COORDINATION 



81 



the will. The afferent neurone may be stimulated by some 

 external agent, such as light, heat, sound, pressure, etc., or 

 by some condition within the body itself, as when diseased 

 or abnormal conditions of the stomach or some other organ 

 induce vomiting. 



It is a common error to suppose that all actions which 

 are not called forth by the will are reflex. The essential 

 feature of a true reflex is the more or less direct action 

 of the afferent impulses 

 on efferent neurones and 

 not merely its nonvoli- 

 tional character. There 

 are, in fact, involuntary 

 actions in which the ef- 

 ferent neurones are di- 

 rectly stimulated not by 

 afferent neurones, but 

 by the condition of the 

 blood or in other ways. 

 Such actions are not re- 

 flex, though they may 

 be either involuntary 

 or unconscious or both. 

 They are known, in gen- 

 eral, as automatic actions, and we shall meet examples of them 

 as we proceed with the study of the various functions of 

 the body. 



12. Actions resulting from stimulation by the will. A wink 

 is not always a reflex action. We can wink " on purpose," 

 or, otherwise expressed, a wink may be called forth by the 

 will and entirely apart from the sudden appearance of some 

 object in front of the eye. Here the muscles of the eyelid 

 act in exactly the same manner as in a reflex wink, which 

 means that they are stimulated in the same way by the same 

 efferent neurones. Thus far the mechanism is the same in 



FIG. 47. The nervous mechanism shown in 



Fig. 46, with the addition of the afferent 



neurone c, from the cornea (see sect. 12) 



