856 A MANUAL OF PHYSIOLOGY 



Whether the sensation of pain can be caused by excessive stimu- 

 lation of the nerves of common tactile sensibility or not, there can 

 be little doubt that afferent * pain ' fibres exist which are anatomically 

 distinct from the fibres of tactile sensation. For the conducting 

 paths in the spinal cord appear not to be the same for tactile and for 

 painful impressions. And in certain cases of disease sensibility to 

 pain may be lost, while tactile sensations are still perceived ; or, on 

 the other hand, pain may be felt in cases where tactile sensibility is 

 abolished. Loss of temperature sensation, however, is almost always 

 accompanied by loss of sensibility to pain. 



Localization of Sensations. We not only perceive sensations of 

 touch, temperature, pain, etc., but are able, more or less accurately, 

 to localize the part of the body from which the sensory impressions 

 come. This power of localization is not equal for all portions of the 

 body nor for all kinds of sensations. It is best developed for touch 

 (in the restricted sense), and all the varieties of common sensation 

 are better localized on the skin than in any of the deeper structures. 

 The precise mechanism of the localization is unknown. But we must 

 suppose that each peripheral area is ' represented ' in the brain, so 

 that the arrival of afferent impulses from it affects particularly the 

 related cerebral area. The brain, therefore, so to speak, associates 

 excitation of a given cerebral area with stimulation of the correspond- 

 ing peripheral area, and thus not only recognises the quality and 

 quantity of the resultant sensation, but also localizes it ; just as a 

 waiter who watches the bell indicator not only learns how a bell has 

 been rung, whether once or twice, peremptorily or languidly, but also 

 in what room it has been rung. If, to pursue the illustration a little 

 farther, he is aware that two rooms are connected with one bell, but 

 that one of the rooms is scarcely ever occupied, he associates the 

 ringing of the bell with a summons from the other room even when 

 it happens to be rung from the usually vacant room. In like manner 

 the brain seems to connect the arrival of sensory impulses from the 

 internal organs, which have few sensory fibres, and these perhaps not 

 often stimulated, with excitation in a related cutaneous region, from 

 which it is constantly receiving sensory impressions. The fact 

 already mentioned (p. 699), that in disease of internal organs the 

 pain is referred to some portion of the skin, may be thus explained. 



Relation of Stimulus to Sensation. 



It is impossible to measure sensation in terms of stimulus. All 

 that we can do is to compare differences in the intensity of stimuli 

 and differences in the resultant sensations, or, in other words, to 

 compare stimuli together and to compare sensations together. And 

 when we determine the amount by which a given stimulus must be 

 increased or diminished in order that there may be a just perceptible 

 increase or diminution in the sensation, it is found that (with certain 

 limitations) the two are connected by a simple law : Whatever the 

 absolute strength of a stimulus of given kind may be, it must be 

 increased by the same fraction of its amount in order that a difference 



