674 AN AMERICAN" TEXT-BOOK OF PHYSIOLOGY. 



sure, heat, and cold. The evidence for those of pressure and heat and cold is 

 the most satisfactory. 



Pain. Upon severe stimulation of the skin or muscles the normal person 

 experiences a distinct sensation of pain. There is, however, great variation 

 in the intensity of this sensation when the same stimulus is applied to different 

 persons. 



If we include abnormal persons, it is found that while in a few cases com- 

 plete absence of painful sensations has been noted the other sensations 

 remaining normal there are at the other end of the scale those cases in which 

 pain is produced by many stimuli, which would not have this effect on persons 

 in ordinary health. The capability of a given stimulus to produce pain is 

 therefore subject to wide variations according to the general condition of the 

 subject. 1 The same stimulus has different effects in a given individual accord- 

 ing to several circumstances. Peripheral irritation, such as an inflammatory 

 process in the skin, greatly increases the intensity of the pain caused by the 

 stimulation of the nerves supplying the locality. Continued stimulation of 

 the sensory nerves of the muscles and viscera has the same effect. 2 Local 

 anesthetics, such as cocaine, may reduce the sensibility to zero, and the same 

 follows the general anesthesia produced by chloroform, ether, nitrous oxide, 

 morphia, and similar drugs. Painful sensations are distinct and powerful 

 only when the stimulus is applied to general sensory nerve-trunks i. e. those 

 mediating cutaneous, muscular, and visceral sensibility while the nerves 

 which mediate the special sensations of light, sound, taste, and smell do not 

 give pain even on excessive stimulation. 



Limiting our observation, therefore, to the nerves of cutaneous sensibility, 

 it is found that the sensations of pressure, heat, and cold may all be present to 

 a normal degree, and yet increasing the stimulus be without effect in causing 

 any painful sensations whatever. This would represent a condition of com- 

 plete analgesia. Moreover, the capacity of the skin to cause abnormal painful 

 sensations upon the adequate stimulation of each of these groups of nerves 

 may be associated (in lesions of the central system) with any one group alone, 

 the abnormal pain-sensations thus produced being either those of excess or 

 deficiency. 



We advance the hypothesis, therefore, that each of these three sensations, 

 if pushed to excess, is usually accompanied by pain of gradually increasing 

 intensity. Therefore it is most probable that these nerves when slightly 

 stimulated mediate their proper sensations, but when this stimulus is pushed 

 to excess they can give rise to pain also, and that in the last instance this sen- 

 sation of pain may prove exclusive of any other. If this view is correct it 

 appears improbable that special pain-nerves exist. 



As various experiments show, increasing either the strength of the periph- 

 eral stimulus, the number of fibres to which it is applied, or the irritability of 

 the terminals of the fibres, will assist in arousing painful sensations. In the 



1 Strong : Psychological Review, 1895, vol. ii. No. 4. 



2 Gad und Goldscheider : Zeitschrift fur klinische Medicin, Bd. xx. 



