400 THE MECHANICAL FUNCTIONS. 



The extensive researches that have been re- 

 cently made in this branch of comparative ana- 

 tomy, have supplied many facts which tend to 

 support the hypothesis that the bony coverings 

 of the brain are the result of the developement 

 of three vertebrae. According to this theory, the 

 first of these supposed cranial vertebrce, beginning 

 our enumeration from the neck, is the origin of 

 the occipital bone, of which the lower part, or 

 that which immediately supports the cerebellum, 

 corresponds to the body of the vertebra ; the two 

 lateral portions, to the leaves; and the upper 

 flat plate to the spinous process. The body of 

 the second cranial vertebra becomes, in process 

 of time, the posterior half of the sphenoid bone, 

 which lies in the middle of the basis of the 

 skull; the temporal 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 sphenoid bone, which is its body, and 

 the frontal bones, which are its leaves. This 

 theory, which originated with Oken, has been 

 further extended to the bones of the face, by 

 Geoffroy St. Hilaire, who conceives them to be 

 likewise dev elopements of several other supposed 

 cranial vertebra ; * but the analogies by which 

 the hypothesis is supported become more feeble 



* In this theory of G. St. Hilaire the number of cranial ver- 

 tebrae is seven, each composed of nine elementary pieces, 



