THE NERVOUS SYSTEM 283 



the control of men fighting a fire, the scene changed ; 

 valuable property was saved, and the fire was stopped. 



513. Every community has certain deliberative and con- 

 trolling bodies ; these may be a board of aldermen, a court 

 of justice, a legislature, or a congress. In a similar way 

 the great number of individual cells which make up the 

 human body must be controlled by some central power, 

 or they will not work in harmony ; nothing will be accom- 

 plished, but a state of anarchy and helplessness called 

 disease, will follow. The seat of this central controlling 

 power is believed to be located in the brain. 



514. Coordination. The nerve cells receive, modify, and 

 send out impulses. By these three kinds of acts the ner- 

 vous system accomplishes what is called coordination. 

 This term signifies not only that the cells work together at 

 the same time, but that they work together as a part of a 

 plan for the accomplishment of some definite result. Sup- 

 pose that a man sees a bright dollar in the road and 

 picks it up. In order that he may do this, the cells re- 

 ceiving through the optic nerve the impression of the 

 shining coin must be in communication, either directly 

 or indirectly, with the muscles of the hand ; and that the 

 hand may reach the coin, the muscles of the arms, legs, 

 trunk, head, and neck must act together. Even the heart 

 and respiratory muscles must modify their action somewhat 

 to suit the movement. 



515. Other Organs besides Muscles must be coordinated. 

 Suppose a boy sees a large red apple. He notices the 

 odor which tells him that it is ripe, and his " mouth waters " 

 for it ; that is, the salivary glands begin to work, even before 

 he puts the apple to his mouth. The chewing and swal- 

 lowing, the secretion and peristalsis in the digestive organs, 

 are all carried on under the control of impulses that reach 

 them through the nerves. Without these impulses, the 

 salivary glands would not work when the mouth was chew- 



