LESSON VIII 

 SENSATIONS AND SENSORY ORGANS 



1. Movement the Result of Reflex Action.— The 



agent by wliich all the motor oi'gans (except the cilia) 

 described in the preceding Lesson are set at work, is 

 muscular fibre. But, in the living body, muscular fibre 

 is, as a rule, made to contract by a cliange which takes 

 place in the motor or eflferent nerve, wliich is dis- 

 tributed to it. This change again is generally effected by 

 the activity of the central nervous system, with which 

 the motor nerve is connected. The central organ is 

 thrown into activity, directly or indirectly, by the in- 

 fluence of changes which take place in nerves, called 

 sensory or afferent, which are ccmnected, on the one 

 hand, with the central organ, and, on the other hand, 

 with some other part, usually on the surface, of the body. 

 Finally, the alteraticm of the afferent nerve is itself pro- 

 duced by changes in tlie condition of the part of the body 

 with which it is connected ; which changes usually result 

 from external impressions brought to bear cm that part. 



Sometimes the central organ enters into a state of 

 activity without our being able to trace that activity to 

 any dii'ect influence of changes in afferent nerves ; the 

 activity seems to take origin in the central organ, 

 and the movements to which it gives rise are called 

 "spontaneous." Putting these cases on one side, it 

 may be stated that a movement of the body or of a 

 part of it, is to be regarded as the efiect of an influence 



