92 



AMPHIBIA. 



less, the body contracted longitudinally into 

 as short a space as possible, the vertebrae very 

 few, and anchylosed or soldered together into 

 a single immoveable piece, and wholly devoid 

 of ribs. 



On comparing with this formation, on the 

 other hand, the extensive developement of 

 the tail, the long flexible body, and gracile 

 form of the newts or aquatic salamanders, 

 and reflecting upon the obvious object of this 

 structure in facilitating their motions in the 

 water, we should hardly be prepared to find 

 that the extraordinary extension of the hinder 

 extremities in the frogs, the primary object of 

 which is to afford the great powers of leaping 

 just alluded to, is made subservient also to 

 their aquatic life, by enabling them to swim 

 with great facility, aided by a web of skin 

 extending between the toes of the hinder 

 feet. 



Now as we shall hereafter see, when treating 

 on the respiration of these animals, that the 

 occasional presence of water, and its applica- 

 tion to the surface of the skin, is equally 

 essential to the well-being of both these 

 forms, it is very interesting to observe how 

 admirably this peculiarity in the general re- 

 quirements is provided for by the very different, 

 and even opposite, construction of their form 

 and limbs, which their individual habits of life 

 demand. 



But to return to the detailed anatomy of the 

 skeleton. On examining the general texture 

 of the bones in this class, there is an obvious 

 advance towards the firm calcareous substance 

 of those of the higher forms of vertebrated 

 animals when compared with the bones of 

 fishes, they being more compact, and less tran- 

 sparent and flexible than in the latter animals. 

 The cranial bones, though they retain to a 

 certain extent the character of those of fishes, 

 in the permanent disunion of the different 

 centres of ossification, at least in many in- 

 stances, yet they do not overlap each other, 

 as in that class, but, on the contrary, remain 

 with their margins at least in contact, and in 

 many cases actually united, though not by 

 true sutures. The elements of which the 

 cranium is essentially composed, and which 

 in a higher grade of organization are found 

 consolidated, are here still exhibited as distinct 

 pieces ; a state of things which is strikingly 

 imitated in the progress of the development 

 of these parts in the highest classes during 

 their growth. It is also to be observed that the 

 bones of the face are more closely united to 

 those of the cranium and to each other in 

 the higher than in the lower forms of the class, 

 exhibiting distinct and obvious links in the 

 development of these parts, which we see so 

 beautifully and gradually perfected in the as- 

 cending series from fishes up to man. 



The following enumeration of the separate 

 cranial bones of the amphibia, as existing in 

 the menopoma alleganiensis, the gigantic sala- 

 mander of America, will illustrate the general 

 relations of the distinct centres of ossification, 

 here remaining permanently detached. 



Fig. 14. 



In figs. 14 and 15 the different elements 

 are thus designated: a. the frontal; b. the 

 exterior frontal ; c. the parietal ; d. the nasal ; 



e. the occipital ; f. the pterygoid ; g. the 

 tympanic ; h. the jugal ; i. the superior 

 maxillary; k. the intermaxillary; /. the vomer; 

 m. the sphenoid ; n. corresponding to the or- 

 bitary processes or alae of the sphenoid. In 

 the frog and most others the palatine bones 

 are distinct. We here find that the separate 

 portions or elements which in this class are 

 permanently detached, correspond almost ex- 

 actly with the number found in the cranium 

 of fishes. 



It will be observed by a reference to the 

 figures, that the intermaxillary bones and this 

 is more or less the case in all the amphibia 

 are much developed transversely, as in the 

 fishes, an affinity which has been already so 

 much insisted on, and which is borne out by 

 the condition of all the other parts of the cra- 

 nium. The lateral extension of the upper and 

 lower maxillary bones, for instance, as well as 

 of the tympanic and jugal, expands the general 

 form of the skull, without involving any ex- 

 pansion of the cavity of the cranium, which is 

 restricted to a small, central, elongated space. 

 The latter bones also terminate in a condyle, 

 which is received into a shallow glenoid cavity 

 of the lower jaw, a peculiarity which offers a 



