THE PRESSURE-SENSE. 927 



Following Lotze, Wundt assumes from a psychophysiological standpoint that 

 each area of the skin sends to the brain, together with tactile impressions, informa- 

 tion as to the localization of the sensation. Hence, each area is able to give to 

 the tactile sensation a local coloring, which is made use of as a local sign. Wundt 

 assumes that this local coloring varies from point to point of the skin. The 

 gradation is abrupt on those parts of the skin in which the spatial sense is highly 

 developed, but gradual in those where it is comparatively poor. Separate im- 

 pressions become fused wherever the gradation of this local coloring is imper- 

 ceptible. As it is possible by exercise and attention to distinguish differences 

 of sensation that cannot ordinarily be appreciated, the reduction in the size of 

 the circles of sensation by practice can be thus explained. The circle of sensation 

 is an area of skin within which the local coloring of the sensation is so little changed 

 that two separate impressions are fused into one. 



Loeb has made experiments upon the tactile area of the hand, that is the 

 total number of points that an individual can reach with the tip of the forefinger, 

 without change in the position of the body. If, with closed eyes, the hands are 

 moved along a cord stretched transversely to the right and the left respectively, 

 an inequality within the distance traversed will be apparent : in right-handed per- 

 sons the distance to the right is generally smaller, and in left-handed the distance to 

 the left. Nervous patients often exhibit marked deviations. In the attempt 

 to make movements of the same extent, the movement executed will be the smaller 

 the more the muscles are already contracted. The perception of the size and 

 the direction of voluntary movements depends upon the voluntary impulse sent 

 to the muscles. 



THE PRESSURE-SENSE. 



Through the pressure-sense information is obtained as to the amount 

 of weight placed upon the skin; v. Frey considers the hair-nerves and 

 Meissner's corpuscles as the organs of the pressure-sense. The pressure- 

 sense is subserved by specific nervous end-organs having a punctate 

 arrangement. These pressure -points possess different degrees of sen- 

 sibility ; in many places (such as the back 

 and the thigh) they are characterized by 



an especially marked after-sensation. . '.'.. :*.'. . : .;-.' 



The distribution of the points corre- J: : VO /.''/.Vv;. :..["';./*'.' 

 spends to that of the temperature- :[-': : .vX' : :. : *-V'!i *!"."" !:";"""" 

 points. The chains of pressure-points % %:V.* 



usually take a different direction from <* b c 



that of thf Vint anH r>n1H nnintc in o-An FlG - 34i- Pressure Points: a, from the mid- 



d cold points , in gen- dle o{ the ^ o{ the foot . bt from the skin 



eral their density is greater; but this of the zygoma; c, from the back (after 



varies in different regions. In places 

 provided with hair the number of pres- 

 sure-points does not correspond exactly with the number of hairs, but 

 the points always lie in circles around the hairs. Hence, the hair when 

 touched can also transmit the pressure-sensation, as it presses like a 

 lever on the nerves of the root-sheaths. The smallest distances at 

 which two pressure-points applied simultaneously can be felt as double 

 have been found to be as follows: on the back, from 4 to 6 mm.; on the 

 chest, 0.8; on the abdomen, from 1.5 to 2 ; on the cheek, from 0.4 to 0.6; 

 on the arm, from 0.6 to 0.8; on the forearm, from 0.5 to i ; on the back 

 of the hand, from 0.3 to 0.6; on the palm of the hand, from o.i to 0.5 ; 

 on the palmar aspect of the distal phalanx of a finger, o.i ; on the dorsal 

 aspect, from 0.3 to 0.5 ; on the leg, from 0.8 to 2 ; on the back of the foot, 

 from 0.8 to i ; on the sole of the foot, from 0.8 to i mm. In the parts 

 of the body devoid of hair, the tactile corpuscles are supposed to transmit 

 the pressure-sensation. 



