1084 THE SENSES 



(Kast and Meltzer). In the intact animal and human being painful 

 impressions can unquestionably be excited in the viscera by adequate 

 stimuli (p. 901). Thus, the spasmodic contraction of the intestines 

 and stomach causes the intense pain of colic and gastralgia. Labour 

 is an example of a strictly physiological function which is the 

 occasion of severe pain. It would appear from the observations 

 of Hertz that the only immediate cause of true visceral pain, as 

 distinguished from referred pain (p. 891) is distension acting on the 

 muscular coat of hollow organs and on the fibrous capsule of solid 

 organs. The sensation of pain in the alimentary canal is due to a 

 more rapid or a greater distension than that which constitutes the 

 adequate stimulus for the sensation of fulness. Visceral sensi- 

 bility seems to be exaggerated in such conditions as hypochondri- 

 asis, neurasthenia, and anaemia. Tissues normally insensible, or, 

 rather, but slightly sensible, to pain may become acutely painful 

 when inflamed. 



The question has been raised whether the sensation of pain can 

 be caused by excessive stimulation of the nerves of common tactile 

 sensibility, or of the nerves that subserve the sensations 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 stih 1 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 explana- 

 tion of the pain sensation is that it, too, is due to excitation of the 

 nervous apparatus for pain. Similarly (as was stated on p. 1082), 

 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 warm spots, and mingle 

 their specific response (cold 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 sensa- 

 tions 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 stimu- 

 lation 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 



