THE BRAIN OF MODERN MAN 791 



reason as digitationes orhilulcs. This entire modification is consonant with 

 other evidences of expansion in the frontal lobe of the iiunian l^rani. It is 

 a feature of the utmost miportanee in distinguishing the ence[)halon of man 

 from ail other species. Inasmuch as the frontal lobe is looketl upon as the 

 area in which the highest syntheses of neural im[)ulses are composed (the 

 gallery of human experience), it is evident that its extreme expansion in man 

 is in direct proportion to his needs in these im|)orlant features of his organi- 

 zation. The olfactor\ tract and bulb are readil\ dt'tachable as tar back as the 

 trigonum ollactorunn, and such detachment re\eals the deep, olfactory 

 sulcus, which in many instances extends forward around the frontal pole of 

 the brain to become continuous w itli a small extension of the superior frontal 

 fissure. The increase in size and prominence of this fissure is another instance 

 of extension in the frontal lobe. In some cases this ollactorv sulcus extends 

 only as far forward as the frontal pole in one hemisphere, but in the opposite 

 hemisphere may attain its continuit\ witii the superior frontal sulcus. By 

 means of the olfactory sulcus, a well-defined gyrus rectus is delimited from 

 the adjacent orbital coinolutions. In those instances in which the olfactory 

 sulcus is continuous with the superior frontal fissure, the gyrus rectus then 

 becomes the orbital expression of the superior frontal convolution. All of the 

 brain features observed in the great anthropoids and even in lower and inter- 

 mediate primates, which in these inferior forms prest-nt themselves as incip- 

 ient landmarks, have in the human brain attained their lull representation. 

 Thus the process of evolution which had its inception in the more primitive 

 anthropoid organization finds its ultimate expression in man. 



The Chiasm. The chiasm shows an angulation with reference both to 

 the nerves and the tracts which identifies it with this region of the anthro- 

 poid brain. Upon the base, however, in the region immediately caudal to the 

 chiasm, the optico-peduncular space in man appears to be much more con- 

 spicuous, by the fact that this region is larger and that its contents are more 



