110 



ESSENTIALS OF PHYSIOLOGY. 



Area. 



Tongue and nose . 

 Lips .... 

 Finger-tip and forehead 

 Palm, arm, thigh 

 Forearm 

 Back of hand 



Abdomen, outside of 'thigh 

 Back of forearm . 

 Loins . 



Grams per sq. mm. 



2 



2'5 



3 



7 



8 



. 12 



. 26 



. 33 



48 



(2) Tactile discrimination is measiired by means of the gesthesio- 

 meter, an instrument somewhat resembling a pair of compasses, one 

 limb of which can be moved along a scale on which the distance between 

 the two points can be read off. When the two points, armed with small 

 pieces of cork, are applied to the skin a sufficient distance apart, the 

 resulting sensation is of two separate stimuli. When the two points 

 are approximated, the degree of approximation varying for different 

 parts of the skin surface, the double stimulus is productive of a single 

 sensation. The minimal distance apart at which the points give rise to 

 separate sensations is shown in the following table : 



Skin region. Distance in mm. 



Tip of tongue . . . . . ., 1-1 



Volar surface of finger-tip . . . . 2 '3 



Palm of hand . . . . .11-3 



Back of hand 31 -6 



Middle of back, upper arm, thigh . . 67' 1 



These figures have no reference to the distances which separate the 

 actual pressure spots, but include on an average about ten such spots 

 in each case between the limbs of the eesthesiometer. 



LOCAL SIGN. 



Just as a localised stimulus of a definite kind applied to the skin 

 invariably evokes the same reflex, so also in a conscious individual a 

 stimulus applied to a particular skin area is referred to the stimulated 

 spot. There must therefore be a specific quality in the impulse 

 conveyed to the association centres according to the skin area in which 

 it originates, and this quality is called "local sign." It has been 

 supposed by some authorities that local sign is due to a recognition of 

 the muscular reflexes which are or may be evoked as a result of the 

 stimulus, but it is more probable that it is developed as the result of 

 experience. If the index and middle fingers be crossed and a pea be 



