NERVOUS SYSTEM. 37 



one side. In a tune on a piano we know that the right 

 keys must be struck ; that each must be struck at the right 

 time, with the proper degree of force, and held for the 

 right length of time, or we have discord instead of har- 

 mony. What the player is to the instrument, the brain 

 is to the body. 



Temporary Loss of Muscular Power. It may have 

 happened to you that after sitting long in one position you 

 attempted to stand, but found that you could not do so. 

 One leg failed to act at the bidding of your will. When 

 the foot is " asleep " we get little sensation from it ; we 

 hardly know whether it is touching the floor or not. Press- 

 ing on it with the other foot causes no pain. 



We try to stand when the foot is asleep, but we are 

 unable to do so. The brain starts the nerve currents, and 

 they run along the nerve as far as the compressed part ; 

 here they stop. They cannot reach the muscles of the 

 leg below. Hence the muscles do not shorten, and we 

 do not rise, no matter how strongly we will to do so. 



Why is it that the nerves and muscles thus sometimes 

 lose their ability to perform their natural activities ? 



Dependence of Nerves and Muscles. This has been 

 explained by saying that owing to external pressure, the 

 nerve has temporarily lost its power of conducting nerve 

 currents. But what beside the nerve has been com- 

 pressed ? What process in the limb has been interfered 

 with by the pressure due to the position in which one has 

 been sitting or lying ? What is the temperature of the 

 benumbed limb ? 



On what are the nerves and muscles so dependent for 

 the maintenance of their activity ? 



