an 
inverse proportion with the size and 
strength of tlieir external organs of motion. 
Serpents, therefore, which entirely want 
these organs, have the most numerous ver- 
tebrae ; sojnetimes more than 300. 
It may be observed in confirmation of 
this remark, that the number of vertebr® is 
very great in fishes of an elongated form j 
viz. in the eel, which has above one hun- 
dred. The porpoise, which has no organs 
of motion which deserve mentioning, has 
between sixty and seventy. 
Birds which have such vast power of lo- 
comotion by means of their wings, have 
very few vertebr®, if we consider the 
anchylosed ones as forming a single piece. 
And tlie frog, with its immense hind extre- 
mities, has a very short spine, consisting of 
still fewer pieces. 
We should naturally conclude, from ob- 
serving the great diversity in the general 
form of fishes, that the structure of their 
skeleton must be equally various. They 
agree together, however, on the whole, in 
having a spine, which extends from the cra- 
nium to the tail-fin ; and in having the other 
fins, particularly those of the thorax and ab- 
domen, articulated with peculiar bones. 
