THE NERVOUS SYSTEM 



213 



Nerve fibers may be only the smallest fraction of an 

 inch in length, or they may be several feet long. Thus, 

 there are nerve fibers which run from the spinal cord to 

 the tips of the toes. The axis cylinders end in branches 

 running to muscle fibers, to glands, or they may end in 

 contact with sense cells in a sense organ. 



Wherever the nerve cells are abundant, the nerve tissue 

 has a gray color ; in other places, it looks white. Most 

 of the gray matter of the brain is on the surface. In the 

 spinal cord, the gray 

 matter lies within the 

 white matter, pre- 

 senting, in section, a 

 crude outline of the 

 letter H (Fig. 140). 



287. Work done by 

 Nerve Cells and Nerve 

 Fibers. The nerve 

 cells are highly active 

 masses of living matter which are nourished by material 

 brought to them by the blood. 



The nerve fibers are conductors of nervous impulses or 

 messages. They serve, not unlike telegraph wires, to con- 

 nect remote parts of the body with central nerve stations. 



Experiment 64. To show the structure of the nerves. Take a 

 small piece of a nerve, which may be easily obtained from the 

 market. Tease it lengthwise with needles on a glass slide. With 

 a hand lens, or even with the naked eye, the nerve is seen to be 

 made up of silky threads. 



Take one of the threads and fray it out as finely as possible on a 

 clean slide. Add a drop of saline solution, and examine under the 

 high power of the microscope. The nerve fibers are now seen as 

 exceedingly slender, white threads with a well-marked wavy outline. 



FIG. 132. Portion of a Medullated 

 Nerve Fiber. 



The axis cylinder is in the center. On either side 

 is seen the medullary sheath, represented by 

 dark lines. The primitive sheath, or neuri- 

 lemma, is on the outside and represented by 

 white lines in which is a nerve corpuscle with 

 an oval nucleus. 



