292 FIRST YEAR COURSE IN GENERAL SCIENCE 



impulse to a nerve center. The nerve center sends another 

 impulse through a motor nerve to a group of muscles, and 

 the toad jumps. These impulses are conveyed so rapidly 

 that before the foot which caused the stimulus is lifted, the 

 toad is a yard away. 



336. Nerve Centers. Nerve centers are groups of cells. 

 They receive impulses brought to them over sensory nerves, 

 and they send out answering impulses over motor nerves. 

 The brain and spinal cord are nerve centers. The brain may 

 be compared to an operator in a telephone exchange; it 

 receives calls from a distant part of the body and connects 

 with a muscle which, on contracting, moves an organ in 

 response to the return message. 



337. The Importance of the Nervous System. The 

 nervous system is of great importance, since it provides for 

 the harmonious working of all the organs of the body. 

 Among the higher animals, no cell of the body does any 

 work except under the direction of the nervous system. 



338. Study of a Vertebrate. Observation of some 

 living vertebrate animal will help greatly in the study of the 

 organs and motions of higher animals. For this purpose a 

 goldfish may be used. It is easily procured from a dealer 

 in household pets, and with a little care, will live a long time 

 in a jar of water. Part of the water should be removed and 

 fresh water supplied once a week. If green water plants are 

 living in the jar, the water need not be changed so often. 

 Prepared fish food, as well as the water plants, can usually 

 be procured where the fish is bought. 



339. The Sense Organs of the Fish. Two parts of 

 the fish's body are specially fitted to receive impressions 

 caused by light. These are the eyes. There is one eye on 

 each side of the head. They are somewhat convex, but do 

 not protrude much beyond the horny frame in which they 

 are set. Experiments show that fish are nearsighted, so they 

 probably cannot perceive objects at any great distance. 



