982 A MANUAL OF PHYSIOLOGY 



which sensibility only to touch and temperature is present, con- 

 forms entirely to neither type. Its sensibility is not alone epicritic, 

 since it responds to extreme temperatures, nor is it purely proto- 

 pathic, since a pin-prick produces no painful sensation. 



Localization of Cutaneous Sensations. We not only perceive the 

 quality and estimate the intensity of sensations of touch, tempera- 

 ture, pain, etc., but are able, more or less accurately, to localize the 

 part of the body from which the sensory impressions come. In 

 other words, two impressions from different parts of the body, 

 although identical in quality and intensity, are nevertheless stamped 

 with a distinctive something, which may be called the local sign. 

 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 par- 

 ticularly the related cerebral area. The brain, therefore, so to speak, 

 associates excitation of a given cerebral area with stimulation of 

 the corresponding 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 which room it has been rung. If, to pursue 

 the illustration a little farther, he is aware that two rooms are con- 

 nected 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 sen- 

 sory 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. 790), that in 

 disease of internal organs the pain is referred to some portion of 

 the skin, may be thus explained. 



It is through the localization of touch sensations that the size 

 and form of objects in contact with the skin are perceived in the 

 absence of other than the cutaneous sensations, and especially in 

 the absence of visual and muscular sensations (stereognosis) . 



Muscular Sensations (Muscular Sense), etc. 



Sometimes, although rather loosely, grouped together as muscular 

 sensations, are a number of forms of sensation of which our knowledge 

 is much less accurate than it is in the case of the fundamental skin 

 sensations. Among these may be mentioned especially (i) the 

 sensations by which the position in space of the body as a whole 

 or of particular parts is recognised in the absence of visual sensa- 

 tions ; (2) the sensations associated with movements, passive as well 

 as active ; (3) the sensations associated with resistance to move- 

 ment. In none of these groups are we dealing with purely mus- 

 cular sensations ; cutaneous tactile sensations and pressure sensa- 

 tions elicited from other structures than muscles are also involved. 



Voluntary muscular movements are accompanied with a peculiar 

 sensation of effort, graduated according to the strength of the con- 



