970 EVOLUTIONAL MODIFICATIONS 



concavities, and becomes gradually less pronounced as this conea\ity is 

 gradually decreased by the expansion of the frontal lobe. Through graded 

 stages in the primates the diminution m promiiieiiee ot the mter orbital keel 

 max be easily traced. It is still a fcatuix' in the brain of the orang, and present 

 also in tiu' chimpanzee, although less promiiu'nt; it may be discerned. less 

 well delined, in the gorilla, but almost entirely disappears in man due to 

 accessions in all portions of the Irontal lobe. 



Conditions in the struetm-es related to the olfactory and the optic nerves 

 denote a similar process of frontal expansion. The ollactorv l)ulb and tract 

 in tarsius are nuich larger than in other primates. Tluar resemblance to the 

 more ])riiniti\e rhinencephalic development of carnivores and ungulates is 

 fairly close. The bull) and the tract appear more as an integral part of the 

 basal surface of the endbrain. The lateral and mesial olfactory roots follow 

 their course to their terminations in a manner characteristic ot the lower 

 mammals. Progressively, howcAcr, in passing upward Irom lemur, the ol- 

 iactory bulb and tract become relatively smaller. They are both more 

 easily detached from the overlying orbital surface of the brain, while the 

 divarication of the lateral and the mesial roots of the olfactory tract becomes 

 less conspicuous. The entire dcAclopment ol' the rhinencephalon (olfactory 

 portions of the brain) is less i^iominent and gixcs e\ idence ol a delloreseenee 

 in its structural elements. Accompan\ ing this ]:)r()cess there is a diminution 

 in the capacity of olfactory function. In the acKance from the lower to the 

 upper end of tlu' ]Mimate series a small fissure is found beneath the olfactory 

 tract. This is the sulcus ollactorius, which determines the existence of the 

 gyrus rectus. Both of these features beeonu' more conspicuous as the human 

 stage of developm lilt is approached. The\ are both indicatixe of progressive 

 difTerenliation in the frontal lobe. These de\ elopmi'Utal changes connected 

 with the sense of smell are of greatest evolutional moment. Their ob\ ions 

 connotation is a diminution of olfaetor\ seiisi' which gave to the hunting 



