482 PHYSIOLOGY 



or electrically, is in the normal individual very thoroughly protected from 

 the possibility of such excitation, so that all impulses arising in the retina 

 may be almost certainly referred to changes in the light-waves which fall on 

 the eye, and by means of the dioptric mechanism of this organ are thrown in 

 a distinct pattern upon the retina. Although the sensation is not a repro- 

 duction of the stimulus, it is a symbol of the stimulus, and can be used to 

 inform us of events occurring in the world around. Like stimuli, falling on 

 the same end-organ, always evoke like sensations, other conditions being 

 equal. An orderly sequence of sensations may therefore be interpreted as 

 indicating a corresponding orderly sequence of physical occurrences in the 

 world around us. 



THE QUANTITATIVE RELATIONSHIPS BETWEEN STIMULUS 

 AND SENSATION 



Since our sensations are merely symbols of the physical conditions which 

 give rise to them, it is important to inquire how far they correspond quanti- 

 tatively to differences in the energy of the afferent stimuli, i.e. how alterations 

 in the strength of stimulus will affect the intensity of the resulting sensation. 

 Whatever form of stimulus be applied and whatever sense-organs be affected, 

 a certain minimum intensity of stimulus is necessary for it to be effective, 

 i.e. to produce a minimum sensation. This strength, which varies with 

 different sense-organs, is spoken of as the ' liminal intensity,' or ' threshold 

 value ' of stimulus or sensation respectively. As the strength of the stimulus 

 is increased above this minimal amount the resulting sensation also increases. 

 The change in intensity of sensation is not, however, indefinite. When 

 the stimulus is increased to a certain amount the resultant sensation becomes 

 maximal, and a further increase in the stimulus evokes no further increase in 

 sensation. In fact fatigue of the sense-organs or recipient centres of the 

 brain rapidly sets in, so that the sensation diminishes even with increasing 

 strength of stimulus. In each sense-organ we can measure the amount of 

 energy which must be applied to it in order to evoke a minimum sensation. 

 This figure varies considerably with the physiological condition of the animal. 

 In dealing with reflexes we have seen that the motor result of stimulation of a 

 receptor organ varies in the same manner. Thus a minimal stimulus is more 

 effective if repeated a few times at definite intervals (summation of stimulus) : 

 the stimulus which is subminimal may become minimal and effective as a 

 result of repetition. 



Another factor which intervenes is that known as ' adaptation ' a 

 process associated to a certain extent with the phenomenon of fatigue. 

 Adaptation is best studied in the case of the eye. Here the dark-adapted 

 eye, i.e. one that has been kept from light for half an hour, will react, and give 

 a visual sensation, to a strength of stimulus which is only one-fiftieth of the 

 minimal stimulus required to evoke sensation in the eye that has been 

 lately exposed to light. 



Another phenomenon which may alter the strength of the liminal intensity 



