i CUTANEOUS SENSIBILITY 51 



clinical point of view. But owing to the imperfect methods 

 employed, the results are very scanty and do not seem worth 

 mention. Taken as a whole they show that the dissimilar sensi- 

 bility to pain of different regions of the skin depends largely 

 upon the varying depth of the horny layer. Not improbably it 

 also depends on the varying number of the pain spots in 

 different regions, but methodical investigation of this difficult 

 and delicate subject is still wanting. 



We saw above that the cornea is rich in pain spots and 

 contains no true touch spots ; the conjunctiva of the eye and the 

 glands are rich in cold and also in pain spots; the mucous 

 membrane of the cheeks, the posterior part of the buccal cavity, 

 the posterior part of the tongue, have little sensitiveness to pain. 

 According to Kiesow's observations, in some parts of the mucous 

 membrane of the cheeks (e.g. those corresponding to the second 

 lower molar) pain spots are entirely absent : pain is not produced 

 here by the strongest mechanical and electrical stimuli. 



Comparison of the results obtained on exciting the pain spots 

 and touch spots respectively shows the following differences : 



(a) The threshold of sensibility to punctiform mechanical 

 stimuli is, generally speaking, higher for pain spots than for touch 

 spots ; but the relation between the two thresholds varies in the 

 different regions and may be reversed (v. Frey). 



(5) The threshold for electrical stimuli (faradic currents 

 applied by the unipolar method) is higher for touch than for 

 pain spots. 



(c) Faradisation of the pain spots at a frequency not exceeding 

 20 shocks per second arouses a continuous sensation, while fara- 

 disation of the touch spots up to a frequency of 130 per second 

 produces discontinuous sensations of vibration. 



(d) The latent time for pain is always much longer than for 

 contact. The after-effect also is incomparably longer (Eichet). 



(e) The sensation aroused by the stimulation of a tactile spot 

 is projected to the surface of the skin, and may be confined to 

 one spot; the sensation of pain aroused on stimulating a pain 

 spot seems to spread superficially as well as deeply, and has no 

 precise local sign. But, according to Ponzo's latest work, pain 

 sensations too are projected to the cutaneous surface and localised 

 there. 



(/) Cooling of the skin produces hyperaesthesia, followed by 

 loss of sensibility. The paralysis of the pain spots invariably 

 precedes that of the touch spots. Cocaine applied to the tongue 

 abolishes first tactile and then pain sensibility. 



When we reflect on the teleological importance of sensi- 

 bility to pain, it is readily seen to be one of the most effective 

 weapons of defence of the organism ; but it appears to be unequally 

 developed at different degrees of the animal scale. We have no 



