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A TEXTBOOK OF PHYSIOLOGY 



In regard to the nature of the stimuli, it has been found that 

 stronger the stimulus the quicker the response (Fig. 4.04). This can 

 be tested upon the pithed frog by various strengths of acid, as 

 shown in Fig. 405. The stronger impulse obtains possession of the 

 final common path, and that painful stimuli (nociceptive) and those 

 evoking sexual feelings are more potent than other forms. The 

 strength of the stimulus required varies according to the length of 

 time of application. Fatigue takes place in the nerve-endings; 

 therefore, after a time, a stimulus applied to another reflex arc may 

 capture the final common path. 



FIG. 404. EFFECT OF INTENSITY or STIMULUS ON SCRATCH REFLEX. 



A, Stimulus is very weak: one small beat of characteristic slowness is evoked after 

 long latent pi-riod. B, increase in intensity of shocks with resulting shorter 

 latent time and a reflex movement of two feeble beats. C, further increase of 

 intensity of stimulus; latent period shorter and a reflex of ten fairly quick and 

 ample beats ensues. The stimulus lasted less than a half-second; the reflex is 

 not completed for more than two seconds after cessation of stimulus. (From 

 Sherrington-'s " Integrative Action of the Nervous System," by permission of 

 Yale University Fress and Messrs. Constable and Co., Ltd.) 



There is one exception, however, to the above rules. This i.s 

 when the stimuli are of the same character and in adjacent areas. 

 It may be exemplified as follows: If, as the result of a stimulus in 

 one part of the saddle, the animal be set scratching, then a similar 

 stimulus applied in an adjacent area of the saddle will cause an in- 

 creased sweep in the scratching movements, so that both irritants 

 will be dealt with and removed at the same time. Similar stimuli 

 in adjacent areas reinforce one another, and increase the general 

 reaction. 



Another important point to be noticed in such reflex responses is 

 that there is reciprocal excitation and inhibition ot muscles (Fig. 407). 

 The response to a stimulus is not simply the contract ion of a muscle or 



