THE NERVOUS SYSTEM 28? 



neuroglia, which is composed of extremely fine fibers and 

 minute cells. Though like connective tissue in function, 

 its chemical composition is different, nor is neuroglia de- 

 veloped from connective tissue (Fig. 193). 



523. How Nerve Fibers terminate. If we could trace 

 toward the central nervous system the course of the various 

 nerve fibers in a nerve like the great sciatic, for example, 

 we should find that every one of its thousands of nerve 

 fibers ends, without exception, in a nerve cell in the spinal 

 cord or brain, or in one of the ganglia near the central 

 nervous system. If we should trace these same fibers 

 away from the central nervous system, they would be found 

 to have various endings. Some enter the muscles where 

 they subdivide, and finally end in the muscle cells. Others 

 follow the blood vessels, and end in the muscle fibers form- 

 ing the middle layer of their walls. Others go to the gland 

 cells ; for example, the sweat glands in the skin, or to some 

 other internal gland. Others, passing to the skin, termi- 

 nate at the roots of the hair, or in curious little bodies com- 

 posed of cells and called touch corpuscles (Fig. 196). 

 Every nerve fiber ends centrally in a nerve cell, and out- 

 wardly in either a muscle cell, gland cell, or sense organ cell. 



524. Motor and Sensory Fibers. Fibers which connect 

 nerve cells with muscles or glands are called motor or 

 efferent (outward bearing), fibers ; those which connect 

 nerve cells with sense organs are called sensory or afferent 

 (into bearing) fibers. There is a third kind, which connects 

 nerve cell with nerve cell in the brain ; they are known as 

 association fibers (Figs. 203, 207). 



525. Nerve Action. If the sensory nerve be irritated at 

 its outer endings in the cells of the other tissues or any- 

 where in its course, an influence will travel to the central 

 nervous system. If the impulse reaches the brain before 

 dying out, it will be felt as a sensation. On the other hand, 

 the brain can originate impulses which travel to the cells of 



