THE PRESSURE-SENSE. 



Method of Examination. (i) Weights of different amount are placed suc- 

 cessively on the parts of the skin to be tested, and the subject is asked to form an 

 estimate of the differences in pressure. In order to exclude, so far as possible, 

 the influence of temperature, displacement and inequality in application, the 

 area of skin should be previously covered by a plate, which is allowed to remain 

 throughout the experiment. The influence of the muscular sense must also be 

 eliminated. (2) A projecting arm from a scale-beam is placed on the skin, 

 and by the addition or removal of weights the differences in pressure are learned 

 that the subject is capable of estimating. (3) In order to avoid the troublesome 

 changing of weights A. Eulenburg constructed his baresthesiometer, an apparatus 

 constructed upon the principle of the spiral-spring balance. It is provided with 

 a small button directed downward, which is depressed by the force of the spring. 

 An indicator marks directly the pressure in grams, and this can at once be readily 

 varied. (4) Goltz employed a pulsating, elastic tube, in which waves of different 

 height could be produced. He determined how high they had to be before they 

 were perceived as pulse-waves on the different areas of skin on which the tube 

 was placed. (5) The mercurial pressure-balance constructed by the author satis- 



fies all requirements completely (Fig. 342). 

 A scale-beam (W) resting on knife-bl 



(W) resting on knife-blades (O O) is supported on the horizontal 

 arm (b) of a heavy stand (T). One arm of the scale possesses a thread (m) 

 on which a balancing weight (S) can be moved to and fro. The other arm (d) , 

 which passes vertically upward, terminates in a graduated tube (R). From the 

 latter there projects downward a pressure-button (P) , to which weights (G) can 

 be added at will, and which rests upon the area of skin to be tested (H). 

 From a buret (B) near by, which is supported on an upright (A), mercury can 

 pass through the hollow arm of the scale, in the direction of the arrow, and mount 

 upward in the tube (R). A thin, readily movable piece of rubber tubing connects 

 the arm (O) with a fixed glass tube, and the latter passes subsequently to the 

 rubber tube (D D) of the buret. If the cock (h) is closed, the mercury moves 

 onward in d and rises in R, thus increasing the pressure of the button (P) when- 

 ever pressure is made on the tube (D D). The weight of the mercury filling one 

 division of the tube (R) is known. The apparatus permits, without any agitation 

 whatever, of rapid or slow variation in pressure, with any selected initial weight 

 (through G) . In the figure a indicates a screw for varying the position of the 

 supporting arm (b) : t is an arrangement with two screws to prevent the scale - 

 arm from tipping over. The more extensively pressure is made upon the tube 

 (D D) , the greater, naturally, will be each increase of pressure. By raising the 

 buret (B), if h is open, the pressure can also be increased. 



If P is at first supported the mercury can be allowed to rise in R to different 

 heights (in order to produce different amounts of pressure) , and after closing the 

 cock (h) , the pressure of the button can be permitted to act suddenly by quickly 

 releasing its support. In general, those methods are to be preferred in which 

 the different pressures act at distinct intervals of time, instead of allowing the 

 original pressure to increase or decrease gradually, because in the latter method 

 the cutaneous nerves are gradually fatigued. Both the pressure-sense and the 

 temperature-sense (to be discussed presently) may be most reliably tested by 

 the principle of the least perceptible difference, that is by permitting different 

 pressures (or temperatures) to act in graduated order, commencing either with 

 great differences or with the smallest ones, and seeking the limit at which or 

 within which a positive recognition of the difference takes place. 



The results of the investigations of the pressure-sense are as follows : 

 i. The minimal pressure that can just be perceived on different 

 parts of the body varies greatly in accordance with the locality. The 

 most delicate areas are the forehead, the temple, the back of the hand, 

 and the forearm, which perceive a pressure of 0.002 gm. The fingers 

 do^ not recognize a pressure of less than from 0.005 to 0.015 gm.; the 

 chin, the abdomen, the nose a pressure of less than from 0.04 to 0.05 

 gm. ; nor the finger-nails of less than i gm. In order to test the pressure- 

 sense of individual small points, they may be pressed upon with a 

 flexible, elastic hair. The most delicate pressure-sensations in these 

 situations were: the face, as low as 0.0007 gm., the arm and the leg, 

 as low as 0.012 gm. Pressure is more readily perceived when applied 



