624 



PROF. R. A. DAllT ON THE 



depression is due to the constricting influence of a small pons, such 

 as was probably present. In other words, the substantia gelatinoaa 

 expanded anterior and posterior to this site to form the promi- 

 nences evident in the cast. More posteriorly the base of the 

 brain passes gradually into the medulla oblongata. In many 

 mammals (e. g., Ungulata) the transverse width of the medulla 

 oblongata is very appreciable, but in these creatures the transr 

 verse width is never so great (relatively) as in animals (e. g., Orni- 

 thorhynchns) which rely to a greater extent upon the " fifth nerve 

 sense." The great increase in transverse width found in the 

 Zeuglodontidse is to be regarded as a corroboration of the obser- 

 vation already made concerning the size of the tuberculum quinti. 

 In the cast there is no trace of the origin of either the third, 

 fourth, or sixth cranial nerves. The origin of the seventh, and 

 eighth is also obscure, though the latter is exposed for a portion 

 of its course through the petrous portion of the temporal bone. 

 The cast ends abruptly before the emergence of the IX, X, and 

 XI complex of cranial nerves. The study of the skeletal parts 



Text-fisnre '4. 



Jugular leash. 



Acoustic nerfe. 



^6us mec//us cercbelli 

 P<>r3f/occo/ui 



Broken margin 

 ■ of csst 



Postorior view of natural endocranial cast of Zeuglodon aensitivtis, sp. iiov. 

 ,M. 12123. About J nat sine. 



confirms what has been stated concerning the splaying apart 

 of the Gasserian ganglia (and incidentally of the posterior parts of 

 the cerebral hemispheres and the lobi flocculares of the cerebellum) 

 by the expansion of the tubercula quinti. 



No new anatomical feature from this aspect (text-fig. 4) is 

 revealed. The natural cast has been severed transversely at the 

 hinder end of the cerebellum. The important fact to be remem- 

 bered is that the lobus anticus and lobus posticus are probably, 

 totally hidden from external view ; and the central mass of the 

 cerebellum, as exhibited in dorsal or posterior view, is composed, 

 entirely of the lobus medius (of Elliot Smith), while laterally on 

 either side lies the lobus fioccularis. Posteriorly to the acoustic 

 nerve and venti-ally from the lobus fioccularis ('• paraflocculus " of 

 the text-figures) is to be seen an ill-defined mass, which represents, 

 the cast of the jugular vein and the associated nerve-structures, 

 which finally emerge at the foramen lacerum posterius. I have, 

 called this region the " jugular leash." 



