AMPHIBIA. 



95 



is admirably adapted to the purpose of swim- 

 ming, which is performed either by a succes- 

 sion of curves, as in the amphiuma and the 

 siren, or by the alternate flexure of the tail, as 

 in the tritons and the menobranchus. 



Having given this general sketch of the os- 

 teology of the amphibia in the adult state, it 

 will be interesting to examine the structure of 

 the skeleton in the tadpole. It has already 

 been observed that in this early condition of its 

 existence the animal resembles fishes in all 

 the most remarkable characters of its organi- 

 zation. We find accordingly that the limbs, 

 which are at first scarcely perceptible by the 

 most minute examination, become gradually 

 developed, passing through a rudimentary 

 form beneath the integuments, from which 

 they do not emerge until they have acquired 

 considerable size and a very defined figure. 

 The hinder legs are first seen, and are early 

 employed as a feeble assistance to the more 

 effective tail, as instruments of progression. 

 The tail is developed, however, to a great 

 degree, occupying the same relative size and 

 situation as it is found to do in fishes. The 

 coccygeal vertebra? are numerous, forming a 

 long column, not ossified, but retaining its 

 cartilaginous structure, at least in those forms 

 in which it is deciduous; but in the salamanders, 

 the tritons, the proteus, and all others of the 

 urodela, it becomes ossified instead of being 

 absorbed. In the frog and other anoura, as 

 the permanent organs of progression acquire 

 their full development, the tail is slowly re- 

 moved by interstitial absorption, not suddenly 

 falling off as some have supposed, but be- 

 coming gradually smaller and smaller until 

 it wholly disappears. The cranium under- 

 goes no other important change than that of 

 the gradual ossification and expansion of its 

 different elements, the centres of ossification 

 being at first wholly disunited as in fishes, 

 and afterwards assuming the more consolidated 

 structure and closer approximation to each 

 other, by which they approach the reptilia. 



II. Muscular system. The similarity which 

 has been already shewn to exist in the osseous 

 system of fishes and of the tadpole and peren- 

 nibranchiate amphibia, would naturally lead 

 to the conclusion that a corresponding affinity 

 would be found in the muscular apparatus. 

 The muscles which are employed for progres- 

 sion in those early forms of vertebrated beings, 

 are found to consist of oblique layers, abutting 

 upon a median line, and extending along the 

 whole length of the tail on each side. A similar 

 general direction obtains in the muscles both of 

 the trunk and tail in the long-bodied forms of 

 the permanently tailed amphibia. The direction 

 of their action therefore is horizontal, and their 

 progression is effected by the alternate action 

 of the muscles on each side. These oblique 

 caudal muscles in the tadpole of the tailless 

 tribe, become absorbed with the vertebrae to 

 which they are attached, as the animal gradually 

 assumes its permanent form ; but its aquatic 

 habits are still provided for by the extraordinary 

 magnitude of the flexors and extensors of the 

 thigh, leg, and foot, which are in perfect ac- 



cordance with the great length of the bones of 

 this extremity, which has been described. The 

 muscles which form this important apparatus 

 of motion are exactly analogous to those which 

 are so peculiarly developed in the human leg. 

 Thus the large glutei extend the femur, the 

 rectus and triceps extend the leg, and by their 

 united and suclden action forcibly throw the 

 whole limb into a straight position, whilst the 

 gastrocnemii, which are here as in the human 

 subject of sufficient size to form a considerable 

 calf of the leg, enable the foot with the wide 

 expanse of its toes, connected as they are by a 

 tense web, to strike with great force and effect 

 the resisting medium in which they live, assisted 

 by the flexors of the toes, which are called into 

 action at the same instant. The same beau- 

 tiful mechanism is no less adapted for the pe- 

 culiar nature of their progression on land ; by 

 it they are enabled to take those long and vigo- 

 rous leaps which particularly characterize some 

 of the genera of the acaudate family of this 

 class. It is obvious that the same sets of 

 muscles must be developed for the performance 

 of the energetic and sudden movements above- 

 mentioned as are required to sustain the upright 

 form of the human subject in its erect position, 

 those, namely, which extend at once the thigh 

 upon the pelvis, the leg upon the thigh, and 

 the heel upon the leg; and hence arises the 

 remarkable similarity in the conformation of 

 the leg in these otherwise remote forms, and 

 hence too the act of swimming in man must 

 be a tolerably accurate imitation of the same 

 effort as exhibited by the frog. 



III. Organs of digestion. The foregoing 

 consideration of the various structures of the 

 organs appertaining to locomotion would pre- 

 pare us for corresponding differences in those 

 belonging to this important office. These 

 variations, however, are not found exactly to 

 follow those which have been described in the 

 former class of organs. The tadpole condition 

 of the higher amphibia does not correspond 

 in the nature of its food, nor consequently in 

 the structure of the alimentary canal, with 

 the class of fishes, nor indeed with that per- 

 manent tadpole, as it may be called, the 

 larviform axoloth. 



The teeth, as has been already stated, vary in 

 the different genera not so much in their size and 

 form as in their situation. Thus the whole of the 

 amphibia have teeth in the palate; the sala- 

 manders have them also in both the upper 

 and lower jaws, the frogs in the upper only, 

 and the toads in neither. In the two latter 

 genera the palatine teeth are placed in a trans- 

 verse line, interrupted in the middle. In the 

 salamanders they form two parallel lines, con- 

 taining not less than thirty on each. In the 

 menopoma they occupy the anterior palatine 

 margin of the vomer, forming a line on each 

 side parallel with the maxillary and inter- 

 maxillary teeth. In the axoloth they are 

 arranged in the quincuncial order, and are nu- 

 merous. But the most remarkable form and 

 arrangement of the palatine teeth is found in 

 the siren, in which they have the quincuncial 

 arrangement ; they are placed on two small 



