650 THE HIGHER ANTHROPOIDS 



which these structures appear attenuated as compared with those of ungu- 

 lates and carnivores. By retracting the olfactory bulb and tract, the gyrus 

 rectus is brousht to view as well as the medial olfactorv fissure. 



FIG. 294A. LEFT HEMISPHERE OF BRAIN. GORILLA. 



[Actual Length 123 mm/ 



A second important character of the basal surface is the relation of the 

 optic nerves and tracts to the chiasm. Their angulation, as is the casethrough- 

 out the primate series, approaches the obtuse, the two optic nerves entering 

 the chiasm at a little less than right angles, while the same angular relation 

 obtains in the optic tracts. This condition is determined by the shortening 

 of the nasal cavity and the protrusion forvvard of the frontal lobe into a 

 position overlying the orbits. 



I n the occipital portion of the basal surface, the decrease in the cerebellar 

 concavity is apparent. This is occasioned by the operation of two factors: 



