RESPONSIVE LIFE OF ORGANISMS 



453 



into action by a single stimulus, work together concordantly 

 to produce results that are in general beneficial. Differen- 

 tiation of parts and the development of a mechanism for 

 intercommunication between the parts would be of no use 

 without coordination. Coordination is illustrated by any 

 act that is neatly performed; all the muscles pull at the 

 right tension, at the right time and in the right order. 

 Want of coordination appears when the nervous system 

 "goes to pieces," as in convulsions; or, when some unusual 

 stimulus is applied ; something of it is seen even as a result 

 of tickling, which may produce useless and unregulated 

 movements. 



Control circuits. Coordination in the nervous system 

 tends to centralization. So long as the ganglia are of equal 

 potency the effect produced in one by a stimulus at one 

 point of the body may be opposed by the effect produced by 

 another stimulus at a distant point. A homely but familiar 

 experience shows how stimuli may interfere with each other. 

 If a stimulus in our nose (which may be accompanied by a 

 tickling sensation) be impelling us to sneeze, we may start a 

 countervailing stimulus by vigorous pressure upon the upper 

 lip, and thus we may keep from sneezing. Stimuli are not 

 of equal importance, nor is their importance always propor- 

 tioned to their intensity. A vibratory stimulus of very 

 slight intensity coming in the form of light to the eye may 

 reveal the near approach of a powerful enemy. It is im- 

 portant that the animal heed this stimulus and make its 

 escape, even though appealed to by the presence of food or of 

 other congenial circumstances. It must run now; it may 

 eat again. In other words, it is often important that all the 

 activities of an animal be concentrated on accomplishing a 

 single purpose be it escape or defense; it is a matter of life 

 or death. Hence some part of the nervous system must 

 dominate the other parts to the extent of determining what 



