STRUCTURE OF THE SKULL IN SERPENTS. 77 



it is perforated by a slit for the eightli pair of nerves ; it 

 articulates below with the basioccipital ; it is excavated 

 in front to lodge the petrosal cartilage, where it articulates 

 with the alisphenoid ; it unites above with the superoc- 

 cipital. The superoccipital, 3, is of a subrhomboidal form, 

 sends a spine from its upper and hinder surface, expands 

 laterally into oblong processes, is notched anteriorly, and 

 sends down two thin plates from its under surface, bound- 

 ing on the mesial side the surface for the cerebellum, and 

 by the outer side forming the inner and upper parts of 

 the acoustic cavities. The superoccipital articulates below 

 with the exoccipitals and alisphenoids, and in front with 

 the parietal, by which it is overlapped in its whole extent. 

 The occipital vertebra is as if it were sheathed in the ex- 

 panded posterior outlet of the parietal one (Fig. 17), the 

 centrum resting on the oblique surface of that in front, 

 and the anterior base of the neural spine entering a cavity 

 in and being overlapped by that of the preceding neural 

 spine : the analogy of this kind of " emboitement" of the 

 occipital in the parietal vertebra with the firm interlock- 

 ing of the ordinary vertebrae of the trunk is very inte- 

 resting: the end gained seems to be, chiefly, an extra 

 protection of the epencephalon — the most important seg- 

 ment to life of all the primary divisions of the cerebro- 

 spinal axis. The thickness of its immediately protecting 

 walls (formed by the basi, ex, and superoccipitals) 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 epencepha- 

 lon also derives a further and equally thick bony covering 

 from the basisphenoid and the parietals, the latter being 

 overlapped by the mastoids, which form a third covering 



to the cerebellum. 



7-x- 



