CUTANEOUS AND INTERNAL SENSATIONS 1085 



of temperature sensations. For the conducting paths in the spinal 

 cord are not the same for tactile and for painful impression^. And 

 in certain cases of disease sensibility to pain may be lost, while 

 tactile sensations are still perceived ; or, on the other hand, pain may 

 be felt in cases where tactile sensibility is abolished. Loss of tem- 

 perature sensation, however, is usually accompanied by loss of 

 sensibility to pain. When a nerve is compressed, the sensibility 

 of the tract supplied by it disappears for cold sooner than for 

 warmth. 



Pain has been denned as ' the prayer of a nerve for pure blood.' The 

 idea is not only true as poetry, but, with certain deductions and limita- 

 tions, true as physiology; that is to say, pain, as a rule, is a sign 

 that something has gone wrong with the bodily machinery; freedom 

 from pain is the normal state of the healthy body. Physiologically, 

 pain acts as a danger-signal. It points out the seat of the mischief, 

 and even, in certain cases, by compelling rest, favours the process of 

 repair. Thus, the surgeon has sometimes looked upon 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 en- 

 tirely 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 aggravated 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 appropriate 

 stimulation of the normal skin, but by study of the defects or altera- 

 tions 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 experi- 

 ments made by skilled observers in whom one or more cutaneous 

 nerves were intentionally divided. 



An extensive investigation was made by Trotter and Da vies. 

 They divided at different times, extending over more than a 

 year, no fewer than seven of their own cutaneous nerves, in- 

 cluding 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 



