THE HUMAN MACHINE 



sound or touch or smell and carry it to the inner nervous 

 system. Here the sensory cells turn it over to another 

 lot of nerve cells and they in turn translate the message 

 to the muscles which act. Such nerves are called motor 

 nerves. In every conscious act not one set of nerve cells 

 but hundreds perhaps thousands take part. General 

 science is not the place to study the details of the nervous 

 system, but rather to learn something about its control and 

 care. In our automobile we do not pretend to know much 

 about the mechanics of the engine under the hood, but we 

 want to know how to drive it carefully. 



Unconscious activities are also controlled by the nervous 

 system. When we sit in the driver's seat and decide 

 where the car shall go, the engine goes purring along. 

 Water circulates through the radiator, supplies of gas 

 and oil are in circulation, spark plugs ignite the gas. All 

 these activities go on if the car is to run. So in the human 

 machine. As you read these words your breathing goes 

 on, movements of the digestive organs take place, the 

 making of enzymes, pumping of the heart, the regulation 



Two kinds of control are illustrated here. Read your text and then try to 

 explain the picture. 



of the body temperature all are going on without any 

 conscious activity on your part. The regular activities 



