62 



ESSENTIALS OF PHYSIOLOGY. 



great importance, since, if they were taking place simultaneously, 



neither would be carried out effectively. 



Facilitation. When a stimulus which is insufficient to produce 



a reflex response is repeated at short intervals, the reflex is often 



ultimately evoked (fig. 15). Evidently the preceding stimuli, though 



causing no visible response, bring 

 about some change in the neurons of 

 the reflex arc whereby the stimulus 

 finally becomes effective. This process 

 is known as facilitation, and forms the 

 basis of habit. Each time a reflex 

 action takes place it becomes easier for 

 it to be brought about on a subse- 

 quent occasion. This is well seen in 

 the case of many skilled movements, 

 which, in the first instance, are learnt 

 by voluntary effort; in time the ad- 

 justment of the impulses concerned in 

 the carrying out of these movements 

 becomes so exact that, in response to 

 a suitable stimulus, they take place 

 without voluntary effort and almost 

 independently of consciousness. 



By these means reflex actions are 

 so adjusted as to bring about a definite 

 movement as rapidly as possible in 

 response to a suitable stimulus. For 

 this purpose the co-existence of con- 

 sciousness is not necessary, indeed it 



is sometimes a hindrance. Many re- 

 ft IG. 14. Scratch reflex temporarily * 



inhibited by application of a flexes, particularly those occurring in 

 painful stimulus to foot. (Star- connect i on w i t h the visceral system, 

 ling's Principles of Physiology.} J . 



take place without affecting conseious- 



Signal A, stimulation of scratch area. 



Signal B, stimulation of paw by strong ness at all. Other reflexes, for instance 



induction shock. 



the closing of the eye when an object 



approaches it suddenly, are associated with consciousness, though the 

 reflex act precedes and takes place independently of consciousness. A 

 third group of reflexes, for example micturition, can to some extent 

 be modified by voluntary impulses. 



The Knee Jerk. Reflex action may be brought about not only by 

 stimuli falling upon the surface of the body, but by stimuli arising 

 within the body itself, for instance in the joints and muscles. Reflexes 



