212 



OUR BODIES AND HOW WE LIVE 



receiving the news, at once sends back its commands as to 

 what must be done. In brief, countless dispatches are sent 

 to and fro with wonderful rapidity and unerring precision. 



Thus, if we accidentally pick up a hot coal, we drop it 

 instantly. A nervous impulse or message is sent from the 

 nerves of touch in the fingers to the cerebro- 

 spinal center, which hurries off its orders to 

 the muscles of the fingers to drop the burn- 

 ing coal. 



286. Nerve Cells and Fibers. Nerve tissue 

 is really made up of a great number of distinct 

 units called nerve cells. Each cell usually 

 contains a large nucleus and gives off one or 

 FIG. 130. more tiny branches, or processes. These cells 

 Nerve Cells var y more in shape and size than any other 



Cdls ln the b d y- Each nei " Ve Cdl haS a 



number of short branches, and many have 

 also one long branch which can be traced for some distance 

 from the cell body. This rootlike process is called an axis 

 cylinder. This is the beginning 

 of a nerve fiber. 



In most fibers a layer of 

 white, fatty substance, called 

 the medullary sheath, protects 

 this soft, gray central core, the 

 axis cylinder, as a kind of insu- 

 lating material, just as electric 

 wires are sometimes covered 

 with waxed paper or thread to 

 prevent the escape of the current. Outside of this is 

 another transparent sheath, or covering, called the neuri- 

 lemma. Some fibers lack the white medullary sheath. 



FIG. 131. Nerve Cells from the 

 Gray Matter of the Brain. 



