RELATION OF SENSATION TO STIMULUS 483 



of stimulus is that known as ' contrast.' A finger plunged into mercury 

 feels a ring of constriction at the level of the surface of the mercury, i.e. 

 where there is a contrast between the pressure of the mercury and the 

 absence of pressure as the finger emerges into the air. 



The strength of the effective stimulus depends also on the number of 

 nerve-endings simultaneously excited. Thus when dealing with tactile 

 sensations, or sensations of pressure, in determining the minimal stimulus 

 we must take into account the area stimulated, and we express the stimulus, 

 just sufficient to produce a threshold sensation, as ' weight per square milli- 

 metre of surface.' Moreover the rapid ' fatigability ' or adaptation of all 

 sense-organs makes the rate at which the stimulus is applied of considerable 

 importance. Thus when weight was applied to the skin of the ball of the 

 thumb at the rate of 0-75 gramme per second over a surface of 21*2 sq. mm., 

 the minimum load necessary to evoke a distinct sensation was 2-5 grammes. 

 When, however, the rate of application of the weight was increased to 5 

 grammes per second, distinct sensation was produced with a load of 0-25 

 gramme. It is evident that the larger the area of the skin stimulated 

 the greater will be the minimal weight required, since this has to be dis- 

 tributed over all the nerve-endings contained in the area of skin which is 

 subjected to pressure, and the larger the area the smaller will be the stimulus 

 applied to each nerve- ending. The following figures were obtained by 

 von Frey on different regions of the skin : 



Volar side of wrist (Subject K) . . . 0-024 - 0-038 grm. 

 Ball of thumb (Subject K) >0-189 - 0-039 



Volar side of wrist (Subject F) . . . >0-236 - 0-055 



Rate of loading 3 grm. per second 

 Stimulated surface 3 5 mm.2 Threshold value of stimulus. 



Ball of thumb (Subject F) 0-200 - 0-045 grm. 



Tip of finger (Subject F) . . . . 0-170 - 0-028 

 Volar side of wrist (Subject F) . . . 0-640 - 0-028 



It will be noticed that when the excited surface is small much greater 

 variations are found from spot to spot in the size of the minimum stimulus. 

 This is probably connected with the fact that the sense-organs for pressure are 

 distributed in points or spots over the surface of the skin, so that when the 

 stimulated surface is small the threshold value of the stimulus will be deter- 

 mined by the number of the tactile spots which are included in the stimulated 

 area. The minimal effective stimuli in the case of the other senses have been 

 determined as follows : 



(a) HEARING. A musical tone can be heard when the variations of 

 pressure in the air ammount to -00000059 mm. Hg. with an amplitude of 

 vibrations of -000000066 mm. It has been calculated that the intensity 

 of the work performed on the drum of the ear by such a minimal tone repre- 

 sents an average of 5-1 x 10~ 15 ergs. In the case of noise the amount of 

 energy required to produce a minimum sensation is still smaller. A distinct 



