Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 283 



several dendrites via axons and collaterals to the dozens of muscles that 

 one uses in jerking away from the pin. 



How are nerve fibers nourished, especially the slender axons that reach far 

 from the cell body? Whether nerve fibers are inside the brain and spinal 

 cord or in the nerves outside, most of them are clothed with a soft, fatty, non- 

 cellular substance, the myelin sheath. Outside the brain and cord, that is, in 



Fig. 16.4. Diagram showing how a few nerve cells may communicate with many 

 others. Responses to one pin prick travel far. Sitting on a pin may cause a high 

 jump. (From The Living Body, Copyrighted 1952 by Henry Holt and Company. 

 Reprinted with their permission.) 



the nerves, each fiber is further protected by a cellular sheath, the neurilemma 

 (Fig. 16.3). The nerve cells within the brain and cord are supported by the 

 processes of neuroglial cells. These resemble nerve cells but have no con- 

 ducting power. The fatty myelin causes the whiteness of nerves and the white 

 matter of the brain and cord, which is composed of great numbers of mye- 

 linated fibers. Regions where no myelin is present appear gray, as in the 

 gray matter of the brain and cord and in certain nerves of the autonomic 

 nervous system. 



Regeneration of Nerve Fibers. A nerve fiber may be completely severed, 

 yet the part between the cell body and the cut may remain alive and regenerate 

 after the injury. On the side of the cut separated from the cell body the fiber 

 disintegrates since no cell body is left to nourish it, but its cellular neurilemma 

 tube persists and takes a remarkable part in the repair. As the regenerating 

 fiber grows longer, it actually enters the empty neurilemma sheath and finally 

 extends through its whole length. Later a new myelin sheath is formed 

 around each fiber, and with this healing of many fibers the function of the 

 nerve is finally restored. Sprouting nerve fibers sometimes cross a distance 

 of several millimeters to reach the neurilemma sheath without which effective 

 regeneration does not occur. In facial paralysis the hypoglossal nerve may be 

 cut and its proximal end sutured to the distal end of the facial nerve. The 



