MAN 187 



of a device that, by introducing flexibility and adapta- 

 bility into the stimulus-response pattern, supplements 

 the operations of the kidney to this same end— to mini- 

 mize the organism's disquietudes. 



The role which the nervous system plays in the per- 

 ception of various sensations, in the integration of per- 

 ceptions, both past and present, and in eHciting motor 

 activity, both voluntary and involuntary, is obvious to 

 everyone who knows the meaning of 'brain' and 'nerve/ 

 But the role of the nervous system as viewed in the 

 long perspectives of evolutionary history can bear a word 

 of elaboration. 



Despite its apparent complexity, the nervous system 

 involves only four basic operations: first, the conduction 

 of a wave of excitation (actually a reversal of the 

 physical-chemical polarization of the protoplasmic sur- 

 face) over the undifferentiated protoplasm of the cell; 

 second, the more rapid conduction of a wholly similar 

 wave of excitation over the long processes (axons) of the 

 specialized nerve fiber; third, the secretion of humoral 

 or chemical agents that serve to excite or inhibit other 

 cells, including other nerve cells ; and fourth, transmis- 

 sion across the 'synaptic' junction between nerve and 

 nerve— and here it must be emphasized that nerve fibers 

 are not connected together anatomically, any more than 

 are other cells, but nerve meets nerve across a sort of 

 physiological spark gap which the physiologist calls the 

 'synapse' (synapsis = conjunction, union). 



A speciahzed cell similar to the nerve fiber first makes 

 its appearance in certain Protozoa; in the lower Meta- 

 zoa, nerve fibers are combined into a nerve net which 

 permits rhythmic activation of successive segments of 

 the body, as in the progressive movements of the legs of 

 the centipede. 



Humoral or chemical excitation and inhibition, ef- 

 fected by such substances as acetylcholine and adrena- 

 line, are widespread among the invertebrates and play 



