120 PHILOSOPHY OF ZOOLOGY. 
The bones of the trunk consist of the vertebrae, mbs, and 
sternum. The vertebr@ form what, in common language, 
is called the back-bone. They vary greatly in number, 
and even structure. They serve to protect the spmal 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 
Jrog, shark, and ray. When present, they are either 
united by the one extremity to the vertebra, 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. 'Vhose 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. 
