148 



ANATOMY OF VERTEBRATES. 



very interesting : the end gained seems to be, in grovelling 

 reptiles liable to have the head bruised, an extra protection of 

 the epencephalon- -the most important segment to life of all the 

 primary divisions of the cerebrospinal axis. The thickness of 

 its immediately protecting walls (formed by the basi-, ex-, and 

 super-occipitals) is equal to that of the same vertebral elements 

 in the human skull ; but they are moreover composed of very 

 firm and dense tissue throughout, having no diploe : the epen- 

 cephalon also derives a further and equally thick bony covering 

 from the basisphenoid and the parietals, the latter being partly 

 overlapped by the mastoids, fig. 97, 8, which form here a third layer 

 of the cranial Avail. 



The basisphenoid, fig. 96, 5, and presphenoid, 9, form a single 



97 



2-2 



Skull of a Python 



bone, and the chief keel of the cranial superstructure. The 

 posterior articular surface looks obliquely upward and backward, 

 and supports that of the vertebral centrum behind, as the posterior 

 ball of the ordinary vertebrae supports the oblique cup of the 

 succeeding one : here, however, all motion is abrogated between 

 the two vertebrae, and the co-adapted surfaces are rough and 

 sutural. The basisphenoid presents a smooth cerebral channel 

 above for the mesencephalon, in front of which a deep depression 

 (sella) sinks abruptly into the expanded part of the bone, and 



there bifurcates, each fork forming a short cul-de-sac in the sub- 



~ 



stance of the bone. The transverse processes from the under 

 and lateral surfaces are well marked, strong, but short, much 

 thicker in the Python than in the Boa. The alisphenoids, 6, form 

 the anterior half of the fenestra ovalis, which is completed by 

 the exoccipitals ; and in their two large perforations for the 

 posterior divisions of the fifth pair of nerves, as well as in their 



