36 ELEMENTARY PHYSIOLOGY. 



If all the muscles were voluntary muscles, 

 Nervous that is, if no movement of the organs of 

 on * the body could be made without a special 

 order from the mind, the continuance and enjoyment 

 of life would be impossible. Every breath, every 

 heart- beat, and many other operations of organs 

 which can scarcely be dispensed with for even a few 

 moments, would need to be constantly thought of 

 and directed. Fortunately, the mind, and even the 

 brain, is relieved from the ordinary control of the 

 operations of organs upon whose regular and con- 

 stant action cur life depends. So the heart goes on 

 beating, the lungs continue breathing and the stom- 

 ach keeps on working, while the mind rests and the 

 brain sleeps. Let us see how this is done. 



The spinal cord may be regarded as a continua- 

 tion of the brain. It is composed of the same two 

 kinds of matter white and gray. We may also look 

 upon the spinal cord as a deputy brain. A deputy 

 is appointed as a substitute for another, and empow- 

 ered to act for him. An officer may have more duties 

 to perform than he can personally attend to. So an 

 assistant is given him, who is entrusted with certain 

 lines of work for which he is held responsible. When 

 serious questions or difficulties arise in the assist- 

 ant's department of work, he appeals for special 

 advice to the chief officer. So in the body, while the 

 brain executes the orders of the mind, and controls 

 the voluntary operations and movements of the body, 

 the spinal cord is entrusted with the control of the 



