258 INTRODUCTION TO NEUROLOGY 



comes into consciousness at all (of course, a large proportion of such reactions 

 are strictly reflex and have no conscious significance). Conversely, the 

 impediment to such discharge, no matter what the occasion, results in a 

 stasis in the nerve centers, the summation of stimuli and the development 

 of a situation of unrelieved nervous tension which is unpleasant until the 

 tension is relieved by the appropriate adaptive reaction. Such a stasis 

 may be brought about by a conflict of two sensory impulses for the same 

 final common path (see p. 59), by the dilemma occasioned by the necessity 

 for discrimination in an association center between two or more possible 

 final paths, by fatigue, auto-intoxication, or other physiological states which 

 lower the efficiency of the central mechanism, and by a variety of other 

 causes. The unrelieved summation of stimuli in the nerve centers, involving 

 stasis, tension, and interference with free discharge of nervous energy, gives 

 a feeling of unpleasantness which in turn (in the higher types of conscious 

 reaction at least) serves as a stimulus to other associated nerve centers to 

 participate in the reaction until finally the appropriate avenue for an 

 adaptive response is opened and the situation is relieved. With the release 

 of the tension and free discharge, the feeling tone changes to a distinctly 

 pleasurable quality (see C. L. Herrick, 1910). 



The fact that the primitive pain path in the spinal cord seems to follow 

 a rather diffusely arranged system of fibers in the fasciculus proprius, fre- 

 quently interrupted by synapses in the gray matter (Fig. 117) with corres- 

 pondingly high resistance to nervous conduction, is perhaps correlated with 

 this general and diffuse quality of unpleasantness. 



Now, pain as a distinct and localizable sensation has not been involved 

 in the situation described in the preceding paragraphs. Pain, considered as 

 a distinct sensation, was, however, born out of this situation or differentiated 

 from it. Certain sensational elements which have a high protective value 

 for the organism are naturally most often involved in such a situation. 

 These are warning calls, and usually necessitate an interruption of the 

 ordinary business of life which may be in process at the time the danger 

 threatens. The free flow of ordinary sensori-motor activity is abruptly 

 checked, and the organism suddenly stops and makes the necessary reaa- 

 justment as quickly as may be. In the interest of increasing the rapidity 

 of this avoiding reaction, which, of course, is frequently of vital importance, 

 the pathways of the exteroceptive pain reactions are well developed and 

 segregated from the more diffuse and poorly organized affective apparatus 

 which we have just been considering. Thus arose pain nerves (if such exist 

 separately) and the pain tract of the spinal cord (whose anatomical dis- 

 tinctness seems well established), and also perhaps a special mechanism 

 for painful reactions in the thalamus. Sherrington has given a graphic 

 statement of the probable history of this process in the following words 

 (Schafer's Physiology, vol. ii, p. 974): 



"The facility of path of these motor reflexes colligated to pain hints at 

 their antiquity, or at their having been formed by some neural method par- 

 ticularly able to, as it were, make a good road. Each reaction that employs 

 a neural path seems to smooth it by sheer act of travel. This is true even 

 of slight impulses light traffic and more true of heavy. Pain reactions 

 are to be regarded as very heavy traffic. Their impressions summate with 



Kculiar ease, take correspondingly long periods to subside, and, to judge 

 their inertia, move generally masses of neural material relatively great. 

 Such impressions might wear a road with quite especial speed. Many 

 spinal reflexes imply, so to say, well-worn habits based on ancient pain 



