THE SENSE OF TOUCH 



CUTANEOUS SENSATIONS 629 



simultaneous stimulation of warm spots and cold spots. When testing the 

 distribution of the temperature sense, it is found that the sense of cold is 

 evoked more promptly than that of warmth. This is interpreted as showing 

 that the end organs for the warm sense are situated more deeply than those 

 for cold. We have no evidence as to the histological identity of these organs. 



By means of the sense of touch we arrive at a conclusion as to the qualities, 

 such as shape, texture, hardness, &c., of the bodies with which the skin 

 is in contact. In this judgment however, very many other sensations 

 are involved besides those which can be regarded as strictly tactile. Thus 

 the hardner, of an object signifies its resistance to deformation, besides 

 its power of deforming the skin surface with which it is in contact ; the 

 former quality, i.e. of resistance, is one which involves the muscular sense, 

 since we judge of it by the extent to which we can move our muscles without 

 causing any alteration of the surface of the object. 



The tactile sensibility of the skin as a whole, like its temperature sensi- 

 bility, is due to the presence in it of a number of touch spots, i.e. small 

 areas which are extremely sensitive, separated by areas almost or entirely 

 insensitive to pressure. The tactile sensibility of any part is proportional 

 to the number of such touch spots present. If the calf of the leg be shaved 

 and then tested by pressing on it with a fine bristle or hair it will be found 

 that the minimal stimulation used evokes sensation only at certain definite 

 points, the ' touch spots.' In a square centimetre of such skin there may 

 be about fifteen touch spots. On thrusting a fine needle into one of these 

 spots a sharply localised sensation of pressure is produced unaccompanied 

 by any painful quality and often described as having a ' shotty ' character, 

 as of a little hard object embedded in the skin and there pressed upon. 

 These touch spots are arranged chiefly around the hairs, lying usually 

 on the side from which the hair slopes. They vary in number according 

 to the part of the body which is the subject of investigation.. Thus the 

 dorsal surface of the finger contains about seven times as many touch spots 

 as an equal area between the shoulders. In some regions, such as the 

 skin over subcutaneous surfaces of bone, as much as one centimetre may 

 intervene between two neighbouring touch spots. They have no relation 

 to the warm and cold spots ; they are entirely absent from the cornea, the 

 glans penis, and the conjunctiva of the upper lid. 



RESPONSE TO DIFFERENT STIMULI. The adequate stimulus 

 for these tactile nerve endings is not so much pressure as deforma- 

 tion of surface. It appears to matter little whether the surface be 

 deformed by pulling *it or by pushing an instrument into it. The 

 ineffectiveness of mere pressure is shown by dipping the finger into a 

 vessel of mercury. The sensation of pressure is noted only at the point 

 where the finger passes through the surface of the mercury, and this is the 

 only part where there is an actual deformation of the skin, due to the sudden 



