THE LIFE OF AN ANIMAL 



291 



parts is composed of groups of cells. A nerve cell consists 



usually of an angular body with thread-like branches. It 



has one very long outgrowth, called an axon. Thousands 



of nerve cells grouped together make a 



nerve center, and the axons of these 



cells form bundles of nerve fibers called 



nerves. Each fiber is protected by a 



covering or sheath. The nerves serve 



to connect the nerve centers with the 



sense organs, muscles, and other active 



organs of the body. 



334. Sense Organs. Some cells of 

 the nervous system are highly sensitive 

 to external changes, and are found on or 

 near the surface of the body. These con- 

 stitute sense organs. The eye is a sense 



organ. In the tips of the fingers there The thread-like 



o rp <,p n P ore-arm projection from the 



0r S ans - main body of the cell 



335. Nerves. There are two classes is one of the fibers 

 of nerves: sensory and motor nerves. 



The sensory nerves are so arranged in 

 the body as to form a connection between the sense organs 

 and the nerve centers, such as the brain and the spinal cord. 

 The motor nerves connect nerve centers with the different 

 organs of the body. Both kinds of nerves are found in all 

 parts of the body. 



A sensory nerve carries inward, to a nerve center from 

 some sense organ, an impulse caused by an external change 

 or stimulus. Motor nerves carry outward, from a nerve 

 center, answering impulses to different organs. Because 

 motion so often results from these answering impulses, the 

 nerves which carry them are called motor nerves. 



Illustration: a footfall jars the ground near a toad. The 

 vibration of the ground arouses or gives stimulus to sensory 

 nerves in the toad, which convey the information as an 



154. 

 A NERVE CELL. 

 (magnified) 



of thread-like sensory 

 and motor nerves. 



