Reflex Actions 157 



the physiological sequence is always in essentials the 

 same, involving (1) sensory, receptor, or afferent 

 nerve-cells (the scouts), (£) associative or inter- 

 nuncial nerve-cells (general headquarters), (3) 

 motor or efferent nerve-cells (the executive officers), 

 and (4) muscle-cells that do the work (the "common 

 soldiers"). If we are humble enough to use the meta- 

 phor we cannot be foggy in regard to the general 

 sequence of participants: (1) Scouts, (2) G.H.Q., 

 (3) Executive Officers, and (4) Men; but it need 

 hardly be said that in a given operation there may be 

 many representatives of each of the four kinds of 

 participants. In ordinary reflex actions the linkage 

 is preestablished, it is part of the hereditary make- 

 up. In learning a new game of skill or a new piece of 

 work we have to establish the linkage by eliminating 

 errors and by frequent repetition. A line of least 

 resistance is established ; the message comes to travel 

 along one particular line rather than along any 

 other; and various improvements may be effected, 

 such as not troubling more than subordinate mem- 

 bers of the general headquarters. But all but the very 

 simplest instances of behavior involve a sequence or 

 simultaneity of actions, each with its four links. Thus 

 (I) S-A-M-E (sensory, associative, motor, effector) 

 leads on to or is associated with (II) s-a-m-e, and 

 this leads on to or is associated with (III) s-a-m-e, 

 and so forth. What habituation does is to make it easy 

 for II to follow or conjoin with I, and for III to follow 

 or conjoin with II, and so on. In some manner that is 

 still very obscure, even on its physiological side, II 



