914 



THE XERVE SYSTEM 



already indicated in the section on brain development, there has occurred, in the 

 evolutionary history of man's vertebrate ancestry, a progressive increase of the 

 secondary fore-brain, with concomitant reduction of the rhincncephalon, or smell 

 brain the most archaic portion because of the important relations of the smell 

 sense to the life history of the earliest vertebrates. 1 



In a mesal view of a hemisected brain (Fig. 669) may be seen the various parts 

 of the brain stem and the cerebellum overlapped by the preponderating^ greater 

 cerebrum. Among the many notable features exposed to view in this brain 



OCCIPITAL 

 POLE 



INTERCEREBRAL 

 FISSURE 



INTERCEREBRAL 

 FISSURE 



FIG. 670. The cerebral hemispheres viewed from above. (Spalteholz.) 



section are certain fibre masses, commissures, extending across the meson, and 

 therefore divided by the knife in this preparation. Of the commissures pertaining 

 to the cerebrum one is conspicuous for its size and firm consistency. This great 

 fore-brain commissure is the corpus callosum already mentioned as being demon- 

 strable in the depths of the intercerebral cleft on divaricating the lips of this 

 fissure. The corpus callosum constitutes a massive system of association fibres 

 for the bilateral coordination of corresponding cortical parts. It is thickened 

 caudally, forming the splenium of the corpus callosum; frontad it bends on itself 



1 For a more thorough discussion on the natural subdivision of the fore-brain, based upon comparative 

 morphology, see the paper by G. Elliott Smith, Journal of Anatomy and Physiology, 1901. 



