THE NERVOUS SYSTEM 409 



sential organs of vision and hearing- (retina and acoustic mac- 

 ula) elaborate series of protective organs become developed, 

 while at the same time the special stimuli are intensified by 

 various accessory organs. 



Thus, while this secondary system of sensory cells, like 

 the rank and file of a modern army, meets the external world 

 with its hazards, the central nervous system, and more espe- 

 cially the brain, like the general staff, remains in safety, 

 though in constant communication with the front. The eyes 

 see, the ears hear, the outer surface receives constant evidence 

 of the external world, while the brain, immured within a 

 dense wall of bone, sits in utter darkness and silence. It 

 neither hears nor sees; no ray of light ever penetrates its 

 obscurity, and even when exposed through injury or operation 

 it is found to have no power of direct perception or even of 

 sensation ; and yet it directs the entire mechanism with the 

 utmost intelligence, sending its messages to the motor system, 

 and causing the entire body to act in the strictest harmony with 

 the external conditions. In the performance of this function 

 it has developed a complexity immeasurably in excess of that 

 of any other organ, and even far beyond that of its own sense- 

 organs, since these latter attain a high degree of development 

 among fishes, while the brain continues its development 

 through amphibians and reptiles, becomes larger and more 

 complex among the mammals, especially along the line leading 

 to the anthropoids, and attains its highest point in the human 

 species, a member of the latter Order, not otherwise to be 

 especially distinguished from the remainder of the group. It 

 is thus to be concluded that the remarkable development of 

 brain characteristic of mammals in general, and the Anthro- 

 poidea in particular, has not been brought about through a 

 greater perfection of the sense-organs, but rather by increasing 

 its own power of receiving the sensory impressions and of 

 recording them through the formation of association paths; 

 and this, like all other structural advances, has been gradually 

 brought about through the w r orking of natural law, as a more 

 perfect adaptation to environment. 



