CHAP, vi.] SOME OTHER SENSATIONS. 1427 



to a given area of skin contains fibres which subserve the sense of 

 touch exercised by that area, and which pass from the terminal 

 organs in that area, whatever their nature, to the parts of the 

 central nervous system, whatever they may be ( 679), which act 

 as centres of touch sensations. If these fibres Avhen directly stimu- 

 lated, apart from their terminal organs, necessarily give rise to 

 touch sensations, stimulation of the nerve itself while running in 

 the subcutaneous tissue should give rise to touch sensations. But 

 experience shews, as we said a little while ago, that this is not the 

 case. Whenever the nerve fibres themselves are directly stimulated, 

 as for instance when the epidermis is removed from the skin or 

 when a nerve is laid bare, then however they be stimulated, be 

 the stimulus weak or strong, if consciousness be affected at all, 

 the affection takes on the form of pain ; psychological examination 

 of the subjective result discloses nothing that can be called a 

 sensation of touch. A familiar instance of the difference between 

 the effects of stimulating a nerve trunk, and those of stimulating 

 the cutaneous terminal organs of special sense, is seen in the effect 

 of dipping the elbow into a freezing mixture. The cold affects 

 the skin of the elbow and gives rise to sensations of cold in that 

 part ; but the cold, if intense enough, also affects the underlying 

 trunk of the ulnar nerve, and by direct stimulation of the fibres in 

 the trunk develops sensory impulses ; these impulses however are 

 those not of sensations of cold, but of pain ; and the pain, in 

 accordance with a principle to which we shall presently call 

 attention, is referred to the terminal distribution of the ulnar 

 nerve on the ulnar side of the hand and arm. In speaking above 

 ( 883) of pain we said that excessive pressure or excessive heat 

 or excessive cold applied to the skin, overrides or annuls pressure 

 and temperature sensations and gives rise to mere sensations of 

 pain ; and it might be urged that when a nerve is directly 

 stimulated the specific sensations of touch and temperature are 

 similarly annulled. But in the case of the skin an excessive or 

 violent stimulation is necessary to produce this effect, whereas a 

 nerve may be directly stimulated by so slight a stimulus as to 

 give rise to hardly more than discomfort without distinct pressure 

 or temperature sensations being felt ; and we can hardly suppose 

 that in such a case these are present but are annulled by an 

 amount of pain so slight as that which is produced. Thus making 

 every allowance for the suggestion that sensations of pain may 

 override and obscure concomitant sensations of touch and tempe- 

 rature, we seem driven to the conclusion that the latter sensations 

 can only be developed by help of special terminal organs, and that 

 a stimulation of the nerve fibres themselves if it produces any 

 effect at all on consciousness gives rise to pain, and to pain alone. 



We are in this way led to conceive of the skin as provided on 

 the one hand with specific fibres ending in specific terminal organs 

 and serving for sensations of touch and temperature, and on the 



F. 91 



