THE NERVOUS SYSTEM 



505 



fibers to and from the cortex. The corpus striatum may be considered 

 as a front porch or main entrance to the higher centers of the brain. 

 Sympathetic functions are ascribed to it. 



Evolution of the Diencephalon. The evolutionary potential of the 

 diencephalon is low, and it changes little from fish to man. The founda- 



TURGENIEFF, 



WEIGHT 

 IN GRAMS. 



'200-1000. 



ORANG OUTAN. 350 G 



Fig. 421. — Variation in human brain size. The range is from 369 grams to 2400 

 grams, both of these extremes being the brains of idiots. The average male brain 

 weighs 1400 grams. (Redrawn after Kahn's "Das Leben Des Mensschen," W. Keller 

 & Co.) 



tions of the human brain are laid in that of cyclostomes. Three regions 

 are distinguishable in the primitive diencephalon of cyclostomes, a dorsal 

 epithalamus, a lateral thalamus, and a ventral hypothalamus. The 

 epithelial roof remains thin, and there is no velum transversum to divide 

 the diencephalon from the telencephalon. Three dorsal outgrowths arise 

 from the epithalamus, an anterior paraphysis, a parietal outgrowth or 



