CHAP. XVII.] EXCITABILITY OF THE RETINA. 51 



image upon it. The image, however, which becomes visible in the 

 experiment on a dead eye, though partly due to the opacity the 

 retina acquires soon after death, is yet an evidence that this mem- 

 brane does not give passage to the light, like transparent glass, or 

 the humors, but rather like ground glass, dispersing and reflecting 

 some portion, as indeed its peculiar texture must dispose it to do. 

 The pink colour of the pupil in albinos shows the reflexion that oc- 

 curs in those cases from the vascular choroid and retina ; and Mr. 

 Gumming has recently pointed out that in the eyes of all persons, 

 where the pupil is tolerably large, a very decided reflexion from the 

 bottom of the eye may be observed under favourable circumstances. 

 To make it apparent he places the individual at a distance of ten 

 feet from a single gas-light or lamp, and directing him to look a little 

 on one side, a strong glare becomes visible to any one standing 

 almost directly between him and the light, In some persons this 

 glare is exceedingly brilliant, like that from burnished brass ; in 

 others it is fainter. This reflexion can hardly be regarded as im- 

 portant in a physiological point of view. It probably proceeds from 

 the hyaloid membrane, the retina, and from the choroid also, but 

 from the latter more or less according to the amount of pigment 

 present in the particular instance. It is remarkable that these re- 

 flexions do not interfere with the perfection of the sense, do not de- 

 range the integrity of the impression resulting from the first passage 

 of the rays. Corresponding but more vivid reflexions from the 

 tapetum lucidum in certain animals serve an useful purpose, by 

 giving an additional stimulus to the retina, where but a feeble light 

 is admitted to the organ. 



Excitability of the Retina, and of the allied nervous parts. When 

 the retina is stimulated, we have the sensation of light, whatever 

 may be the nature of the stimulus applied. Pressure, for instance, 

 made on the side of the eye in the dark, gives rise to the sensation 

 of a spot of light, the situation and size of which will be determined 

 by those of the point of the retina touched. The same is true of 

 the optic nerve, and of certain parts of the encephalon with which 

 the nerve is connected. The sensation of light, then, consists in a 

 recognition by the mind of a certain condition or affection of these 

 nervous parts, and this condition may be induced by the applica- 

 tion of any of the ordinary stimuli of nerves. The retina, however, 

 is capable of being affected in this way by the luminous rays ; and 

 perhaps this capacity is dependent on the peculiar manner in which 

 the nervous matter is spread out in this part. However that 

 may be, the light incident on the retina is the only stimulus which 



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