CHAP. V] THE CRANIA OF THE SIMI1DAE (PRIMATES) 231 



reference has been therefore made to exact angular measurements; 

 nor to the exact details of procedure that should be followed in 

 drawing the lines by which such angles are included. Nor has 

 special reference been made to the cranio-facial axis as such : this 

 would have necessitated reference to the nasal bones with con- 

 sequent complication of the description. 



Such then are the chief features of interest in the sectionized 

 skull, and their enumeration and description may be not unfitly 

 followed by an indication of their significance. Assuming that 

 the conformation of the cranium is largely expressive of the 

 conformation and development of the encephalon which it encloses, 

 it is to the latter that an appeal must be made in elucidating the 



Fig. 156. 



differences in the several crania investigated. Nor will the appeal 

 be made in vain. For in the lowly Mammal and lowly Primate, 

 the encephalon is still, in the great majority of cases, relatively 

 small, and the cerebral hemispheres have not assumed the 

 exuberance of growth which is a characteristic of the higher 

 forms. When this tendency to cerebral growth has been initiated, 

 it is found on the inferior aspect of the cerebrum that the base 

 of the brain, the floor of the third ventricle and the allied and 

 adjacent structures remain comparatively passive, while the cere- 

 bral hemispheres tend to expand in all directions, anteriorly, 

 posteriorly, and laterally, upwards and downwards. 



A glance at the series of diagrams of the sections of skulls will 

 shew how such expansion implies pressure in those directions ; 

 acting anteriorly, this will force the cribriform fossa first forwards 



