i CUTANEOUS SENSIBILITY 35 



the number of superficial cutaneous elements on which the 

 pressure acts. The effect seems to be greater or more easily 

 perceived in indirect relation to the area of skin compressed. 

 Von Frey's method of determining the tactile sensibility in 

 different regions of the skin by hairs is based on this fundamental 

 observation. 



With this method it is easy to prove that the parts most 

 sensitive to mechanical agents are the tip of the tongue, the 

 red part of the lips, and the ends of the fingers. Even in these 

 parts there is a threshold of stimulation which must be crossed 

 to evoke a sensation. This shows that the parts where the sense 

 of pressure is most delicate do not coincide with those in which 

 thermal sensibility is most developed, which is a valid argument 

 in favour of the theory that the two forms of sensation arise in 

 different end-organs. 



We know nothing about the nature of the process by which 

 the excitation takes place. Possibly it may consist in a discharge 

 of energy caused by a chemical change within the end -organ, 

 due to a displacement of fluid a change in the concentration 

 of the dissolved substances which acts as a chemical stimulus. 

 We know that continuous pressure of a certain intensity on a 

 sensory nerve produces a continuous sensation, which is difficult 

 to explain as a mechanical effect, because the work required to 

 stimulate an exposed nerve is probably a thousand times greater 

 than that sufficient to excite those nerves adequately when it 

 acts on their terminal end-organs. We have consequently no 

 reason to reject the hypothesis that mechanical stimuli can only 

 9 excite the nerve indirectly, and that excitation is always due to 

 alteration of the chemical structure or to osmotic pressure of 

 the tissue fluids (v. Frey and Kiesow). At the same time we 

 cannot exclude another simpler though more indefinite hypothesis, 

 according to which excitation depends upon a purely physical 

 process, by which the mechanical stimulus is changed into another 

 form of energy, to which Meissner's corpuscles are far more 

 sensitive. 



As regards the mode of action of a mechanical stimulus in 

 producing sensations of contact and pressure, v. Frey and Kiesow 

 (1899) showed that it is not compression in itself that determines 

 the excitation, but the deformation of the skin surface, which 

 produces an alteration in the pressure (Druckgefalle], and this 

 again indirectly produces an active reaction of the terminal organ. 

 This alteration is produced both by compressing the skin with a 

 point or small surface, and by pulling on a small body or disc 

 attached to the surface of the skin. In the first case the pressure 

 is greatest at the compressed surface, and diminishes in the deeper 

 parts and in the surrounding areas of the skin ; in the second the 

 pressure is least in the part of the skin drawn up, and increases 



