IO6 ESSENTIALS OF PHYSIOLOGY, 



in other words, each sense has its own specific quality or modality. 

 This characteristic was stated by Miiller in the form of a law, which 

 he called the " law of specific nerve energy." It is better, however, in 

 view of the restricted modern use of the word energy, to speak of the 

 law of specific irritability. The quality of the sensation aroused might 

 be determined by the nature of the receptive end organ, by the con- 

 ducting apparatus, or by the area of the cortex to which the impulse 

 is transmitted. Direct stimulation of the central portion of a divided 

 sensory nerve gives rise to the specific sensation, so that the modality 

 is not determined by the end-organ. For example, pressure on the 

 ulnar nerve trunk excites a pricking sensation referred to the distribu- 

 tion of the nerve, and section of the optic nerve in a conscious patient 

 is accompanied by the sensation of flashes of light. Again, there is 

 no reason to believe that the conducting nerve fibres have any more 

 influence on the nature of the impulse they convey than an electric 

 wire has in determining the nature of a telegraphic message. It is, 

 therefore, in the cortex of the cerebral hemisphere that the ex- 

 planation of the specific character of the sensation is to be sought, 

 and this conclusion is supported by the fact that sensations may be 

 aroused in the absence of any stimulation of the end-organs, e.g. in 

 dreams or hallucinations. 



(2) In the case of each sense a certain minimal strength of stimulus, 

 known as the threshold stimulus, is necessary to evoke a sensation. 

 The exact strength of the threshold stimulus for any particular sense 

 varies in different individuals, and also in ^ the same individual at 

 different times. A succession of subliminal stimuli, that is, of stimuli 

 each of which is below the threshold value, may excite a sensation by 

 a summation effect, just as the summation of subminimal stimuli may 

 excite a reflex action. The threshold value will vary with the condition 

 of the sense organ. For example, the mechanism may be fatigued, and 

 will then be less responsive to stimulation. This is well illustrated 

 in the case of smell. The air of a closed room, which is occupied, 

 becomes disagreeable, but the occupants of the room do not notice the 

 unpleasant smell, which is at once apparent to anyone coming in from 

 the outer air. The threshold value also varies with the state of 

 adaptation of the sense organ. Thus an eye which has been exposed 

 to light is said to be light-adapted, while one that has been in 

 darkness for a time is dark-adapted. The threshold value of the 

 light stimulus for the dark-adapted eye is much lower than, actually 

 about one-fiftieth of, that required to produce a sensation in the light- 

 adapted eye. 



(3) The increase of stimulus necessary to cause a difference in the 



