CHAPTER III. 



THE GENERAL FUNCTIONS OF THE NERVOUS 

 SYSTEM. SENSATION AND MOTION. 



WE have seen that the muscles have the power of short- 

 ening ; that in shortening they act on the bones as levers to 

 produce our varied motions. But what makes the muscles 

 shorten ? 



Some motions we will to make. We will to sit, to stand, 

 to walk, to run, to stretch out the hand, to take pencil and 

 paper, to write. 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 voluntary or involuntary, are dependent upon 

 the nervous system. In the experiment with the frog's mus- 

 cle we made it shorten by pinching the sciatic nerve. Let us 

 look at the nervous system of the rabbit, which is larger than 

 the frog's, and essentially like our own. 



The Brain and Spinal Cord of the Rabbit. It will be 

 found helpful to have at hand a well-mounted skeleton of a 

 cat or rabbit. Note carefully (a) the cavity of the cranium, 

 (b) the cavity in the spinal column, and (c) the sides of each 

 neural ring where the bone is to be cut by the bone forceps, 

 as indicated in Fig. 10. 



It is best to remove the skin completely before beginning 

 the work, as the fur is likely to be troublesome. 



Cut away the muscles from the back of the neck and along 



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