THE SENSES 969 



The simplest form of tactile sensation is that of mere contact, 

 as when the skin is lightly touched with the blunt end of a pencil. 

 This soon deepens into the sensation of pressure if the contact 

 is made closer ; and eventually the sense of pressure merges into 

 a feeling of pain. Most physiologists agree that in the skin itself 

 four fundamental qualities of sensation are represented touch 

 in the restricted sense (the sensation elicited by light contact), 

 warmth, cold, and pain. Pressure is mainly a sensation con- 

 nected with the stimulation of structures deeper than the skin 

 e.g., the sensation of contact is abolished in cicatrices where the 

 true skin has been destroyed, while sensibility to pressure 

 persists although the sensation of light pressure may be to some 

 extent represented in the skin itself in association with touch. 

 In a somewhat diagrammatic sense it may be said that the sur- 

 face of the skin is divided into a great number of very small areas, 

 each of which is related especially to one or other of the four 

 fundamental sensations. Areas concerned in one sensation are 

 everywhere mingled with areas concerned in the others. By 

 appropriate methods it has been found possible to determine the 

 existence on the skin of the trunk and limbs of not less than 

 30,000 ' warm-spots/ which always react to stimulation by a 

 sensation of warmth ; 250,000 ' cold-spots/ which react by a 

 sensation of cold ; and half a million touch-spots, whose specific 

 reaction is a sensation of touch. It is more difficult to localize 

 definitely bounded ' pain-spots/ partly because of the very rich 

 supply of pain- fibres to the skin. Yet there is reason to believe 

 that pain, like touch, warmth, and cold, is subserved by separate 

 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 

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

 quantitative observations^ have been made by means of v. Frey's 



