i CUTANEOUS SENSIBILITY 37 



takes place. A deformation rapidly produced lias more effect, HUM 

 one produced slowly. It follows that the effect of the stimulus 

 is not dependent on the mechanical work performed, because 

 different amounts of mechanical work may produce identical 

 -1'iisutions, and rice versa. 



(d) If on stimulating one and the same tactile spot the surface 

 area of the stimulus is altered, then to obtain approximately the 

 same effect the weight and rapidity of stimulation must be 

 correspondingly altered. Hence the results obtained with different 

 methods can only be compared when the increment of weight for 

 the unit of time and surface, i.e. the rate of pressure, remains 

 constant. 



The results which v. Frey and Kiesow obtained on exciting 

 large and moderate cutaneous areas show that the threshold 

 values of the weights do not increase in proportion with the 

 increase of the surface deformed. This can only be explained on 

 the theory that the excitation which causes a sensation of pressure 

 depends on the. alteration in pressure produced within the skin, 

 and that any pressure that is equal on all sides produces no effect 

 at all. As the excited surface grows larger the fall of pressure 

 in the skin becomes less. On the other hand, the smaller the 

 stimulating surface, the more rapid will be the alteration in 

 pressure ; an increase in surface pressure then becomes necessary 

 to produce a change in pressure at the level of the nerve-organ 

 adequate to excite it. 



On the strength of these investigations it is easy to explain 

 the well-known experiment of Meissner. If the hand is dipped 

 into a fluid water or mercury of the same temperature as the 

 hand a pressure sensation is not felt over the whole surface of the 

 submerged skin, but only at the boundary between the parts 

 compressed and those not compressed. If, e.g., one finger is dipped 

 into mercury at the same temperature as the finger, a sensation 

 is felt of a ring compressing the finger. This sensation is referred 

 to the level at which there is an alteration in the pressure, while 

 there is no sensation over the whole surface that is exposed to a 

 gradual and slowly increasing pressure, since the variation is so 

 slight that it remains below the effective threshold of stimulation. 



Kiesow (1904), in a long series of patient and delicate re- 

 searches by the most modern methods, attempted to estimate as 

 accurately as possible how the sensibility to touch and pressure 

 alters upon the different parts of the surface of the body. He 

 investigated in two directions. He determined separately the 

 number of the touch spots (or pressure spots of Blix and Gold- 

 scheider) in the surface unit, and then the liminal stimulus for 

 the touch spot, that is the mean value of the threshold, obtained 

 by a series of separate observations in different cutaneous regions. 

 These estimations were made by means of v. Frey's hairs. 



