THE RECEPTORS 863 



stimuli, acting together on sense organs which give rise to sensations of 

 different quality. The nature of the resulting sensation is consequently 

 modified. Water heated to 40 C. feels warm to the hand and stimulates 

 only the heat spots. At 45 C. the cold spots and pain spots are also 

 excited, and the resulting sensation takes on a distinctly different qual- 

 ity which we call "hot." Since pain is excited only by extreme in- 

 tensities of stimuli which can set up other qualities of sensation, it is 

 natural to find painful sensations varying considerably in their quality, 

 depending on the sensations which occur in association with them. 

 Thus a throbbing pain is due to the simultaneous pressure produced by 

 dilated blood vessels. 



It is interesting to find that certain qualities of sensation are incompa- 

 tible with one another. When stimuli capable of setting up two such incom- 

 patible sensations occur at once, one sensation is suppressed or inhibited by 

 the other. An interesting experiment is described by Head which illustrates 

 this. The tip of the glans penis is supplied with receptors for cold and pain, 

 but may be devoid of heat spots. If it is dipped into water at 40 C. the 

 pain spots alone are stimulated and a disagreeable, painful sensation re- 

 sults. If the temperature is raised to 45 C. the cold spots also are stimu- 

 lated, the pain is displaced by a vivid sensation of cold. About the corona 

 of the penis heat spots also occur. If this region is also immersed, the 

 quality of the sensation changes to one of exquisitely pleasant warmth. If 

 the water employed in the experiment is at a temperature higher than 

 45 C. the painful sensation persists and no sensation of warmth is felt. 

 The sensations of pain and pleasant warmth are incompatible and cannot 

 occur simultaneously. Which one will succeed in gaining control of 

 consciousness and in suppressing the other depends on the relative 

 strength of their stimuli. An entirely analogous phenomenon occurs in 

 the competition of incompatible reflexes for the control of a common 

 motor path (page 947). 



Noxious stimuli give rise to an exceedingly impelling sensation and 

 to reflexes which can dominate over any others which may be set up 

 at the same time. The response to painful stimulation is of a protective 

 character and it is imperative that such stimuli and their sensations 

 should control the activity of the organism when they arise with in- 

 tensity. On the other hand, if the noxious stimuli are near the thresh- 

 old value of intensity, and little danger is threatened, it is an advan- 

 tage that its effects be suppressed so that the organism may react with 

 a discretion based on data derived from other sensations as well. 



The Distribution of Sensitivity in the Body 



fSensations of touch, heat, cold, and pain are felt generally throughout 

 the surface of the body, with the exception of certain limited areas such 

 iis we have referred to, from which one or another quality of sensation 



