278 THEORETICAL BIOLOGY 



Much more frequently the control occurs inside the body. 

 Here there are two cases to be distinguished : either the move- 

 ment of the effector muscles is received by special sensory 

 nerves, as the accompanying diagram indicates > — 0^=0 — t 



or else the excitation conveyed to the effector nerves is par- 

 tially taken up by special central receptors and conducted 



back to the mark-organ. ^^^O'^^^^^Ov* . These receptors 



form the central sense-organ of Helmholtz, which, anatomically 

 speaking, is still undiscovered. 



In the human being all three kinds of receptor occur : 

 he has the power of controlling his own movements, firstly, 

 by the eye or the sense of touch ; secondly, by muscular sen- 

 sations ; and thirdly, by direction-signs. 



Our ignorance of the relations of the central receptors 

 prevents any dealing with their relations to the organ for 

 giving direction in space that lies in the semicircular canals. 



I shall bring together all controlled actions under the 



r 1 R MOI->AOI E 

 common formula j — <- — j. 



THE RECEPTOR ACTION 



If I lay before a draughtsman an unfamiliar arabesque, 

 and after it has been taken away, he is able to copy it, that 

 is an action based on experience, essentially indistinguishable 

 from the imitative piping of the bull-finch that has heard 

 a new tune. 



The draughtsman, however, must, in receiving, execute a 

 movement, and this makes his action more difficult. In 

 listening to a piece of music, the hearer does not have to make 

 any movement, whereas the draughtsman, as he observes, 

 moves his eye to and fro with his eye-muscles, so that his 

 glance follows along the line of the arabesque ; and it is this 

 directing of the sight that must be formed anew like a melody. 



