I04 o THE SENSES 



receptors. The simplest assumption which will satisfactorily 

 account for the distribution of the four fundamental cutaneous 

 sensations is that the skin is supplied with four kinds of nerve- 

 fibres, anatomically as well as functionally distinct. Some fibres 

 minister to the sensation of cold, others to that of warmth, others 

 to that of touch, and others still to pain. And just as stimulation 

 of the optic nerve gives rise to a sensation of light, so stimulation 

 of any one of the cutaneous nerves gives rise to the specific sensa- 

 tion proper to the group to which It belongs. The existence of 

 different forms of sensory end-organs in the skin and other tissues 

 (tactile or touch-corpuscles, corpuscles of Pacini, end-bulbs of 

 Krause, etc.) points in the same direction. The end-organs of the 

 touch sensations are believed to be the ring-like arrangements of 

 non-medullated nerve-fibres encircling the hair-follicles, and in 

 parts of the skin devoid of hairs the corpuscles of Meissner (v. Frey). 



Touch-spots can easily be demonstrated by touching the skin lightly 

 with some small object such as a hair. The most exact quantitative 

 observations have been made by means of v. Prey's hair aesthesiometer. 

 This consists of a handle in which hairs of different diameters can be 

 fixed. The area of the cross section of each hair is measured under the 

 microscope, and the pressure necessary to bend it is determined by 

 pressing it upon the scale-pan of a balance. The pressure in milli- 

 grammes, divided by the cross section in square millimetres, gives the 

 pressure per square millimetre, which, according to v. Frey, permits 

 hairs to be chosen so as to give a uniform intensity of stimulation or a 

 variable intensity, according to the object of the investigation. Many 

 observers, however, believe that it is more accurate to take no account 

 of the pressure per unit of area, but to graduate the hairs according to 

 the total pressure needed to bend them. When touch-spots ascer- 

 tained in this way are excited by an inadequate stimulus e.g., an 

 alternating current of minimal strength, applied by the unipolar 

 method through the head of a pin as an electrode they still respond 

 by their characteristic or specific reaction namely, a sensation of 

 touch in the case supposed, a vibrating sensation like that caused by 

 a tuning-fork in contact with the skin. In the spaces between the 

 touch-spots the sensation produced by the same strength of current, or 

 even by a weaker current, is not one of touch, but a painful pricking 

 sensation which has no vibratory character, but is permanent as long 

 as the current lasts. 



The spots most sensitive to touch lie close to the hairs on their 

 ' windward ' side i.e., on the side away from which they slope. The 

 minimum pressure necessary to evoke a sensation of contact is not the 

 same for every portion of the skin. The forehead and palm of the 

 hand are most sensitive. 



If two points of the skin are touched at the same time there is a 

 double sensation when the distance between the points exceeds a cer- 

 tain minimum, which varies for different parts of the sensitive surface. 



Practice increases the acuity of touch for the two points test. Even 

 in a few hours it may be temporarily quadrupled on some parts of the 

 skin. Since at the same time it is increased in the corresponding part 

 of the opposite side of the body, it is argued that the modification takes 

 place in the central nervous system, not in the end-organs themselves. 



