480 PHYSIOLOGY 



substance is more marked than the sweet taste of another. The question 

 arises how far these differences in sensation correspond to and are a measure 

 of differences in the physical events by which they have been evoked. A 

 very little consideration suffices to show that there is no resemblance between 

 a sensation and the stimulus, and that one and the same physical event 

 applied to different sense-organs will evoke absolutely distinct sensations ; 

 while different modes of stimuli applied to one sense-organ will always 

 evoke the same sensation. Thus if light falls on the retina it causes a 

 sensation of light. If the same radiant energy, consisting of transverse 

 vibrations in the ether, be allowed to fall on the skin, it either produces no 

 sensation at all, or, if concentrated by means of a burning-glass, may give 

 rise to a sensation of warmth, heat, or pain. If we take a tuning-fork which 

 is vibrating 100 times per second and apply it to the surface of the skin, we 

 get simply a sensation of vibration, i.e. a series of tactile impressions repeated 

 at rapid intervals. If the same tuning-fork be applied to the head, its 

 vibrations are imparted to the bones of the skull and thence to the auditory 

 nerve-endings and arouse in our consciousness a tone-sensation of a certain 

 note. The same thing happens if the vibrations of the tuning-fork are 

 conducted by the ear to the auditory nerve- endings in the ordinary way 

 through the external and middle ear. On the other hand, a sensation of light 

 may be aroused not only by the incidence of radiant energy of a certain 

 wave length on the retina, but also by electrical or mechanical stimulation 

 of the retina. If the eye be turned inwards and the finger be pressed on 

 the eye through the outer canthus of the lids, a sensation of light is aroused 

 and we see a coloured circle which we refer to some spot lying to the nasal 

 side of the eye stimulated. The character of the sensation bears therefore no 

 resemblance to the physical events by which the sensation is evoked, but 

 depends entirely on the nature of the sense-organ which is stimulated. 

 A sensation of light may be produced by any stimulation of the retina, 

 or of the optic nerve, or of the terminations of the optic nerve in the brain. 

 In the same way stimulation of an auditory nerve or its intracranial endings 

 gives rise to sensations of sound. 



Where the question has been investigated it has not been found possible 

 to evoke different qualities of sensation by different modes of stimulation 

 of nerve fibres, and it has therefore been concluded that the quality of any 

 sensation depends simply and solely on the termination of these nerves 

 in the central nervous system, and that where sensations of different quality 

 are produced there must be also difference of nerve fibres. This idea was 

 formulated by Miiller, and is often alluded to as Miiller's ' law of specific 

 irritability.' The law states that every sensory nerve reacts to one form 

 of stimulus and gives rise to one form of sensation only, though if under 

 abnormal conditions it be excited by other forms of stimuli, the sensation 

 evoked will be the same. 



Although the different forms of sensation must be regarded as dependent 

 on the integrity of the brain, and of its connections with the peripheral sense- 

 organs, sensations are not referred to the brain, but are localised as proceed- 



