974 A MANUAL OF PHYSIOLOGY 



of coolness and warmth. It is true that when the skin is lightly 

 touched in the region of a touch-spot with a small object at its 

 own temperature the sensation is one of pure touch. As the 

 pressure is increased, a sensation of pressure, quite distinct from 

 that of contact, may be felt ; and if the pressure is still further 

 increased, a sensation of pain may be elicited. It seems to be 

 quite clearly made out that the pressure sensation in this case 

 is due not to excessive stimulation of the touch-nerves, but to 

 stimulation of the specific pressure-nerves when the threshold 

 is reached. The most natural explanation of the pain sensation 

 is that it, too, is due to excitation of the nervous apparatus 

 for pain. Similarly (as was stated on p. 972), if the skin 

 is raised to higher and higher temperatures, the response 

 is at first a pure sensation of warmth, increasing in intensity 

 without changing its quality. When a certain temperature 

 (about 45 C.) is exceeded, the sensation changes to ' hot,' either 

 because a pain element is now added to the pure thermal 

 sensation, or because the cold spots are now stimulated as well 

 as the wa r m spots, and mingle their specific response (co)d 

 sensation) with that of the warm spots. Further increase 

 of the temperature will cause distinct pain, the sensation 

 assuming a burning character. When a cold spot is tested 

 with decreasing temperatures, an analogous series of sensations 

 is run through, the pure sensation of coolness eventually giving 

 place to cold, intense cold, and finally pain. Here, also, it is 

 simplest to assume that the pain sensation is caused not by 

 excessive stimulation of warm or cold spots, but by excitation 

 of the specific pain-spots. In any case, there is no doubt that 

 afferent ' pain ' fibres exist which are anatomically distinct from 

 the fibres of tactile and of temperature sensations. For the 

 conducting paths in the spinal cord are not the same for tactile 

 and for painful impressions. 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 temperature 

 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 

 limitations, 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. Physio- 

 logically, 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 



