374 THE MECHANICAL FUNCTIONS. 



these fishes remains in the incipient stage of ossifi- 

 cation, being more or less cartilaginous ; and where 

 the ossific process has begun, it has not advanced 

 the length of producing union between the pieces 

 formed from the separate centres of ossification. 

 Where they meet without uniting, they form no 

 sutures, but overlap one another. Thus the bony 

 structures are detached, and often completely iso- 

 lated ; affording to the speculative physiologist an 

 opportunity of studying the earlier stages of this 

 interesting process, and marking with distinctness 

 the number of the elements of each bone, and the 

 relative situations of their centres. 



It is here, more especially, that we obtain evi- 

 dence in favour of the theory that the cranial bones 

 are derived from vertebrae analogous to those of the 

 spine. The occipital bone, in particular, corre- 

 sponds to a spinal vertebra in all its essential ele- 

 ments. In many fishes, the body of this bone, 

 being lengthened out to form the posterior part of 

 the basis of the skull, becomes the basilar portion. 

 We find, on its posterior surface, the same cup-like 

 cavity as in the true vertebrae ; and it is joined to 

 the next vertebra in the same manner as the spinal 

 vertebrae are joined to each other. Its crest has 

 the exact shape of a spinous process. In front, the 

 basilar bone is united to the sphenoid bone, which, 

 with the vaulted roof that springs from the sides of 

 both these bones, like the leaves and spinous pro- 

 cesses of the vertebrae, form together a long cranial 

 cavity. This cavity is placed in a direct line with 

 the spinal canal, and contains the nervous tubercles 

 which constitute the brain. Yet the brain does 

 not completely fill this cavity ; for a space is still 



