THE NERVOUS SYSTEM 45 



Man's special mechanism of adaptation to environ- 

 ment is his nervous system, which coordinates each 

 part of the body with every other part by means of the 

 brain, the spinal cord and a labyrinthian network of 

 nerve fibers and peripheral nerve-endings. Recent 

 developments in the sciences of biology, physics, 

 chemistry and physiology suggest that this nerve 

 mechanism is merely a highly specialized pathway 

 for the transmission of impulses set up by environ- 

 mental stimuli. In higher beings these impulses meet 

 and coordinate, or impinge and interfere in a central 

 organ of the mechanism, the brain, where schemes 

 or patterns of action are formed automatically accord- 

 ing to the lines of least resistance which have been 

 established by the evolution of the organism and the 

 species. 



A Specialized Pathway for Stimuli 



The response to external stimuli by adaptive reac- 

 tion is not limited to animals endowed with a nervous 

 system, but is common to all living protoplasm. For 

 example, unicellular organisms respond to the stimuli 

 in their environment by moving toward food and 

 away from danger, these actions being paralleled by 

 the manner in which many of the component cells in 

 pluricellular organisms respond to stimuli. In the 

 intestines of certain animals the cells throw out separate 

 prolongations of protoplasm which, like the pseudopodia 

 of amoebae, seize minute drops of fatty matter and draw 

 them within the main body mass. 



Thus in their search for food and in other activities, 

 the free-living protozoa furnish examples of adaptive 



