8 BIOLOGY AND SOCIAL PROBLEMS 



and complexity of their arrangement, they 

 can be grouped conveniently under the three 

 classes, afferent, efferent, and internuncial. 



The general arrangement of the neurones 

 in any complex nervous system, as indicated 

 in the preceding classification, foreshadows in 

 a way the simplest complete nervous act, the 

 reflex. When an animal is stimulated, it usu- 

 ally responds almost immediately by a move- 

 ment. In most animals this operation is carried 

 out by the nervous system and the appended 

 motor organs, the muscles. Such a simple 

 neuromuscular operation is called a reflex. It 

 involves the stimulation and activity of an 

 afferent neurone, whereby a nervous impulse 

 is sent into the central organ from which issues 

 over an efferent neurone a motor impulse that 

 brings a muscle or a group of muscles into 

 play. Such an action can be conceived to be 

 restricted to the two classes of neurones men- 

 tioned, namely, the afferent and the efferent, 

 but probably in the great majority of actual 

 occurrences internuncial neurones are included, 

 and these strictly central elements mediate be- 

 tween the central ends of the efferent and of 

 the afferent neurones. The reflex is thus a 

 physiological unit, so to speak, in the action 



