REFLEX ACTION 341 



(technically termed a stimillus) applied directly, or in- 

 directly, to the ends of afferent nerves, and giving. rise to a 

 modification of the condition of the particles or molecules 

 which form the suVjstance of the nerve fibres, i.e., to a 

 molecular change, which is propagated from molecule 

 to molecule along the fibres to the central nervous system 

 with which these are connected. The molecular activity 

 of the afferent nerve sets up changes of a like order in the 

 fibres and cells of the central organ ; from these the dis- 

 turbance is transmitted along the motor nerves, which pass 

 from the central organ to certain muscles. And, when 

 the disturbance in the molecular condition of the efferent 

 nerves reaches the endings of those nerves in nmscular 

 fibres, a similar disturbance is communicated to the sub- 

 stance of the muscular fibres, whereby, in addition to the 

 production of certain other phenomena to some of which 

 reference has already been made (Lesson VII.), the parti- 

 cles of the muscular substance are made to take up a new 

 position, so that each fibre shortens and becomes thicker, 

 and a movement ensues. Thus, for instance, if we un- 

 intentionally prick one of our fingers or touch some very 

 hot object the hand is jerked away almost before we are 

 aware of what has happened. 



Such a series of molecular changes as that just described 

 is called a reflex action : the disturbance in the aft'erent 

 nerves caused by the irritation being as it were reflected 

 back, along the efferent nerves, to the muscles. But the 

 name is not a good one, since it seems to imply that the 

 molecular changes in the afferent nerve, the central organ, 

 and the efferent nerve are all alike, and differ only in 

 direction ; whereas there is reason to think that they differ 

 in many ways. 



The several structures necessary for the carrying out of 

 a muscular contraction, resulting in movement, in the 

 way we have described may be made clear by the following 

 diagram. 



The stimulus is applied to a sensory surface (S) j the 



