772 CUTANEOUS SENSATIONS [CH. LIII. 



Varieties of Cutaneous Sensations. 



The surface of the skin is a mosaic of tiny sensorial areas ; but 

 these areas are not set edge to edge as in the retina, but separated 

 by relatively wide interval which are not sensitive to stimuli just 

 above liminal intensity. If the stimuli are made nearly minimal, 

 the individual fields are reduced to small spots. Each of these spots 

 subserves a specific sense, touch, cold, heat or pain, and each 

 doubtless coincides with the site of some special end-organ, placed 

 either singly or in clusters. The "touch spots," "cold spots," 

 "heat spots," and "pain spots" are intercommingled. In some 

 districts one variety predominates, in others another. " Pain spots " 

 are the most and "heat spots" the least numerous. It is a 

 matter of common experience that the sensitiveness of these varieties 

 of cutaneous sensation differs in different parts of the body. The 

 tip of the finger, which is very sensitive to the true tactile sense 

 (sense of pressure or contact), is not nearly so sensitive to alterations 

 of temperature as the forearm or cheek, to which a washerwoman 

 generally holds her iron when forming a judgment of its temperature. 

 Some parts of the skin are more sensitive to pain than others, and 

 in the cornea we have an instance of a surface in which " pain spots " 

 alone are present. 



For the more accurate exploration of the skin, cesthesiometers of 

 various kinds have been invented. The sense of pressure may be 

 estimated by the ability of the skin to distinguish different weights 

 placed upon it; there must be no lifting of the weight, or the 

 motorial sense is brought into play. The fraction which by Weber's 

 law represents the differential threshold (see p. 763) varies 

 from -gV to more than -^ in different parts of the body. It does not, 

 however, follow that the acuteness of the pressure sense varies 

 exactly as the ability of accurately localising sensations ; for instance, 

 the skin of the forearm is as sensitive to pressure changes as that 

 of the palm ; and the tip of the tongue, which is the most discrimi- 

 native region of the body for locality, is not so for pressure. For 

 pressure stimuli which are near the limen or threshold of sensa- 

 tion, the hair sesthesiometer is much used; this is a hair suitably 

 mounted in a holder ; the hair can then be shifted backwards or for- 

 wards in the holder, and the amount of pressure it exercises can 

 thus be varied. It is used for the exploration of " touch spots," and 

 these are found most numerously around the hair follicles. The 

 touch spots are more numerous in some parts than in others, but 

 fifteen for each square centimetre of skin is a rough average. To 

 explore " pain spots " a stout hair or needle is used ; in the latter 

 case the needle shifts up and down in the holder, and works 

 against a spring which registers the amount of pressure exerted to 



