SKELETON OF VERTEBRATA. 



279 



in process of time, the posterior half of the sphenoid bone, 

 which lies in the middle of the basis of the skull; the tem- 

 poral bones being formed by its leaves, and the parietal bones 

 by the lateral halves of its spinous process. The third cra- 

 nial vertebra is constituted by the anterior half of the sphe- 

 noid bone, which is its body, and the frontal bones, which 

 are its leaves. This theory, which originated with Oken, 

 has been farther extended to the bones of the face, by Geof- 

 froy St. Hilaire, who conceives them to be likewise deve- 

 lopments 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 regarded as ac- 

 cessory 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 development. In or- 

 der to obtain a standard of comparison by which to estimate 

 all their gradations of evolution, it will be best to consider 



* In this theory of G. St. lliluire, the number of cranial vci-tebra; is seven, 

 each composed of nine elementary pieces. 



