418 THE MECHANICAL FUNCTIONS. 



surface of the skull, have no relation to this cavity, 

 and are merely extended over the temporal muscles, 

 which are of very large size, occupying the whole 

 of the spaces, s, s ; which spaces are completely 

 surrounded by these bones. It would appear that 

 the same tendency to lateral expansion, which 

 exists in the spinous processes of the dorsal verte- 

 brae, prevails also among those which contribute to 

 form the skull. The parietal bones, which repre- 

 sent the spinous processes of the second cranial 

 vertebra, after having performed their primary 

 office of protecting the hemispheres of the brain by 

 closing over them, still proceed in their develope- 

 ment, forming first a crest on the upper part of the 

 real cranium, and then separating to the right and 

 left, and expanding horizontally into the upper roof 

 (p, p) already mentioned, for the protection of the 

 temporal muscles. This great breadth of the head 

 in the turtle gives the animal an aspect of superior 

 intelligence, to which character, from the really 

 diminutive size of its brain, it is in no respect en- 

 titled. As the turtle is unable to withdraw its head 

 within the carapace, such extraordinary protection 

 appears to have been necessary ; for it is not met 

 with in the tortoise, which has a carapace suffi- 

 ciently capacious to give shelter to the head when- 

 ever occasion may require. The analogy of the 

 spine of the occipital bone with that of a vertebra, 

 is confirmed by this bone extending backwards to 

 a considerable length, exactly in the manner of the 

 spinous processes of the cervical vertebrae in other 

 animals. 



This arrangement of the expanded spinous pro- 

 cesses and ribs in the Chelonia gives rise to a sin- 



