THE SENSES 975 



pain as " Nature's splint.' But, as a matter of fact, a certain amount 

 of pain occurring at intervals is not incompatible with high health ; 

 and probably nobody, even when accidents and indiscretions of all 

 kinds are avoided, is entirely free from pain for any considerable 

 time. Sometimes, indeed, the mere fixing of the attention on a 

 particular part of the body is sufficient to bring out or to detect a slight 

 sensation of pain in it ; and it is matter of common experience that a 

 dull continuous pain, like that of some forms of toothache, is aggra- 

 vated by thinking of it, and relieved when the attention is diverted. 



As to the sensations of tickling and itching, it is enough to 

 say that physiologists are not agreed whether they represent 

 specific sensibilities subserved by special nerves distinct from 

 those of touch and pain, or merely modifications or mixtures of 

 these sensations. 



Phenomena observed after Section of Cutaneous Nerves. 

 The innervation of the skin can be explored not only by appro- 

 priate stimulation of the normal skin, but by study of the defects 

 or alterations of sensibility which follow section of a cutaneous 

 nerve, and which may be observed at different stages in its 

 regeneration. In recent years this has proved a fruitful method, 

 especially in experiments made by skilled observers in whom 

 one or more cutaneous nerves were intentionally divided. 



Quite recently a very elaborate investigation has been made 

 by Trotter and Davies. They divided at different times, ex- 

 tending over more than a year, no fewer than seven of their own 

 cutaneous nerves, including the internal saphenous at the knee, 

 the great auricular, three divisions or branches of the internal 

 cutaneous of the arm just below the elbow, and a branch of the 

 middle cutaneous of the thigh. The operations were purposely 

 done at such intervals as would allow the experience gained in 

 investigating one area to be applied to others. About a quarter 

 of an inch was cut out of each nerve, and the ends then sutured 

 together. ' In each case the area of skin supplied by the nerve 

 showed defects in seven distinct functions : four sensory namely, 

 sensibility to touch, cold, heat, pain and three motor namely, 

 vaso-motor, pilo-motor, sudo-motor (sweat-secretory). The 

 sensory changes showed a central area of profound loss, an area 

 of moderate extent surrounding this of partial loss, and a large 

 area in which a qualitative change could be alone detected.' 

 The maximal extent of change, and therefore the outer boundary 

 of this third area, can be mapped out by getting the subject to 

 determine by light, stroking touches the area which feels in any 

 way unnatural when he touches it himself. The most common 

 feeling is that the skin has become smoother at the boundary as 

 the stroking finger crosses it, coming from the normal skin. This 

 area is always much larger than the area included in it, in which 

 by quantitative methods e.g., the use of a very fine camel's- 



