CUTANEOUS AND INTERNAL SENSATIONS 



1039 



of change of temperature, or of warmth and cold. The sensation 

 of pain, although it cannot be absolutely separated from these, ought 

 not to be grouped along with them. It is called forth by the stimula- 

 tion of afferent nerve-fibres in their course; and it may originate, 

 under certain conditions, in internal organs which are devoid of 

 tactile sensibility, and the functional activity of which in their 

 normal state gives rise to no special sensation at all. The peculiar 

 sensation associated with voluntary muscular effort, to which the 

 name of the muscular sense has been given, also deserves a separate 

 place ; for although it may in part depend on tactile sensations set 

 up through the medium of end-organs situated in muscle, tendon, 

 or the structures which enter into the formation 

 of the joints, other elements are, in all proba- 

 bility, involved. 



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 re- 

 presented touch in the restricted sense (the 

 sensation elicited by light contact), warmth, 

 cold, and pain. Pressure is mainly a sensation 

 connected 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 re- 

 presented in the skin itself in association 

 with touch. In a somewhat diagrammatic 

 sense it may be said that the surface 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 con- 

 cerned in one sensation are everywhere mingled with areas con- 

 cerned 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 



Fig. 441. Tactile Cor- 

 puscle from Skin of 

 Finger (Smirnow). 

 (Golgi preparation.) 

 The winding and in- 

 tersecting black lines 

 are the non-medul- 

 lated endings of the 

 one or more nerve- 

 fibres that enter the 

 corpuscle. 



