SKELETON OF VEUTEBRATA. 357 



them to be likewise developements of several other 

 supposed cranial vertebrae;! but the analogies by 

 which the hypothesis is supported become more 

 feeble and confused as we recede from the middle 

 of the spinal column. 



All the other parts of the skeleton may be re- 

 garded as accessory to the spine ; and they are far 

 from exhibiting the same constancy either in form 

 or number, as the vertebral column. In some in- 

 stances, as in serpents, these accessory parts are 

 altogether wanting ; in others, they exist only in 

 rudimental states ; and it is but in a few that they 

 can be considered as having reached their full 

 developement. In order to obtain a standard of 

 comparison by which to estimate all their grada- 

 tions of evolution, it will be best to consider them 

 first in their more perfectly developed forms, as 

 they are presented in the higher classes of quadru- 

 peds. In the following descriptions, the skeleton 

 of the Hog (Fig. 181) will be taken for the purpose 

 of reference. 



The ribs consist of arches of bone, affixed at their 

 upper ends to the bodies of the vertebrae, and also, 

 by a separate articulation, to their transverse pro- 

 cesses ; where, in general, they are allowed a slight 

 degree of motion. Their primary use is to defend 

 the vital organs situated in the region of the chest, 

 or thorax (namely, the heart and the lungs) ; but 

 they are subservient also to the function of respira- 

 tion, by the alternate movements which are given 

 to them by their muscles. The two parts of which 

 they are composed often form an angle by their 



\ In this theory of St. Hilaire the number of cranial vertebrse is 

 seven, each composed of nine elementary pieces. 



