THE NERVOUS SYSTEM 



119 



FIG. 104. LARGE NERVE TRUNK, 

 such as supplies the muscles. 

 Cross-section (magnified 6 diam- 

 eters), showing bundles of nerve 

 fibers. (Peabody.) 



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 dcndrons or 



dendrites (SQQ 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 



X \ / I 



} I II 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. 

 104). 

 White and Gray Fibers (Fig. 105).- 



6 W 



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, are w }^ te anc j glistening, and are called 



two gray fibers 



(without sheath), w kite fibers. Others are without this fatty 



