994 CUTANEOUS SENSATIONS. 



question, such a mechanism of pain have originated? Or, when origin- 

 ated, be preserved intact (although unused whole generations through), 

 in order occasionally, perhaps once in a lifetime, to come into use ? 



This difficulty is in the main, though not entirely, avoided hy the view that 

 visceral pain results from excessive stimulation of afferent structures normally 

 employed for reflex actions, accompanied only in some instances hy sensations 

 at all, and not, under usual circumstances, by feelings of physical pain. 



Further, it must be remembered that some of the most salient pains of 

 visceral disease are of the nature of "sympathies," "associate" or "referred" 

 sensations (Mitempfindungen). 1 These are clearly merely adjuncts of excessive 

 neural reaction, not always the effect of excessive stimulation, and yield no 

 support to a hypothesis of specific visceral pain-nerves. The visceral nerves 

 seem connected with the central neural mechanism for pain only through a 

 path offering high resistance. The central paths of pain are probably in all 

 cases high-resistance paths, but the resistance seems different in degree in 

 different pain-paths. It seems lowest in a certain set of the cutaneous paths. 

 The resistance, it seems, can be overcome in various ways, by intensifying the 

 stimulus, and also by repeating it (summation). When nerve impulses are en- 

 abled, e.g. by summation, to travel high-resistance paths, they show a tendency to 

 escape to neighbouring channels. Borrowing language from electrical termino- 

 logy, the tension of the charges seems to be high, and the ordinarily efficient 

 insulation then allows some leakage. In physiological parlance, by virtue of the 

 neural laws of irradiation and intermingling (overlap), the visceral afferent nerves, 

 thrown into excitement by excitation excessive in intensity or duration (condi- 

 tions in which pains arise) impart excitement to juxtaposed nervous apparatus, 

 and especially to the dolorific nerves of the skin. There is nothing in the so- 

 called referred pains of the viscera, either in regard to their mechanism or their 

 occurrence, that argues for the existence of specific pain-nerves in the viscera. 



As far, therefore, as analogy from the pains of the viscera and the 

 organs of muscular sense can guide, we are not given warranty for an 

 expectation that cutaneous pain shall be the outcome of the stimulation 

 of specific " pain-nerves." And if that supposition be adopted, logical 

 justice leads to the following dilemma. The adequate stimuli for 

 touch and thermal sense can evoke pain from the skin. A persistent 

 touch, e.g. the light pressure of a hat, not painful at first, may in time 

 become so. According to Weber, the merely areal increase of applica- 

 tion of heat to the skin can convert a non-painful heat stimulus 

 into a painful. Those who infer that painful cutaneous sensations 

 are called up only by specific pain-nerves will be consequent, 

 if they suppose that the skin possesses at least two species 2 of 

 pain fibres, equipped with specific end-organs, for which tangible and 

 thermal stimuli respectively are adequate. Analgesia for tactile stimuli 

 sometimes co-exists in the same skin with hyperalgesia for thermal 

 stimuli. 3 Painful mechanical stimuli have been in similar experiments 

 noted to elicit wholly different reflexes from those evoked by painful 

 thermal stimuli. 4 



Weber felt this difficulty when he adopted the long-current opinion that 

 nerves of " touch " and thermal sense yield pain when excited not mediately 



1 See above, p. 981. 



2 From the analogy of the existence of separate nerve-endings in the skin for sensation of 

 " cold " and "warmth " it would be probable that more than two, namely, three species of 

 pain-endings in skin would be demanded for pains from mechanical and thermal stimuli. 

 Two is, however, taken in the text as the smaller hypothesis. 



3 Strong, Psychol. Rev., N. Y. and London, 1896, vol. iii. 



4 E. Bruecke, " Vorlesungen ueber Physiologic," Wien, 1874, Bd. ii. S. 240. 



