viii TOUCH 355 



separate "sense" of pain. But there are certain facts 

 which show that sensations of pain are distinct from other 

 sensations. In the first place there is a protopathic 

 sensation of pain, in many places in which there is no such 

 sensation of contact. Again there are certain situations 

 in the body which are incapable of appreciating touch, 

 but which readily feel pain, and in many diseases of the 

 nervous system, such as locomotor ataxy, the sensitive- 

 ness of the skin to touch may be almost entirely wanting, 

 while pain is readily felt. Further, observation shows 

 that the impulses giving rise to pain, as also those 

 resulting from heat and cold, pass along the spinal cord 

 on their way to the brain by paths which are distinct 

 from those which convey the impulses resulting from mere 

 touch. 



(iv) The Localisation of Tactile Sensations. — Certain 

 very curious phenomena appertain to the sense of touch ; 

 some of these are probably in part due to varying 

 anatomical arrangements, to the varying thickness of the 

 epidermis, and to the abundance or scantiness of special 

 end-organs. Not only is tactile sensibility to a single im- 

 pression much duller in some parts than in others — a cir- 

 cumstance which might in many cases be accounted for by 

 the different thickness of the epidermic layer — but the 

 power of distinguishing double simultaneous impressions is 

 very different. Thus, if the ends of a pair of compasses 

 (which should be blunted with pointed pieces of cork) are 

 separated by only one-tenth or one-twelfth of an inch, 

 they will be distinctly felt as two, if applied to the tips 

 of the fingers ; whereas, if applied to the back of the 

 hand in the same way, only one impression will be felt ; 

 and, on the arm, they may be separated for a quarter of 

 an inch, and still only one impression will be per- 

 ceived. 



Accurate experiments have been made in different 

 parts of the body, and it has been found that two points 



A A 2 



