SKELETON OF THE AMPHIBIA.
Turtles and tortoises have no teeth : but their jaws are covered with a horny substance, 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 circum stance is still more remarkable in the cro codile, where, in an individual measuring 13 or 14 feet, this cavity v. ill 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 trunk 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 intermaxillary bone; and the lateral portions of the lower maxilla are formed of several pieces join ed together. The lower jaw is articulated in a peculiar manner in these animals : it has an articular cavity, in which a condyle of the 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 Albinus:) this, at least, is the case in the 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 motion, which has been adopted even by such anatomists as Vesalius and Co lumbus, has perhaps arisen from this pe culiar mode of articulation. An examina tion of the cranium 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 proportion ally easier in the present instance than in any other animal, both on account of the very great relative size of the lower jaw, as well as from its anomalous mode of ar ticulation. There is, however, no motion of the upper jaw bone, only upon the bones of the cranium, similar to that which occurs in most birds, serpents, and fishes.
The most surprising singularity in the skeleton of the crocodile consists in an abdominal sternum, which is quite differ ent from the thoracic sternum, and ex tends from the ensiform cartilage to the pubis, apparently for the purpose of sup porting 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 import ant and clearly defined difference, which distinguishes.the poisonous species ofser pents from the much more numerous in noxious tribes.
The latter have, in the upper jaw, four maxilliary bones, oeset with small teeth, which form two rows, separated by a considerable interval from each other. One of these is placid 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 wanting to the poi sonous species ; which have in their stead, much larger tubular fangs, connected with the poison bladder, and consti tuting, in reality, bony, excretory ducts, which convey the venom into the wound inflicted by the bite of the animal It appears, in general, that the number of vertebra, in red-blooded animals, is in an inverse proportion with the size and strength of their external organs of mo tion. Serpents, therefore, which entire ly want these organs, have the most nu merous vertebra; sometimes more than 3UP.
It may be observed, in confirmation of this remark, that the number of vertebra is very great in fishes of an elongated form, viz. in the eel, which has above one hundred. The porpoise, which has no organs of motion which deserve mention ing, has between sixty and seventy.
Birds, which have such vast power of locomotion by means of their wings, have very few vertebra, if' we consider the anchylosed ones as forming a single piece. And the frog, with its immense hind extremities, has a very short spine, consisting of still fewer pieces.