18 EVERS'S COMPARATIVE ANATOMY. 



AMPHIBIA. 



In the Batrachia the ribs are almost wholly absent ; it is only 

 in the rana pipa and the salamanders that small cartilaginous 

 appendages are found attached to the transverse processes of some 

 of the vertebrae. In these animals the spine consists of dorsal, 

 sacral, and caudal portions — the distinction being marked by the 

 connection of one of the vertebras to the ilium on each side. The 

 salamander has fourteen dorsal, one sacral, and twenty-seven cau- 

 dal vertebrae ; in the common frog there are only nine vertebrae ; 

 and in the rana pipa but eight; whilst the siren has forty-three in 

 the trunk, and about forty-five in the tail ; from the second to the 

 eighth inclusive have rudimentary ribs attached to them. The 

 bodies of the vertebrae are concave on their surfaces; so that, when 

 in apposition, they give rise to the existence of an intervertebral 

 oval cavity filled by a gelatinous mass. 



When we consider the habits, food, and mode of progression, of 

 the frog — that its movements, which are chiefly on land, are both 

 active and extensive, as well for the purpose of seizing its prey as 

 of escaping from danger — it becomes obvious why its spine should 

 be short and firmly anchylosed — how the presence of a tail would 

 be worse than useless, and why a necessity exists for a full deve- 

 lopment of the posterior extremities. The absence of ribs will 

 hereafter be accounted for by an interesting peculiarity in the func- 

 tion of respiration. 



Cranium. — The elements composing this cavity are remarkable 

 for remaining permanently ununited, a state of things which is 

 strikingly imitated during the progress of the development of these 

 parts in the higher orders of animals. The maxillary, intermaxil- 

 lary, tympanic, and jugal bones are greatly expanded, in the 

 transverse direction, giving the face a flattened appearance. The 

 lower jaw consists of three pieces on each side. 



Of all parts of the osseous system of this class, there is none so 

 interesting as the os hyoides, on account of the changes to which 

 it is subjected during the transit of the amphibia from the pisciform 

 to the reptile state. The branchiae are external, supported by car- 

 tilaginous arches connected with os hyoides. As the age of 

 the tadpole increases, the branchiae disappear, the lungs become 

 developed, and the os hyoides grows, from a small rhomboidal 

 point, to the large size and peculiar from which it presents in the 

 full-grown frog. 



Anterior extremity. — The shoulder of a frog consists of scapula, 

 clavicle, and coracoid bone, all of which unite to form the glenoid 

 cavity. The humerus is short and thick, having a round head 

 above, received by the glenoid cavity, and also a spherical extremity 

 below, to articulate with the bones of the fore-arm, which consists 

 of radius and ulna so united that a faint line indicates their former 

 separation. The carpal bones are six ; the metacarpal, four. The 



