420 THE MECHANICAL FUNCTIONS. 



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 spi- 

 nous process. In front the basilar bone is united 

 to the spenoid bone, which, with the vaulted roof 

 that springs from the sides of both these bones, 

 like the leaves and spinous processes of the ver- 

 tebrae, 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 com- 

 pletely fill this cavity; for a space is still left, 

 which is occupied by a pulpy substance. In like 

 manner, the accordance of the other cranial bones 

 with vertebrae, has been attempted to be traced ; 

 but in proportion as we recede from the central 

 parts of the spine, this correspondence is less 

 distinct, in consequence of the various degrees 

 of developement which these several elements 

 have received, in order to adapt them to particu- 

 lar purposes relating to sensation, to the prehen- 

 sion and deglutition of the food, and also to 

 aquatic respiration. It is impossible, however, 

 without exceeding the limits within which I 

 must here confine myself, to enter into the details 

 of structure which would be requisite in order to 

 render this subject sufficiently intelligible. 



The rest of the skeleton of fishes is extremely 

 simple. In many, as in the Ray and Tetrodon, 



