]48 



ANATOIMY OF VEETEBRATES. 



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 

 ]irimary 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 wall. 



The basisi^henoid, fig. 96, 5, and presphenoid, 9, form a single 



97 



^'^i'l^bkM^kkUI'Usi 



SkiiU t,i a Pytlion 



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

 posterior articular surface looks obliquely upward and backward, 

 and supjiorts that of the vertebral centrum behind, as the jJosterior 

 ball of the ordinary vertebra?, su]iports the oblique cup of the 

 succeeding one : here, however, all motion is al.trogatcd between 

 the two vertebra?, and the co-adapted surfaces arc rouiih and 

 sutural. The basisphenoid presents a smooth cerebral cliannel 

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

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

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

 stance of the bone. The transverse processes from tlic under 

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

 thicker in the Python tlum in tlie Boa. Tlic alisphcnoids, «, form 

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

 tlic exoccipitals ; and in their two large perforations for the 

 ])cisterior divisions of the fifth pair of nerves, as well as in their 



