SKELETON OF VERTEBRATA. 401 



and confused as we recede from the middle of 

 the spinal column. 



All the other parts of the skeleton may be 

 regarded 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 instances, 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 gradations of evolution, it will 

 be best to consider them first in their more 

 perfectly developed forms, as they are pre- 

 sented in the higher classes of quadrupeds. 

 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 processes ; where, in general, they 

 are allowed a slight degree of motion. Their 

 primary use is to defend the vital organs si- 

 tuated 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 that are given 

 to them by their muscles. The two parts, of 



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