288 LOCALIZATION OF TOUCH. 



regions are able to detect the least pressure, it is found that 

 the forehead is most sensitive, and nearly equally so are the 

 temples, back of the hand, and forearm. 



The ability to detect differences of pressure is tested by 

 finding what is the least addition to a weight required to 

 make it seem heavier. For instance, if a weight of 11 grains 

 is just perceptibly heavier than one of 10 grains, it does not 

 follow that 1 grain added to a weight of 100 grains will 

 give any palpable increase. To 100 grains must be added 10 

 grains before additional pressure is felt ; that is, whatever the 

 weight, there must be the same ratio of increase to increase 

 the sensation. This is part of the law, already stated, of the 

 relation of stimulus and sensation. The law is true only in 

 a general way, and will not apply in extreme cases. It is 

 stated that the forehead, the lips, and temples appreciate an 

 increase of one-fortieth to one-thirtieth of the weight esti- 

 mated, while the skin of the hand, the fingers, and the forearm 

 require an increase of one-twentieth to one-tenth for its per- 

 ception. 



Discrimination of difference of pressure is more keen if 

 weights are applied in rapid succession on the same spot than 

 when applied at different places or after an interval of time. 

 The weights used in these tests should press on equal areas of 

 the skin. 



The lingering effect of pressure, or After-Pressure, may be 

 noticed after taking off a tight hat, skate-strap, shoe, or glove. 



The Localization of Touch-Sensations. We can usu- 

 ally locate very accurately any skin sensation. " If a point of 

 the skin is touched, certain tactile corpuscles are irritated ; 

 these, in turn, set up impulses in sensory nerve fibers, and 

 these impulses are carried by the fibers, first to the spinal 

 cord, and then to the brain, where the fibers end in ganglionic 

 masses in the gray matter of the cerebral cortex. There are 



