480 PHYSIOLOGY 



another. Thus sound applied to the ear is adequate but to the eye is not. 



The sense organs appear to be elaborated from a very simple type, and 

 the higher they are developmentally the more is one kind of stimulus adequate 

 and the less are all others. 



Thus the end organs in the cornea react alike by a sensation of pain to 

 touch, electricity, heat, inflammation, &c., while the ear reacts to sound 

 waves only. This specific property of the end organs is also attended by an 

 increase in the strength of response to the chosen stimulus. Thus the retina 

 of the eye can be stimulated mechanically and electrically, but has its greatest 

 sensitiveness to light. It is estimated that the eye is 30,000 times as 

 sensitive to light as any instrument that has been so far constructed. This 

 specialisation of the end organs makes the information which we obtain 

 from them more detailed and complete, but at the same time sets a limitation 

 to the range of stimulus to which each can respond. Thus a pain end organ 

 can react alike to heat rays, visible rays and ultra-violet rays, while the retina 

 of the eye responds to visible rays only. 



THE LAW OF SPECIFIC IRRITABILITY. 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 snisat ion of light may be produced by 

 any >t inmlation of the retina, or of the optic nerve, or of the termiiuit ions 

 of the optic nerve in the brain. J n the same way stimulation of an auditory 

 nerve or its intracraninl nulm-- gives rise to sensations of sound. 



Where the question li;i- IHMMI iim><t iu.-ifed it has not been found p 



