XIII 



PHYLUM CHORDATA 



521 



anteriorly and posteriorly to such an extent that the entire length 

 of the cavity greatly exceeds that of the basi-cranial axis. A re- 

 sult of the posterior bulging of the brain-case is that the foramen 

 magnum (fan) is no longer situated at the posterior extremity of the 

 skull as in other Mammals, but assumes a position further forwards 

 towards the middle of the base. The anterior expansion, causing 

 -a strong arching forwards of the frontal region, brings about an 

 .alteration in the position of the ethmoidal plane, which, instead of 



Pa 



AS 



OS 



so 



f/i 



a 



TIG. 1107. Skull of Man. Letters as in Fig. 1086. In addition, a. angle of mandible ; 

 e.g. crista galli, a process of the mesethmoid ; s.t. sella turcica. (After Flower.) 



being perpendicular or inclined to the basi-cranial axis, becomes 

 horizontal, and the cribriform plate forms the middle part of the 

 floor of the anterior extension of the cranial cavity. The fossa for 

 lodgment of the cerebellum lies entirely beneath the posterior 

 portion of the cerebral fossa : the olfactory fossa is comparatively 

 small. (See Fig. 1057, D.) 



The outer surface is smooth and rounded, devoid of any prom- 

 inent ridges or crests. The occipital crest of lower Mammals 

 is represented merely by a rough raised line the superior cicrved 



