SKELETON OF VERTEBRATA. 403 



one only being central and surrounded by the 

 rest. Few subjects in comparative osteology are 

 more curious and instructive than to trace the 

 developement of these several elementary parts 

 in the different classes of animals, from the ru- 

 dimental states of this bone as it occurs in 

 fishes, to its greatly expanded conditions in the 

 tortoise and the bird, which severally exhibit 

 the most opposite proportions of these elements. 

 Last in the order of constancy come the bones 

 of the extremities. As we ascend in the scale of 

 animals we may observe the prevalence of a ten- 

 dency to the concentration of organs, and conse- 

 quently to the diminution of their number. 

 While in animals of the inferior orders, which 

 are possessed of extremities, we find a con- 

 siderable number of legs; in all the animals 

 comprised in the class of true insects nature 

 has limited the number to six ; and in the 

 vertebrata it never exceeds four. As in insects 

 we observed that all the legs are divided into the 

 same number of parts ; so we find among qua- 

 drupeds a striking correspondence in the bones 

 of the fore and the hind extremities. Both the 

 one and the other are connected with the spine 

 by the intermedium of large and broad bones, 

 which are intended to serve as a basis for their 

 more secure attachment, and for giving, at the 

 same time, extensive and advantageous purchase 

 to the muscles, which are to move the limbs. 



