160 THE NERVOUS SYSTEM. 



other extremity, within the brain, communicating with a 

 nerve-cell, the latter, in its turn, is excited, and the sensa- 

 tion of sight is produced. 



21. What sort of change takes place in the nervous 

 tissue when its excitability is aroused, is not known ; 

 certainly none is visible. On this account, it has been 

 thought by some, that the nerve-fibre acts after the man- 

 ner of a telegraph wire : that is, it transmits its messages 

 without undergoing any material change of form. But, 

 though the comparison is a convenient one, it is far from 

 being strictly applicable ; and the notion that nerve-force 

 is identical with electricity has been fully proved to be 

 incorrect. 



22. The Functions of the Nerves. The nerves 

 are the instruments of the two grand functions of the 

 nervous system, Sensation and Motion. They are not the 

 true centres of either function, but they are the conductors 

 of influences which occasion both. If the nerve in a limb 

 of a living animal be laid bare, and irritated by pinching, 

 galvanizing, or the like, two results follow, namely: the 

 animal experiences a sensation, that of pain, in the part to 

 which the nerve is distributed, and the limb is thrown into 

 convulsive action. WJien a nerve in a human body is cut 

 by accident, or destroyed by disease, the part in which it 

 ramifies loses both sensation and power of motion ; or, in 

 other words, it is paralyzed. We accordingly say that the 

 nerves have a twofold use, a sensory and motor function. 



23. If a nerve that has been exposed be divided, and the 

 inner end, or that still in connection with the nerve-cen- 

 tres, be irritated, sensation is produced, but no movement 

 takes place. But if the outer end, or that still connected 

 with the limb, be irritated, then no pain is felt, but mus- 



*v 



21 Change in the nervous tissues ? Nerve force and electricity ? 



22. Functions of the nerves ? In the case of the nerve of a living animal ? Of 

 the human body ? 



23, If an exposed nerve be divided? What is proved? The course of the sensory 

 set of fibres ? Of the motor set ? To what are they likened ? 



