62 PLAIN FACTS FOR OLD AND YOUNG 



impulses to these various organs, by which they are 

 made to act. Thus, when we wish to move the hand, 

 the cells in the brain which communicate with the mus- 

 cles of the arm, send down an order to the muscles to 

 contract, and do our bidding. The heart beats, the 

 lungs breathe, the liver makes bile,— all in obedience 

 to the commands received from the nerve cells which 

 preside over them. The nerve cells are divided into 

 classes, each of which has its particular work to do. 



dueer Thinking.— The principal business of nerve 

 cells is to think and feel. The nerve cells found in the 

 spinal cord are chiefly of two classes. One class receives 

 impressions brought through their fibers from the out- 

 side of the body. These impressions they communicate 

 to another class of cells, which, through their branches, 

 send out impulses to the parts from which the impres- 

 sions came, or to other parts, causing them to act. For 

 example, when the bottom of the foot is tickled, the 

 limbs draw up in spite of us. It does the same thing 

 if we are asleep, or if the limb is paralyzed so that we 

 cannot control it by the will. This is a very simple 

 kind of thinking, which is done by the nerve cells of 

 the spinal cord. There is a special collection of nerve 

 cells in the enlarged upper end of the spinal cord, 

 called the medulla, which does a little higher grade of 

 thinking. These cells regulate the activity of the heart, 

 bowels, liver, and other internal organs. At the base 

 of the brain a large collection of nerve matter consti- 

 tutes what is called the little brain, which has for its 

 important function the duty of regulating the action 

 of those nerve cells that control the muscles, so that 

 the muscular movements may be orderly and able to 

 accomplish what is desired of them. 



Arranged at the under part of the brain are found 



