130 SCIENCE PRIMERS. ' [§ x. 



longer or shorter sojourn in the body, having done its 

 work, having built up this or that part, having helped 

 the muscle to contract or the liver to secrete, having 

 by its burning given rise to work or to heat, goes back 

 powerless to the earth and air from which it caro'^. 

 And so the tale is told. 



HOW WE FEEL AND WILL. § X. 



56. One other matter we have to note before we 

 have given the full answer to the question why we 

 move. 



We have seen that we move by reason of our 

 muscles contracting, and that in a general way a 

 muscle contracts because a something started in the 

 brain by our will passes down from the brain through 

 more or less of the spinal cord, along certain nerves 

 till it reaches the muscle. It is this something, which 

 we may call a nervous impulse, which causes the 

 muscle to contract. 



But what leads us to exercise our wills ? What 

 starts the nervous impulse ? 



All the nerves in the body do not end in muscles. 

 Many of them end, for instance, in the skin, in those 

 papillae of which I spoke a little while ago. These 

 nerves cannot be used for carrying nervous impulses 

 from the brain to the skin. By an effort of the will 

 you can mal ^ your muscles contract ; but try as much 

 as you can, you cannot produce any change in your 

 skin. 



What purpose do these nerves serve, then ? If you 

 pricfc or touch your finger, you feel the prick or touch ; 

 you say you have sensation in your finger. Suppose 



