^STHESIOMETRY. 



813 



A. B. 



Fig. 617. 

 Aristotle's experiment. 



Smallest Appreciable Distance. The preceding statement gives the smallest 

 distance, in millimetres, at which two points of a pair of compasses can still be dis- 

 tinguished as double by an adult. The corresponding numbers for a boy twelve 

 years of age are given within brackets. 



Illusions of the sense of locality occur very frequently; the most marked are : (1) A uni- 

 form movement over a cutaneous surface appears to be quicker in those places which have the 

 finest sense of locality. (2) If we merely touch the skin with the two points of an sesthesio- 

 meter, then they feel as if they were wider apart than when the two points are moved along the 

 skin (Fechner). (3) A sphere, when touched with short rods, feels larger than when loDg rods 

 are used (Tourtual). (4) When the fingers of one hand are crossed, a small pebble or sphere 

 placed between them feels double (Aristotle's experiment). [When a pebble is rolled between 

 the crossed index and middle finger (fig. 617, B), it feels as if two balls were present, but with 

 the fingers uncrossed single.] (5) When pieces of skin 

 are transplanted, e.g., from the forehead, to form a 

 nose, the person operated on feels, often for a long 

 time, the new nasal part as if it were his forehead. 



Theoretical. Numerous experiments were made by 

 E. H. Weber, Lotze, Meissner, Czermak, and others 

 to explain the phenomena of the sense of space. 

 Weber's theory goes upon the assumption, that one 

 and the same nerve-fibre proceeding from the brain 

 to the skin can only take up one kind of impression, 

 and administer thereto. He called the part of the 

 skin to which each single nerve-fibre is distributed 

 a "circle of sensation." When two stimuli act 

 simultaneously upon the tactile end-organ, then a 

 double sensation is felt, when one or more circles of 

 sensation lie between the two points stimulated. 

 This explanation, based upon anatomical considera- 

 tions, does not explain how it is that, with practice, 

 the circles of sensation become smaller, and also how 

 it is that only one sensation occurs, when both points 

 of the instruments are so applied, that both points, although further apart than the diameter of 

 a circle of sensation, at one time lie upon two adjoining circles, at another between two others 

 with another circle intercalated between them. 



Wundt's Theory. In accordance with the conclusions of Lotze, Wundt proceeds from a 

 psycho-physiological basis, that every part of the skin with tactile sensibility always conveys 

 to the brain the locality of the sensation. Every cutaneous area, therefore, gives to the tactile 

 sensation a " local colour" or quality, which is spoken of as the local sign. He assumes 

 that this local colour diminishes from point to point of the skin. This gradation is very sudden 

 in those parts of the skin where the sense of space is very acute, but occurs very gradually 

 where the sense of space is more obtuse. Separate im- 



pressions unite into a common one, as soon as the gra- .-...:; ***.**.*" *'.: 



dation of the local colour becomes imperceptible. By *"""..*<, . : : , .'V*;* : . .!*/':>>*' 

 practice and attention differences of sensation are expe- **.";'/';.."* " ;"** *''/* "":&*'" 

 rienced, which ordinarily are not observed, so that he .*'" ": ' r '*."''*; ""'' 



explains the diminution of the circles of sensation by 



practice. The circle of sensation is an area of the skin, a be 



within which the local colour of the sensation changes so Fig. 618. 



little that two separate impressions fuse into one. Pressure-spots, a, middle of the sole 



427 PRESSURE SENSE. By the sense of of * he 5>*; & > . skin of zygoma; 

 , . . ,j ft c c, skin ot the back, 



pressure we obtain a knowledge ot the amount or 



weight or pressure which is being exercised at the time on the different parts of 

 the skin. 



A specific end-apparatus arranged in a punctated manner is connected with the 

 pressure sense (fig. 618). These points or spots are called "pressure-spots" or 

 "pressure-points" (Blix), and are endowed with varying degrees of sensibility; 

 at some places (back, thigh) they are distinguished by a markedly pronounced after- 

 sensation. The arrangement of the pressure-spots follows the type of the arrange- 

 ment of the temperature-spots. The pressure-spots have usually another direction 

 than that of hot and cold spots, as a rule, they are denser. The minimal distance 

 at which two pressure-spots, when simultaneously stimulated, are felt as double, is 



3G 



