200 



Dr. H. Head. 



from the neighbourhood of the index knuckle, the direct effect of the hot 

 stimulus in contact with the thumb reasserted itself as pleasant warmth and 

 the pain disappeared. 



The behaviour of the normal glans penis ([9], p. 274) forms another 

 excellent illustration of such inhibition. It is not uncommon to find that 

 the punctate end-organs with which it is endowed are not uniformly dis- 

 tributed ; the tip of the glans around the meatus may be devoid of heat- 

 spots, but sensitive to cold and to pain. In such a case we carried out the 

 following remarkable experiment. The end of the penis was dipped into a 

 glass containing water at 40° C. ; since no heat-spots were present and this 

 temperature has no effect upon the cold-spots, the only sensation evoked was a 

 peculiarly disagreeable pain. When, however, the temperature of the water 

 was raised to 45° C, pain was to a great extent displaced by a vivid sensation 

 of cold, due to stimulation of the cold-spots. Instead of increasing the 

 discomfort an elevation of temperature ceased to be strictly painful because of 

 the appearance of the specific sensation of " paradox cold." But around the 

 corona the penis is always well furnished with heat-spots in addition to those 

 for cold and for pain ; as soon then as the water at 45° C. covered the corona 

 without reaching the foreskin, both cold and pain disappeared, giving place to 

 an exquisitely pleasant sensation of heat. 



In none of these cases was the process of selective inhibition in any way 

 conscious. The sensation evoked was a definite one of heat, of cold, or of 

 pain; but it was the final issue of a struggle between incompatible and 

 mutually antagonistic afi'erent impulses. The whole process takes place on 

 the physiological level, and would have remained incomprehensible, had it not 

 been for our knowledge of the phenomena of sensory dissociation. 



Many impulses capable of forming the basis of a sensation are prevented 

 under normal circumstances from reaching the highest centres ; or, if their 

 forward path is not completely barred, they pass on in a profoundly modified 

 form, in consequence of the concurrent activity of other sensory end-organs. 

 The utility of this arrangement is obvious, especially with those impulses 

 which underlie discomfort or pain. 



Pain is the oldest defensive reaction and potentially unpleasant stimuli are 

 the basis of most primitive reflexes. It is, therefore, most important that the 

 impulses they evoke should be diminished in extent, or actually inhibited in 

 favour of those sensory impressions more capable of leading to discriminative 

 action. The mechanism underlying the production of pain and discomfort 

 must, however, remain in full physiological activity, so that it can play its 

 part in defence of the organism, when noxious stimuli reach a high grade of 

 intensity. 



