CHAPTER V. 



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 shortening 

 they act on the bones as levers to produce our varied mo- 

 tions. 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, both voluntary 

 and involuntary, depend upon the nervous system. 



The Cerebro-spinal Nervous System. This consists of 

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



The Brain. There are two main parts of the brain, the 

 cerebrum, which fills all the upper part of the cranium, 

 and the cerebellum, very much smaller, in the lower back 

 part of the cranium. The cerebrum is divided into right 

 and left hemispheres by a lengthwise groove. The sur- 

 face of the cerebrum is covered with ridges called convo- 

 lutions. The outside of the brain is of gray matter, 

 consisting of cells, while the inside is white, consisting of 

 nerve fibers. 



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