THE NERVOUS SYSTEM 



119 



called protoplasm. The protoplasm in nerve cells possesses 

 the most marvelous and varied powers of any known sub- 

 stance, for the nerve cells are 

 the seat of the mind. 



Nerve Cells and Fibers. 

 The many branches of nerve 

 cells make them the most 

 remarkable of all cells for 

 irregularity in shape. Since 

 the protoplasm of the cell con- 

 tinues into the fibers, it is 

 plainly wrong to consider the 

 nerve cell as something apart 

 from its fibers. It is not a 

 complete cell without them. 

 A cell usually has many short 

 branches called dendrons or 



FIG. 104. LARGE NERVE TRUNK, 

 such as supplies the muscles. 

 Cross-section (magnified 6 diam- 

 eters) , showing bundles of nerve 

 fibers. (Peabody.) 



dendrites (see Fig. 103) for communicating 

 with near-by cells, and one long branch 

 called an axon (Fig. 103) for communicat- 

 ing with distant parts. The axons form 

 the fibers that go to the skin, muscles, 

 and other organs. 



A Nerve. These long branches, or 

 axons, of nerve cells go all over the body 

 and are often bound together into visible 

 cords called nerves, or nerve trunks (Fig. 



V) 



White and Gray Fibers (Fig. 105). 

 Some fibers have a fatty covering sur- 



FlG. 105. c , a white 



fiber with its fatty rounding the thread of protoplasm ; they 



sheath (dark) ; d, ^ & white and glistening and are called 

 two gray fibers 



< without sheath), white fibers. Others are without this fatty 



