THE NERVOUS SYSTEM 



285 



not yet been settled by physiologists whether the ends of 

 the dendrons of communicating cells actually touch. 



518. The Axis Cylinder, or Axon. In most of the fully 

 developed nerve cells, one of the branches is very greatly 

 prolonged as a fine thread of protoplasm, 



which becomes the core of a nerve fiber 

 (Fig. 193). Every nerve fiber has such a 

 core. In most of them this central thread 

 of protoplasm becomes covered with two 

 coats ; an outside protective coat forms 

 a tube, called the connective sheath : and 

 between the walls of this tube and the 

 central thread is a coat of semiliquid, fatty 

 substance called the medullary sheath, 

 which, shining through this outer sheath, 

 gives a silvery white appearance to the 

 fiber. Such fibers are called medullated 

 fibers, or white fibers (Fig. 194). Some 

 fibers lack the medullary sheath, and are 

 called non-medullated) or gray fibers (Fig. 



194). 



519. A Neuron. One nerve cell to- 

 gether with all its branches, both dendrons and axon, is 

 called a neuron. A neuron is the unit, and the nervous 

 system is built up of an enormous number of these units, 

 together with supporting tissue (Fig. 193). 



520. A Nerve. Many nerve fibers may be bound to- 

 gether by connective tissue until they form a cord large 

 enough to be seen by the unaided eye (Fig. 195). A bun- 

 dle of nerve fibers is called a nerve. The sciatic nerve in 

 the thigh is as large as the end of the little finger and strong 

 enough to support the weight of the body. The fatty 

 sheaths of the fibers are supposed to insulate the fibers so 

 that a nerve impulse cannot go across to another fiber; they 

 serve the same purpose as the insulating substances with 



FIG. 194. 



c, medullated or white 

 nerve fiber; rf, two 

 non-medullated or 

 gray fibers. 



