538 PHYSIOLOGY 



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 con- 

 siderably 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 (summa- 

 tion 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 of stimulus is that known as ' contrast.' A finger plunged 

 into mercury feels a ring of constriction at the level of the surface of 

 the mercury, i.e. where there is a contrast between the pressure of the 

 mercury and the absence of pressure as the finger emerges into the air. 



The strength of the effective stimulus depends also on the number 

 of nerve- endings simultaneously excited. Thus when dealing with 

 tactile sensations, or sensations of pressure, in determining the minimal 

 stimulus we must take into account the area stimulated, and we 

 express the stimulus, just sufficient to produce a threshold sensation; 

 as * weight per square millimetre of surface.' Moreover the rapid 

 ' fatigability ' or adaptation of all sense-organs makes the rate at which 

 the stimulus is applied of considerable importance. Thus when 

 weight was applied to the skin of the ball of the thumb at the rate of 

 0-75 gramme per second over a surface of 21*2 sq. mm., the minimum 

 load necessary to evoke a distinct sensation was 2-5 grammes. When, 

 however, the rate of application of the weight was increased to 

 5 grammes per second, distinct sensation was produced with a load of 



