CH. LII.] VARIETIES OF CUTANEOUS SENSATIONS 725 



points of a pair of compasses (maintained at a constant distance 

 apart) are slowly drawn over the skin of the cheek towards the lips, 

 it is almost impossible to resist the conclusion that the distance 

 between the points is gradually increasing. When they reach the 

 lips they seem to be considerably further apart than on the cheek. 

 Thus, too, our estimate of the size of a cavity in a tooth is usually 

 exaggerated when based upon sensation derived from the tongue 

 alone. Another curious illusion is the following: If we -close 

 the eyes, and place a marble between the crossed fore and middle 

 fingers, we seem to be touching two marbles. This illusion is due 

 to an error of judgment. The marble is touched by two surfaces 

 which, under ordinary circumstances, could only be touched by two 

 separate marbles ; hence, the mind, taking no cognizance of the fact 

 that the fingers are crossed, forms the conclusion that the two 

 sensations are due to two marbles. 



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 intervals 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, warmth 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," 

 " warmth spots " and " pain spots " are intercommingled. In some 

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

 are the most and "warmth 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 

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

 law represents the discriminative sensibility (see p. 717) varies 

 from -.yV to more than T V in different parts of the body. It does not, 



