326 THE MECHANICAL FUNCTIONS. 



it from behind, as is shown in Fig. 215, we soon discover 

 that the real cavity in which the brain is lodged, and to 

 which the aperture at f leads, is very small, only just admit- 

 ting the end of the finger, and that the broad plates of bone, 

 p, p, which form the upper surface of the skull, have no re- 

 lation to this cavity, and are merely extended over the tem- 

 poral muscles, which are of very large size, occupying the 

 whole of the spaces s, s; which spaces are completely sur- 

 rounded by these bones. It would appear that the same ten- 

 dency to lateral expansion, which exists in the spinous pro- 

 cesses of the dorsal vertebrae, prevails, also, among those 

 which contribute to form the skull. The parietal bones, 

 which represent the spinous processes of the second cranial 

 vertebra, after having performed their primary office of pro- 

 tecting the hemispheres of the brain by closing over them, 

 still proceed in their development, 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 tem- 

 poral 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, entitled. As the turtle is unable to withdraw 

 its head \vithin the carapace, such extraordinary protection 

 appears to have been necessary: for it is not met with in the 

 tortoise, which has a carapace sufficiently capacious to give 

 shelter to the head whenever occasion may require.* 



This arrangement of the expanded spinous processes and 

 ribs gives rise to a singular inversion in the position of the 

 scapula; for it is here placed on the inside of the ribs and 

 sternum, that is, between the carapace and plastron.t The 



* The analogy of the spine of the occipital bone with that of a vertebra is 

 farther shown by this bone extending- backwards to a considerable length, 

 exactly in the manner of the spinous processes of the cervical vertebrae in 

 other animals. 



I The anomalous situation of these bones, and the strangely disguised 

 f<jrms which their several parts assume, render it veiy difficult to recognise. 



