232 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



tion of the sensory nerves of the muscles and viscera has the same effect. 1 

 Local anaesthetics, such as cocaine, may reduce the sensibility to zero, and 

 the same follows the general anaesthesia produced by chloroform, ether, nitrous 

 oxide, morphine, and similar drugs. Painful sensations are distinct and 

 powerful only when the stimulus is applied to general sensory nerve-trunks — 

 /'. <■., 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 in exceptional cases that the sensations of pressure, heat, and cold 

 may all be present to a normal degree, and yet increasing the stimulus he 

 without effect in causing any painful sensations whatever. This would 

 represent a condition of complete analgesia. Moreover, the capacity of the 

 >kin to cause abnormal painful sensations upon the adequate stimulation of 

 each of these groups of nerves may he associated (in lesions of the central 

 system) with any one group alone, the abnormal pain-sensations thus pro- 

 duced being either excessive or deficient. 



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 

 sensation of pain may prove exclusive of any other. H 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 last analysis the physiological condition for pain is excessive stimulation, 

 which by all analogy must mean excessive discharge within the central 

 system. The changes following this discharge into the central system are 

 not such as lead to co-ordinated muscular responses, but to convulsive reac- 

 tions of a very irregular character. Where this process takes place in the 

 central system we do not know. As to normal analgesia, it must be looked 

 upon as dependent on a condition in which excessive stimulation cannot be 

 produced ; and we find this condition normally present only in the case of 

 the nerves of special sense. 



Since in the pathological conditions one sort of sensibility may lie lost 

 while the others remain, it has been inferred that there are separate fibres 

 for the conveyance of each sort of sensation. This idea was expressed in the 

 law of the specific energies of nerves as formulated by Joannes Muller, who 

 pointed out that in many cases the same nerve might be stimulated in any 

 way — mechanically, electrically, or chemically, as well as in the normal physi- 

 ological ma nner; and that in all cases the mode of the response was the same — 

 a sensation of light or taste or contact, as the case might be. Hence it was 

 1 Gad mill Goldscheider : Zeitschrift fiir klinische Me'dicin, Bd. xx. 



