282 THE NERVOUS SYSTEM 



of subjective sensation, in whatever way that nerve is 

 stimulated. This apphes not only to the nerve-ending but 

 to the nerve-fibre. The optic nerve, however stimulated, 

 only conveys a sensation of light ; the auditory nerve only 

 one of hearing. This is the law of specific irritability, first 

 enunciated by Miiller. The reason why normally a par- 

 ticular nerve only responds to a particular stimulus is 

 partly because it is so situated in the body that only the 

 appropriate stimulus can excite it, and partly because it 

 is endowed with a higher susceptibility to that mode of 

 stimulation than to all others. 



Certain conditions materially affect the subjective sensa- 

 tion arising from a stimulus. One of these is the duration 

 of the stimulus. The sense organs on prolonged stimula- 

 tion become fatigued, and the resulting sensation becomes 

 fainter. Another is the action of a previous stimulus. 

 Hot water, for instance, feels hotter to the hand after 

 cold. For these and other reasons sensations are never 

 an accurate judge of stimuh. Attempts have been made 

 to relate the intensity of the stimulus with the intensity 

 of sensation, but the only law which is to any degree 

 estabhshed is that of Weber, according to which the least 

 increase in stimulus which can be appreciated bears a 

 constant relation to the whole stimulus. If, for instance, 

 a person can only just appreciate the difference between 

 10 grms. and 11 grms., he can only just appreciate the 

 difference between 100 ^rms. and 110 grms. 



1.— CUTANEOUS AND DEEP SENSATION 



The three sensations which may be aroused by contact 

 of the skin with an object are touch, pain and temperature. 

 When an area of skin is carefully examined it is found that 

 the appreciation of these sensations is confined to certain 

 spots. There are spots for touch, for pain, for heat and 

 for cold. These spots are bizarre in shape and distribu- 

 tion. Some overlap one another ; others are separated by 



