RESULTS OF THE PRESSURE SENSE. 835 



closed, whenever pressure is exerted on the tube (D, D), the mercury rises through d into R, 

 and increases the pressure on P. We measure the weight of the mercury corresponding to 

 each division of the tube (R). This instrument enables rapid variations of the weight to be 

 made without giving rise to any shock. In estimating both the pressure sense and temperature 

 sense, it is best to proceed on the principle of " the least perceptible difference," i.e., the 

 different pressures or temperatures are graduated, either beginning with great differences, or 

 proceeding from the smallest difference, and determining the limit at which the person can dis- 

 tinguish a difference in the sensation. 



Results. 1. The smallest perceptible pressure, when applied to different parts of 

 the skin, varies very greatly according to the locality. The greatest acuteness of 

 sensibility is on the forehead, temples, and the back of the hand and fore-arni, 

 which perceive a pressure of 0*002 grm. ; the fingers first feel with a weight of 

 0*005 to 0*015 grm. j the chin, abdomen, and nose with 0*04 to 0*05 grm. ; 

 the finger nail 1 grm. (Kammler and Aubert). 



The greater the sensibility of the skin, the more rapidly can stimuli succeed each other, and 

 still be perceived as single impressions ; 52 stimuli per second may be applied to the volar side 

 of the upper arm, 61 on the back of the hand, 70 to the tips of the fingers, and still be fel 4 

 singly (Block). 



2. Intermittent variations of pressure, as in Goltz's tube, are felt more acutely by 

 the tips of the fingers than with the forehead. 



3. Differences between two weights are perceived by the tips of the fingers when 

 the ratio is 29 : 30 (in the fore-arm as 18*2 : 20), provided the weights are not too 

 light or too heavy. In passing from the use of very light to heavy weights, the 

 acuteness or fineness of the perception of difference increases at once, but with 

 heavier weights, the power of distinguishing differences rapidly diminishes again 

 (E. Hering, Biedermann). This observation is at variance with the psycho-physical 

 law of Fechner ( 383). 



4. A. Eulenburg found the following gradations in the fineness of the pressure 

 sense : The forehead, lips, dorsum of the cheeks, and temples appreciate differences 

 of -fa to ^V (200 : 205 to 300 : 310 grm.). The dorsal surface of the last phalanx 

 of the fingers, the fore-arm, hand, 1st and 2nd phalanx, the volar surface of the 

 hand, fore-arm, and upper arm, distinguish differences of j- 1 ^ to -$ (200 : 220 to 

 220 : 210 grm.). The anterior surface of the leg and thigh are similar to the fore- 

 arm. Then follow the dorsum of the foot and toes, the sole of the foot, and the 

 posterior surface of the leg and thigh. Dohrn determined the smallest additional 

 w r eight, which, when added to 1 grm. already resting on the skin, was appreciated 

 as a difference, and he found that for the 3rd phalanx of the finger it was 0*499 

 grm. back of the foot, 0*5 grm. ; 2nd phalanx, 0*771 grm.; 1st phalanx, 0*02 

 grm.; leg, 1 grm. ; back of the hand, 1*156 grm. ; palm, 1*018 grm. ; patella, 1*5 

 grm.; fore-arm, 1*99 grm.; umbilicus, 3*5 grms. ; and the back, 3*8 grms. 



5. Too long time must not elapse between the application of two successive 

 weights, but 100 seconds may elapse when the difference between the weights is 

 4 : 5 (E. H. Weber). 



6. The sensation of an after-pressure is very marked, especially if the weight i 

 considerable and has been applied for a length of time. But even light weights, 

 when applied, must be separated by an interval of at least ^^ to ffo second, in 

 order to be perceived. When they are applied at shorter intervals, the sensations 

 become fused. When Valentin pressed the tips of his fingers against a wheel 

 provided with blunt teeth he felt the impression of a smooth margin, when the teeth 

 were applied to the skin at the intervals above mentioned ; when the wheel was 

 rotated more slowly, each tooth gave rise to a distinct impression. Vibrations of 

 strings are distinguished as such when the number of vibrations is 1506 to 1552 

 per second (v. Wittich and Grilnhagen). 



7. It is remarkable that pressure produced by the uniform compression of a part 

 of the body, e.g., by dipping a finger or arm in mercury, is not felt as such ; the 



