120 PHILOSOPHY OF ZOOLOGY. 



The bones of the trunk consist of the vertebra?, ribs, and 

 sternum. The vertebra; form what, in common language, 

 is called the back-bone. They vary greatly in number, 

 and even structure. They serve to protect the spinal mar- 

 row, and to give stability to the ribs and the extremities. 

 They obtain different names, according to the situation in 

 which they occur, as cervical, dorsal, lumbar, sacral, and 

 caudal. But, in many animals, these distinctions can scarce- 

 ly be retained with propriety, in consequence of the absence 

 of the parts from which they have derived their names. 



The ribs are wanting in several vertebral animals, as the 

 frog, shark, and ray. When present, they are either 

 united by the one extremity to the vertebrae, and by the 

 other to the sternum, forming true ribs ; or they are only 

 fixed by a joint at one of their extremities, either to the 

 vertebrae or sternum, and are then called false ribs. 



The sternum, or breast-bone, is wanting in many animals, 

 as serpents ; and, where it does exist, it varies greatly in its 

 form and dimensions. 



The extremities, where perfect, as in man, are four in 

 number, two arms and two feet ; termed by anatomists the 

 anterior and posterior extremities. In quadrupeds they are 

 termed fore and hind legs ; in birds, wings and legs ; and 

 in fishes, pectoral and ventral fins. The bones of the an- 

 terior extremity consist of those of the shoulder, the arm, 

 the fore-arm, and the hand. Those of the posterior extre- 

 mities, are the hip, the thigh, the leg, and the foot. In 

 some animals, as serpents, the extremities are wanting ; in 

 whales, the posterior extremities cannot be distinguished 

 from the tail ; and in many fishes they are wanting. 



Besides these bones, which are considered as more imme- 

 diately connected with the skeleton, there are many others, 

 such as those of the tongue in quadrupeds, and of the fins 

 in fishes, which are not intimately united with the skele- 

 ton, but which serve to give strength and support to the 

 parts where they occur. 



