CENTRAL NERVOUS SYSTEM 



531 



until eventually it acquires a plasticity and adaptability sufficient to form 

 a structural basis for the physical dexterity and intellectual supremacy of 

 man. In contrast with the types of nervous s^^stem we have just discussed, 

 the activities of which are expressed in simple and immediate reflexes 

 concerned with the ready transformation of afferent impulses into somewhat 

 stereotyped responses, it makes provision to an increasing degree for the 

 appreciation of broadly correlated sensory patterns, for individual adjust- 



FiGS. 699 TO 702. — The Evolution of the Bkain of Vertebrates. 



Fig. 700.— The stage 

 of three primary 

 cerebral vesicles. 



Fig. 701.— The stage 

 of five cerebral 

 vesicles. 



Fig. 702.— The final 

 division of the 

 telencephalon. 



Fig. 699.— The ini- 

 tial archencepha- 

 lon ; a vesicular- 

 shaped swelling at 

 the ujjper end of 

 the medullary 



tube. 



The fore-brain and its derivatives are dotted ; the mid-brain is in solid black ; 

 the hind-brain is cross-hatched. 



CB, cerebellum ; CH, cerebral hemispheres ; CS, corpus striatum ; DE, 

 diencephalon ; FB, fore-brain (prosencephalon) ; HB, hind-brain (rhombence- 

 phalon), divided into metencephalon and myelencephalon in Figs. 701-2 ; MB, mid- 

 brain (mesencephalon) ; MO, medulla oblongata ; OL, olfactory lobe ; OT, optic 

 thalamus; OV, vesicle which becomes the iter or aqueduct of Sylvius: TE. telen- 

 cephalon ; I, II, III and IV, ventricles. 



ments in response and eventually for the emergence of thought and per- 

 sonality. Within the Vertebrates, however, this process of evolution was 

 slow but conformed to a general plan whereby the reflex mechanism in the 

 lower levels became gradually subordinated to the controlling and integrating 

 influence of a cerebral cortex. In the process, changes affecting the central 

 visual mechanism played a predominating part, and the gradual transference 

 of the sensory activities of vision to the highest level, leaving the reflex 

 photostatic functions at a lower level, formed the pivot around which the 

 nervous system of the higher Vertebrates eventually became reorganized. 



As in the lower C'hordates, the central nervous system of Vertebrates is 

 formed from the dorsal ectoderm by the infolding of the medullary groove 



