292 THE MECHANICAL FUNCTIONS. 



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 processes of the vertebrse, 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 occu- 

 pied 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 cen- 

 tral parts of the spine, this correspondence is less distinct, in 

 consequence of the various degrees of development which 

 these several elements have received, in order to adapt them 

 to particular purposes relating to sensation, to the prehen- 

 sion and deglutition of the food, and also to aquatic respira- 

 tion. It is impossible, however, without exceeding the li- 

 mits 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, there are no ribs. 

 Where these bones exist, they are articulated with the ex- 

 tremities of the* transverse processes of the vertebrse, of 

 which they appear to be merely continuations, or appendi- 

 ces. There is generally no sternum to which they can be 

 attached below: in a few fishes only, such as i\iQ herring 

 and the dory, we find rudiments of this bone, consisting of 

 a few pieces placed in a line on the lower part of the trunk.^ 



The parts of the skeleton of fishes, which correspond to 

 the arms and legs of quadrupeds, are the pectoral and ven- 

 tral fins (marked respectively by the letters p and v in Fig. 

 184.) The former are met with, with but few exceptions, 



• The bony arches arising from the skull, which support the bronchiae, or 

 gills, have been considered as the bones corresponding to the ribs of terres- 

 trial quadrupeds; and if this view were taken of them, it would tend to con- 

 firm the analogy of the cranial bones to the spinal vertebra. 



