THE SENSE OF TOUCH. 28 1 



weight estimated, while the skin of the head, the fingers, 

 and the forearm require an increase of one twentieth to 

 one tenth for its perception. 



After-Pressure. The lingering effect of pressure, or 

 after-pressure, may be noticed after taking off a tight hat, 

 skate strap, shoe, or glove. 



Local Sign. " If a point of the skin is touched, certain 

 tactile corpuscles are irritated; these, in turn, set up im- 

 pulses in sensory nerve fibers, and these impulses are car- 

 ried 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 thus pro- 

 jected, as it were, on the cortex of the brain, tactile centers 

 for the hind leg, fore leg, neck, eye, ear, trunk, etc. ; and 

 it follows that each point of the skin has a corresponding 

 point in the cerebral cortex. Thus for each stimulation of 

 a point of the cerebral cortex there is a local sign, and so 

 we localize tactile impressions." 



Accuracy in locating Touch Sensations. The accu- 

 racy varies, and is ordinarily keenest where the nerves are 

 most numerous. Where the sense of locality seems to be 

 improved by cultivation, this appears to be due to keener 

 discrimination in the brain cells, and not to changes in the 

 nerves or nerve endings. This is indicated in the fact that 

 if the fingers of one hand become more discriminating by 

 practice, it will be found that the fingers of the other hand, 

 without special training, are also improved. 



Test by Compass Points. The delicacy of localizing 

 touch is usually tested in this way. The blunted points of 

 a light pair of compasses are allowed to rest gently on the 

 skin of various parts of the body. If the two points are 



