The Nervous System 



209 



/> 318, The Functions of the Nerve Cells and Nerve Fibers. 

 The nerve cells are a highly active mass of living material. 

 They find their nourishment in the 

 blood, which is supplied to them in 

 abundance. We may think of the 

 nerve cells as a sort of a miniature 

 manufactory, deriving their material 

 from the blood, and developing from 

 it nervous energy. 



' The nerve fibers, on the other hand, 

 are conductors of nervous energy, They 

 furnish a pathway along which the 

 nerve energy generated by the cells 

 may travel. The fibers can also gener- 

 ate energy, yet it is their special func- 

 tion to conduct impulses to and from 



the cells 



319. The Nervous System compared 

 to a Telegraphic System. It will help 

 us the better to understand the com- 

 plicated functions of the nervous system 

 if we compare it to a telegraph system. 

 The brain and the spinal cord are the 

 main offices, and the multitudes of nerve 

 fibers branching off to all parts of the 

 body are the wires. By means of these, 

 nerve messages are constantly being sent 

 to the cerebro-spinal center to inform it 

 of what is going on in various parts of 

 the body, and asking what is to be done 

 in each case. The cerebro-spinal center, 

 on receiving the intelligence, at once 

 sends back the required instructions. Countless messages are 

 sent to and fro with unerring accuracy and marvelous rapidity. 



Thus, when we accidentally pick up something hot, it is 

 instantly dropped. A nerve impulse passes from the nerves of 



?IG. 113. Medullated 

 Nerve Fibers. 



a medullated nerve fiber, 

 showing the subdivision 

 of the medullary sheath 

 into cylindrical sections 

 imbricated with their 

 ends; a nerve corpuscle 

 with an oval nucleus is 

 seen between the neuri- 

 lemma and the medullary 

 sheath. B, a medullated 

 nerve fiber at a node or 

 constriction of Ranvier; 

 the axis cylinder passes 

 uninterruptedly from one 

 segment into the other, 

 but the medullary sheath 

 is interrupted. 



