CHAPTER III. 



THE GENERAL FUNCTIONS OF THE NERVOUS SYSTEM. 

 SENSATION AND MOTION. 



What makes Muscles Shorten ? We have seen that 

 the muscles have the power of shortening ; that in shorten- 

 ing they act on the bones as levers to produce our varied 

 motions. What makes the muscles shorten ? 



Voluntary and Involuntary Motions. Some motions 

 we will to make. We will to sit, to stand, to walk, to run, 

 or to stretch out the hand. Such motions, originating in 

 a brain activity, are called Voluntary. Other motions are 

 Involuntary. The will does not control the heart beat. 

 Most persons cannot keep from winking when a quick 

 motion is made toward the face, even if they know they 

 will not be hit. But all of these motions, whether volun- 

 tary or involuntary, are dependent upon the nervous 

 system. 



The Cerebro-spinal Nervous System. This consists 

 of the brain, the spinal cord, and the spinal nerves. The 

 brain will be described later. 



The Spinal Cord. The spinal cord is a cylindrical 

 body extending from the brain along the cavity of the 

 spinal column. Its diameter is not uniform throughout. 

 Between the shoulders is an enlargement called the cer- 

 vical enlargement, where the large nerves are given off to 



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