AMPHIBIA. 



93 



still further illustration of the proximity of this 

 class to the fishes. The lower jaw consists of 

 three distinct pieces on each side, an anterior, 

 a lateral, and a posterior or articular portion. 

 The anterior bone supports the teeth in those 

 genera which have teeth in the lower jaw, and 

 unites with its fellow at the symphysis. In 

 frogs the lower jaw is devoid of teeth, but they 

 are found in the upper jaw, bordering the in- 

 termaxillary and the maxillary bones ; and the 

 vomers are also furnished, each with a trans- 

 verse row of teeth ; but in the salamander, the 

 menopoma, the proteus, and others, they are 

 found occupying the margin of the lower jaw. 

 In the toads there are no teeth in the lower jaw, 

 but the edge of the jaw-bone is serrated. The 

 second bone of the inferior maxilla occupies 

 the side, and is larger even than the former. It 

 has at the posterior part a coronoid process, 

 behind and within which is placed the third 

 bone, which forms the medium of articulation 

 with the cranium. 



It is to the os hyoides that the principal interest 

 attaches in the present class, as it is that bone 

 which undergoes the most remarkable changes 

 in its form and relations during their transforma- 

 tion, passing from the office of supporting the 

 branchial organs into a true os hyoides, and 

 thus offering, as Cuvier has beautifully shewn, 

 an elucidation of the true nature of this ap- 

 paratus in fishes. As this bone, however, has 

 a direct relation with the respiratory functions, 

 I shall explain these changes while treating on 

 that part of the subject. 



The spinal column varies exceedingly in the 

 different forms of the amphibia. In the highest 

 form the vertebrae are fewer than are found in 

 any other animals. In the common frog 



Fig. 16. 



there are but nine, and in the pipa only eight. 

 Of the nine vertebrae in the frog, the first, the 

 atlas, a, has no transverse processes ; there are 

 two articular surfaces situated anteriorly, by 

 which it is articulated to the two occipital 

 condyles. In the seven following vertebrae the 

 anterior articular surfaces of the bodies are 

 concave, and the posterior convex. This con- 

 vex tubercle, which enters the concavity of the 



next vertebra, consists of the intervertebral car- 

 tilage converted into bone. In the tadpole 

 condition of the animal (and this remains per- 

 manently the case in the perenni-branchial 

 forms, as the menobranchus, the proteus, &c.) 

 this intervertebral substance retains the soft con- 

 sistence which characterises it in fishes ; and, 

 as in that class, it is contained in the circum- 

 scribed cavity formed by the cup-like hollows 

 of the two articular surfaces of contiguous 

 vertebrae. The elongated fish-like form of 

 those amphibia which retain their branchiae 

 throughout life, requires that this structure 

 should also be permanent ; and we have thus 

 another beautiful example of that perfect chain 

 of organisation which is manifested by this 

 class of animals, from the fish upwards to the 

 reptilia. 



The vertebrae of the adult frog have long 

 transverse processes (fig. 16, 6), but are wholly 

 destitute of ribs a class of bones which 

 would be utterly useless in the particular 

 modes of locomotion to which these animals are 

 restricted, and the absence of which implies 

 a peculiarity in the act of respiration, which 

 will be described hereafter. The spinous pro- 

 cesses are very short ; the articular are oblique, 

 the posterior of each being placed above the 

 anterior of the following one. 



The last or sacral vertebra has large transverse 

 processes (fig. 16, c) directed a little back- 

 wards, to which the ilia (fig. 16, d) are at- 

 tached; and to the body of this vertebra is 

 united by two tubercles, a long single bone, 

 extending backwards to above the anus. This 

 bone (fig. 16, e) is considered by Cuvier as a 

 second sacral vertebra ; but by Schultze, 

 Altena, Dr. Grant, and others, it is regarded 

 as the coccyx. The vertebral canal occupies 

 the anterior third of a carina or crest, which 

 runs along the upper surface of this bone, 

 diminishing gradually in its course until it 

 wholly disappears. 



The spinal column in the other orders of 

 the class differs in a remarkable degree from 

 that which has been just described. In the 

 salamander there are thirteen dorsal, two sacral, 

 and about twenty-five caudal vertebrae, which in 

 the genus molge or newt are increased to upwards 

 of thirty. In these the anterior surface of the 

 body is convex, and the posterior concave, a 

 contrary arrangement to that which occurs in 

 frog. The transverse processes are directed 

 a little backwards, each, excepting the atlas, 

 supporting a small rib, which is scarcely curved. 

 The menopoma has a similar arrangement. In 

 the siren are found forty-three vertebrae in the 

 trunk, and forty-four or more in the tail. They 

 all retain in a great measure the form of those 

 of fishes and of the tadpole of the higher orders 

 of this class, particularly in the existence of the 

 intervertebral cavity or double cone, formed 

 by the apposition of two hollowed surfaces of 

 their bodies, and filled by a semi-cartilaginous 

 mass or intervertebral substance. Eight only 

 of the vertebrae, commencing with the second, 

 bear ribs, which are extremely small, and in 

 fact merely rudimentary. In the tail the trans- 



