THE BRAIN 117 



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 

 apparent movement or parallax of these objects is a 

 familiar condition of our life. If we are placed where the 

 nearest rocks and trees are many yards away, before 

 or below us, the seeming movement of these landmarks 

 is much less than that to which we are accustomed. 

 We miss one of the clues to our own success or non-suc- 

 cess in keeping our poise. No doubt it is much easier 

 to stand at the brink of a chasm 10 feet wide than on 

 the edge of a broad abyss of the same depth. The dan- 

 ger of a fall is no greater in the second case than in the 



