COMPARATIVE ANATOMY. 



it slips in with S^TIC force, like the end 

 of a dislocated bone. 



SKELETON OP THE AMPHIBIA. 



Turtles and tortoises iiave no teeth : 

 but th-jir jaws art; covered with a horny 

 substance, somewhat resembling the 

 horse's noof in the mode of its connection 

 iviih liit- bone. The cavity containing 1 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 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 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- 

 cd 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 



VOL Tir. 



abdominal srenitim, which is quite differ- 

 ent from tiie thoracic sternum, and ex- 

 tends from the e.'isn'or.n cartilage to the 

 pubis, apparently ior the purpose of sup- 

 porting the abdomi'iul viscera. 



Tlie serpents have an upper jaw, un- 

 connected with the rest of the skuii, and 

 more or less moveable of itself. 



We find in iheir teeth th*e import- 

 ant and clearly defined difference, which 

 distinguishes.the poisonous species of ser- 

 pents from the much more numerous in- 

 noxious tribes. 



The latter have, in the upper jaw, four 

 maxiiliary bones, oeset with small teeth, 

 which form two rows, separated by a 

 considerable 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 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 vertebrae, 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 vertebrae ; sometimes more than 

 300. 



it may be observed, in confirmation of 

 this remark, thai the number of vertebrae 

 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 vertebrae, 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. 



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. Tiiey 

 agree together, however, on the whole, in 

 having a spine, which extends from the 

 cranium to the taii-fin ; and in having the 

 other fins, particularly those of the thorax 

 and abdomen, Articulated with peculiar 

 bones, destined to that purpose. They 

 have in general many more bones uncon- 

 Q q 



