8oo MAN 



brain is a wcll-clc'liiH-tl nu'sial <z;t'niculatr body ccjiinccted witli the inlcrior 

 colliculus by means ot the braciimni conjimctix Litn posticuni. The decrease 

 HI proninience of the colliciilar eminences in the rool ol the midbrain is 

 significant of that general transference of \isual and auditory function w hich 

 has been in process throughout the primate series. It attains its ultimate 

 expression in man. Confirmatory evidence of this supersedence of the mid- 

 brain tectum by the areas in the cerebral hemispheres is easily obtained by a 

 surve\ of those regions in the cerebral cortex assigned to the functions of 

 hearing and vision. 



Internal Structure of the Brain Stem 



It is diflicult to approach the description ol the internal structure of 

 the human brain without a certain feeling of conviction that the sections 

 clearly depict the culmination of that evolutionary process w hicli has been 

 traced from its primate beginnings. What is most impressive about these 

 sections of the human l)rain is the fact that every feature hitherto seen in 

 the apes comes at length to its sharp and final focus. The entire brain stem, 

 like tile hemispheres, manifests an enlargement w hich seems somewhat out 

 of proportion to the stature ol man. The oblongata, the pons Varolii and the 

 midbrain lia\c a little less than twice the dimensions of the corresponding 

 structures in gorilla, an animal which in point of size is considerably larger 

 than man. 



LEVEL OF THE PYRAMIDAL DECUSS.\TION (fiG. 342) 



Here the features are those lamiliar througiiDut the entire primate series 

 incident to the decussation of tlu' jiyramidal libers. The section shows the 

 ventromedian sulcus, somewhat dellected to one side under the inllucnce 

 of the crossing fibers. On its dorsal aspect is a long dorsonuxlian se]jtum 

 extending from the surface into the central gray (Cen). The pyramid, 



