CUTANEOUS SENSATIONS' 439 



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. 



THE SENSE OF TOUCH 



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 hardness 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 body. 



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 



i spots a sharply localised sensation of pressure is produced unaccompanied 



I by any painful quality and often describecLas 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. 



The adequate stimulus for these tactile nerve-endings is not so much 

 pressure as deformation 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 only noted 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 

 passage from the pressure of the mercury to the ^negligible pressure of the 

 outside air. The tactile apparatus is smarter in it*s response than any other 

 of the sense-organs. On this account stimuli are still perceived as discrete, 



16* 



