248 EXPERIMENTAL GENERAL SCIENCE 



carried to the brain by the nerves, but the nerves themselves 

 are insensible to such sensations. When a nerve is cut or 

 otherwise injured, it sends a sensation such as it ordinarily 

 sends to the brain, and this is referred back to the end organs 

 of the nerve, though these may perhaps be a long way from 

 the injury. Cutting the nerve which carries the sensation of 

 sight to the eye would give the sensation of a blinding flash 

 of light, but the nerve would feel no pain. The seat of sensa- 

 tion, therefore, is in the brain. If the part of a nerve which 

 carries sensations from any part of the body to the brain be 

 cut, all further sensation in that part of the body will be lost 

 but it may still be capable of motion. If the part of a nerve 

 that carries motor impulses from the brain be cut, all motion 

 in that part of the body served by the nerve will cease. Al- 

 though all sensations are felt in the brain alone, this organ is 

 insensible to injuries to itself and may be cut without causing 

 pain. 



213. Sleep. Like the muscles, the brain is improved by 

 proper exercise. Hard study for a reasonable length of time 

 is not harmful, but such study for prolonged intervals, or 

 carried on late at night when the brain should be resting, is 

 objectionable. After a long period of work with the brain, 

 the cells have a shrunken appearance and a period of rest is 

 required to restore them to their normal condition. The 

 best rest for the brain is sleep. An adult requires from seven 

 to eight hours of sleep daily and growing children require 

 somewhat more. Healthy babies sleep a large part of each 

 day. It appears to make no difference whether one sleeps 

 in the daytime or at night, provided the period of sleep is 

 uninterrupted. Night, however, is usually the better time 

 because of the greater quiet. 



Practical Exercises 



1. Cross your first and middle fingers and touch your nose with them. 

 Why does it feel like two noses? 



