CUTANEOUS AND DEEP SENSATION 283 



patches of skin which seem to be totally insensitive. Each 

 of these spots when stimulated causes but one kind of 

 sensation, however stimulated ; a cold spot touched with 

 a hot object feels cold. 



Touch. — -The touch-spots are arranged especially aromid 

 the roots of the hairs. Hairs considerably increase the 

 sensitiveness of the skin to touch, by their leverage stimu- 

 lating the nerve-endings which are in intimate association 

 with their roots. 



The number of touch-spots per unit area varies in 

 different parts of the body, and with this is associated a 

 corresponding variation in the power of accurately locahsing 

 the point stimulated and of discriminating between one 

 stimulus and two stimuh apphed at the same time. The 

 power of discrimination is greatest at the tip of the tongue, 

 where two stimuli about 1 mm. apart are distinguished, 

 least on the back, where two spots touched are not 

 recognised as two unless they are about 70 mm. apart. 



A rough estimation of the degree of sensitiveness to 

 touch can be measured by means of Von Frey's hairs. 

 These are hairs of different thickness mounted on handles. 

 Knowing the pressure which just bends the hairs we can 

 tell the pressure required to evoke a sensation. 



Pain. — Pain is the affective aspect of a stimulus which 

 is harmful and which therefore tends to evoke a pro- 

 tective motor response. The different kinds of pain are 

 probably due to the coincident stimulation of other sense- 

 organs. A tingling pain, for instance, would be caused by 

 the coincident stimulation of pain- and touch-spots. Loss 

 of sense of pain without loss of other forms of sensation 

 is known as analgesia. 



Temperature. — The sense of temperature is more acute 

 in some parts of the body than in others. In general it 

 may be said to be less acute on the exposed parts and in 

 the mouth. 



Several forms of nerve-endings are present in the skin. 



