RECEPTOR MECHANISMS CUTANEOUS SENSATIONS 589 



stimuli are successively applied influences the judgment. Generally 

 speaking, the slower the rate of change, the higher must be the stimulus 

 value. It is also easier to judge of a difference in weight than to 

 say whether such a weight is heavier or lighter than another. It is 

 easier to detect an increase in weight than a decrease. The experi- 

 mental results are so variable that it cannot be said that Weber's 

 law has been proved to hold good for tactile impressions. 



The most sensitive parts are those which we use habitually as 

 organs of touch. Thus, the under-surfaces of the tips of the fingers 

 and the palms of the hand are far more sensitive than the skin of 

 the gluteal region. Sensitiveness is also marked in the parts of the 

 body habitually moved, and increases from the joints towards the 

 extremities. It is also greater along the transverse axis of a limb 

 than in the same distance along the long axis. 



The following table shows in millimetres the distance in various 

 parts of the body at which the points of a compass are appreciated 

 as separate: 



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



Under surface of tip of finger . . 2-3 



Red part of lip . . . . . . 4-5 



Under surface of second phalanx of 



finger . . . . . . . . 4-5 



Upper surface of tip of finger . . 0-8 



Tip of nose . . . . . . . . H-8 



Ball of thumb <>-5-7 



Centre of palm . . . . . . 8-9 



Under surface of tip of great toe . . 11-3 

 Upper sufrace of second phalanx of 



finger 11-3 



Eyelid 11-3 



According to these degrees of sensitiveness, it is possible to locate 

 with precision the exact spot touched. Most people are unable to 

 localize a touch on the second, third or fourth toe to the exact toe 

 touched. By education the local sign is developed. 



The sensitiveness of the skin is increased after massage or salt 

 baths, blunted by cold, anaemia, venous congestion, or after the appli 

 cation of solutions of certain drugs, such as atropine, chloral, etc. 

 In regard to absolute sensitiveness, the most sensitive parts of the 

 body are the forehead, temples, back of hand, and forearm. These 

 are said to be able to detect a pressure of 0-002 gramme. Many ex- 

 periments have been made to determine the frequency with which 

 pressure stimuli must follow each other to produce a fused sensation, 

 but the results are discordant. The length of time a sensation per- 

 sists after removal of the stimulus varies greatly in different parts. 

 In the finger it vanishes almost at once, on the forehead it persists 

 for some little time, after a moderately strong stimulus. 



The Information derived from Tactile Sensations. From the 

 tactile sensations, associated with the kinsesthetic sensations, which 

 occur when the object presses heavily or is moved, we derive in- 

 formation as to form, size, and nature, of the body touched. It may 

 be small or big, smooth or rough, etc. When a large area of the 



Under surface of -lower third of 



forearm . . 

 Cheek 

 Temples 

 Forehead . . 

 Back of head 

 Back of hand 

 Knee 



Buttock . . 

 Forearm and leg 

 Neck 

 Upper arm, thigh, centre of back 



15-0 

 15-8 

 22-6 

 22-6 

 27-1 

 31-6 

 36-1 

 44-6 

 45-1 

 54-1 

 67-1 



