426 ELEMENTARY PHYSIOLOGY 



passes straight into the outer molecular layer of the 

 retina (Fig. 140, ('.). 



8. The Sensation of Light.— The most notable 

 property of the retina is its power of converting the 

 vibrations of ether, which constitute the physical basis of 

 light, into a stimulus to the fibres of the optic nerve. 

 The central ends of these fibres are connected with 

 certain parts of the brain which constitute the visual 

 sensorium, just as other parts, as we have seen, constitute 

 the auditory sensorium. The molecular disturbances set 

 up in the fibres of the optic nerve are transmitted to the 

 substance of the visual sensorium, and produce changes 

 in the latter, giving rise to the state of feeling which we 

 call a sensation of light. 



The sensation of light, it must be understood, is 

 the work of the visual sensorium, not of the retina ; 

 for, if certain parts of the brain be destroyed or 

 affected, no sensation of light is possible even though 

 the retina and indeed the whole optic nerve be intact ; 

 blindness is the result, because the visual sensorium 

 cannot work. 



Light, falling directly on the optic nerve, does not 

 excite it ; the fibres of the optic nerve, in themselves, are 

 as blind as any other part of the body. But just as the 

 peculiar hair-cells of the cochlea, are contrivances for 

 converting the delicate vil)rations of the perilymph and 

 endolymph into impulses which can excite the auditoi-y 

 nerves, so the structures in the retina appear to be adapted 

 to convert the infinitely more delicate pulses of the 

 luminiferous ether into stimuli of the fibres of the optic 

 nerve. 



9. The "Blind Spot."— The sensibility of the difterent 

 parts of the retina to light varies very greatly. The point 

 of entrance of the optic nerve is absolutely blind, as may 

 be proved by a very simple experiment. Close the left 

 eye, and look steadily with the right at the cross on 



