SENSE OF SPACE. 925 



the points fixed at a distance at which they can be perceived as separate they are 

 moved over other parts of the skin, and the subject is asked whether the points 

 seem to move closer or further apart. (3) Two compasses with their points 

 separated unequally are placed on two different portions of the skin, and the 

 subject is asked to state when they seem to be equally separated: Fechner's 

 method of equivalents. Thus, a separation of four lines on the forehead seems to 

 be equal to a separation of 2.4 lines on the upper lip. Camerer found, in general, 

 that the separation of the points applied to a portion of the skin endowed witn 

 delicate tactile sensitiveness is equivalent to a much greater separation in a 

 less sensitive area. (4) A portion of the skin can be touched with a blunt rod, 

 and the subject with his eyes closed be asked to indicate exactly where he was 

 touched. 



Investigation has yielded the following results: The spatial sense 

 in a given portion of skin is the more highly developed : 



1 . The more numerous the tactile nerves that ter- 

 minate in the area in question. 



2 . The greater the mobility of the part ; hence it 

 is most delicate in the extremities, toward the fingers 

 and toes ; also in parts of the body that are moved 

 with great rapidity. 



3. In the extremities the sensitiveness is greater 

 in the transverse than in the long axis. It is one-eighth 

 greater on the flexor surface of the upper arm, and one- 

 fourth greater on the extensor surface. Likewise, the 

 flexor surface is more sensitive than the extensor 

 one-sixth more in the upper extremity. 



4. The method of application of the compass-points 

 has an influence : (a) if they are applied in succession, 

 instead of together, or if they are considerably warmer 



or colder than the skin, or if they are unequally warm, FIG. 339. Compasses for 

 it is possible to distinguish a separation of shorter 

 distances; (>) if the examination is begun with the 

 points far apart and the distance is gradually lessened, it is possible 

 to recognize shorter distances than when the examination is begun with 

 the points separated by an indistinguishable distance and the distance is 

 gradually increased; (c) if one point is cold, and the other hot, two im- 

 pressions are felt when the minimum distance is exceeded, but it is im- 

 possible to determine their relative position. 



5. The spatial sense can be sharpened by practice; hence its delicacy 

 in the blind, and the improvement is always bilateral. 



6. Moistening the skin with indiffer- 

 ent fluids increases the delicacy of the 

 m^i . . . rc 5 ^ , spatial sense. If, however, the skin be- 



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HI tween two points that are still recognized 



FIG. 340. Sieveking's Esthesiometer. as separate be gently tickled or be tra- 

 versed by imperceptible electrical cur- 

 rents, the impressions become fused. 



The spatial sense is sharpened at the kathode on applying the constant 

 current, likewise by congestion of the skin in consequence of irritation, 

 and also by slight stretching of the skin ; further after carbonated baths 

 or warm sodium chlorid baths, and temporarily by the use of caffein. 

 7. Anemia (induced by elevation of the extremities) and venous 

 hyperemia (induced by compression of the veins) impair the spatial 

 sense ; likewise too frequent repetition of the tests (as a result of fatigue). 



