SPATIAL QUALITY OF TOUCH. 937 



great, it cannot be reduced so speedily or largely as in the fingers. 1 

 Education of an area of skin in one lateral half of the body reduces the 

 average liininal distance, not only in and around the area itself, but in 

 the symmetrically placed area of the other half of the body as well. 

 Similarly, the learning of a movement by the right hand imparts a partial 

 ability to perform it with the left. 2 



Delicacy of tactual space perception at any point of the surface of 

 the body seems in part a function of its mobility (C. Vierordt). 3 It 

 depends both on the variety of direction and on the linear amplitude of the 

 movements of the part. It increases with the distance of the part from 

 the axis of rotation. In the arm the delicacy increases continuously 

 from acromion to finger-tip, but at unequal rate between the different 

 joints. If the localising power for touch at the acromion be valued at 

 100, it increases to 2582 at the finger-tips. In the upper arm it 

 increases to 151, in the forearm to 272, in the hand exclusive of digits 

 to 659, in the thumb to 2417, and in the middle digit to 2582. These 

 values in each segment {e.g. forearm) of the limb can be considered the 

 sum of two quantities. One of the quantities depends on the distance 

 from the main axis of movement of the limb as a whole, e.g. from 

 shoulder. The other is determined by the distance from the axis 

 of movement of the segment itself, e.g. from elbow. Even in each 

 separate finger the rate of increase of delicacy is different in the 

 different segments and greatest in the terminal. The relation 

 between delicacy of sense and degree of mobility is least easily 

 examined in the head, because of the complexity of movement of that 

 region. The difference is only as 100 to 160 between all mobile (but 

 not motile) parts of the head's surface. Yet Vierordt's theory seems to 

 apply very perfectly there also. 4 Thus there is a continuous and regular 

 decrease of average liminal distance along the cheek from ear to lip. In 

 the head and in the arm the average liminal distance is not only 

 inversely as the mobility of the part, when passively partaking in move- 

 ments of the head upon the neck, but is further reduced by the active 

 movements of the part itself, if mobile. Thus the average liminal 

 distance decreases rapidly from base of tongue to tongue-tip. Again, its 

 measure is less for the surface of the lower than the upper lip, for the upper 

 than the lower eyelid ; lower eyelid and upper lip are the less mobile. 



Among conditions affecting the fineness of localisation by touch, are 

 the following. It is decreased by antemia of the skin and by venous 

 hyperemia ; 5 also by too frequent testing, and by mental fatigue, — thus, 

 three hours of school classes will double some threshold distances on the 

 face ; 6 also by cold and by excessive stretching of the skin. 7 In relation 

 to the last-named point, there is an experiment of some theoretical interest. 

 If the skin of the neck be stretched by throwing back the head as far as 

 possible, the distance of two skin-points can in certain regions of the 

 neck lie doubled. The average liminal distance in that region is, how- 

 ever, not doubled, but only increased 8 per cent. 8 The passage of a 



1 Funke, Hermann's " Handbuch," 1880, Bd. iii. Abth. 2, S. 382. But see Volkmann, 

 op. cit. 



2 Weber, loc.cit. 3 Arch. f. d. ges. Physiol., Bonn, 1869, Bd. ii. S. 297. 



4 Riecker, Ztschr. f. Biol., 1873, Bd. ix. S. 95 ; 1874, Bd. x. S. 177. 



5 Alsberg, " Inaug. Diss.," Marburg, 1863. 



6 Griesbach, Arch. f. Hyg., Munehen u. Leipzig, Bd. xxiv. S. 124; Varmod, "These 

 inaug.," Berne, 1896. 



7 v. Frey, loc. cit. 8 G. Hartniann, Ztschr. f. Biol., 1875, Bd. xi. 



