THE BRAIN 115 



soles which are responsive to stimulation by pressure. 

 The afferent currents which result have a long course 

 to run to reach the brain. In the gray matter of the 

 cerebellum, and probably in other places too, they 

 generate efferent impulses which play upon the motor 

 cells of the cord and secure purposeful contractions of 

 various muscles tending to restore the balance. 



2. Any swaying of the body alters the tension? of 

 many muscles and their associated connective tissues. 

 The bearing upon one another of the small bones in 

 the feet is subjected to change, the stresses in all the 

 leg and trunk muscles are modified, the weight of the 

 head puts special and varying strain upon the muscles 

 of the neck. We must never forget that the motor 

 equipment of the body is at the same time a great re- 

 ceptor system. No muscle is contracted without regis- 

 tering its action by returning impulses to the cord and 

 brain. When a muscle is stretched by an external 

 force the same thing is true. One of the chief services 

 of impulses thus generated is to guide the adaptive 

 reactions that make for equilibrium. 



3. If the head moves the retinal pictures are shifted. 

 Our attention is not usually fixed upon this matter but 

 if we attend to it at all we are quick to interpret the 

 experience as due to the swaying of the body. Whether 

 we realize the displacement of the images or not we 

 may not doubt that it is one of the sources of stimula- 

 tion on which we depend to steady ourselves. No one, 

 probably, can stand quite so steadily with the eyes 

 closed as he can when he is using them. Reliance on 

 the eyes is more absolute when the other mechanisms 

 are at fault; there are cases of nervous disease in which 

 the victims reel and fall when the eyes are bandaged, 

 though able to stand by their aid. 



The giddiness and alarm which we feel when at the 

 verge of a precipice can be explained in this connection. 

 It is not wholly the result of imagined danger. We are 

 used to the presence of objects in front of us and the 



