1 1 oo THE SENSES 



glosso-pharyngeal nerve, may be assumed to be specifically related 

 to it. Under ordinary conditions the sensation is elicited through 

 the afferent nerves of the pharynx when the mucous membrane 

 becomes dry, as when dry or salt food is eaten, or dry and dusty 

 air inhaled, and local moistening of the area in question gives 

 temporary relief, even when no water is swallowed. When water 

 is long withheld, the water-content of all the tissues sinks, and a 

 more intense and distressing thirst, which cannot be allayed in any 

 way except by the ingestion of water, ensues. Probably in this 

 case afferent impulses originating in many organs, and conditioned 

 in some way by the abnormally low water-content of the blood and 

 tissues, as well as a more direct action of the loss of water upon the 

 (unknown) centre in which the sensation is represented, are re- 

 sponsible. 



Relation of Stimulus to Sensation. It is impossible to measure 

 sensation in terms of stimulus. All that we can do is to compare 

 differences in the intensity of stimuli and differences in the resultant 

 sensations, or, in other words, to compare stimuli together and to com- 

 pare sensations together. And when we determine the amount by 

 which a given stimulus must be increased or diminished in order that 

 there may be a just perceptible increase or diminution in the sensation, 

 it is found that (with certain limitations) the two are connected by a 

 simple law: Whatever the absolute strength of a stimulus of given kind 

 may be, it must be increased by the same fraction of its amount in order 

 that a difference in the sensation may be perceived (sometimes called 

 Weber's law). Thus, a light of the strength of one standard candle 

 must be increased by T ^ th candle, a light of 10 candles by ^ Q , and in 

 light of 100 candles by a candle, in order that the eye may perceive 

 that an increase has taken place, just as the weight necessary to turn 

 a balance increases with the amount already in the pans. The fraction 

 varies for the different senses. It is about ^^ for light, for sound. 

 But it would appear that Weber's law does not hold for the pressure 

 sense, nor for the other senses above and below certain limits. Fechner, 

 making various assumptions, has thrown Weber's law into the form 



y=k - t where y is the intensity of sensation, x the intensity of 



*o 



stimulation, x the smallest intensity of stimulus which can be perceived 

 (liminal intensity), and k, a constant. This so-called psycho-physical 

 law of Fechner states that the sensation varies as the logarithm of the 

 stimulus. But Fechner's law has been subjected to serious criticism, 

 and the subject cannot be further pursued here. 



