1052 



THE SENSES 



under local anaesthesia it has been seen that the parietal peritoneum 

 is quite insensitive to touch, pressure, and temperature stimuli, in- 

 cluding extreme temperatures (Ramstrom), while pain is caused by 

 traction on it. Its sensibility is therefore neither purely epicritic nor 



Eurely protopathic in Head's sense. In .like manner the mucous mem- 

 rane of the mouth, in which sensibility only to touch and temperature 

 is present, conforms entirely to neither type. Its sensibility is not 

 alone epicritic, since it responds to extreme temperatures, nor is it 

 purely protopathic, 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, temperature, 

 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 localiza- 

 tion is not equal for all portions of the body nor for all kinds of sensa- 

 tions. 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 localiza- 

 tion 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 stimula- 

 tion of the corresponding peripheial area, and thus not only recognizes 

 the quality and quantity of the resultant sensatign, 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 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. 863), that in disease of internal organs the pain is re- 

 ferred 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 recog- 

 nized in the absence of visual sensations ; (2) the sensations associated 

 with movements, passive as well as active ; (3) the sensations associated 

 with resistance to movement. In none of these groups are we dealing 

 with purely muscular sensations; cutaneous tactile sensations and 

 pressure sensations elicited from other structures than muscles are 

 also involved. 



Voluntary muscular movements are accompanied with a peculiar 



