SEAT OF VISION. 255 



The fact, observed by Mariotte, was not suffered to remain in repose. 

 A new hypothesis of vision was framed upon it ; and, as he considered 

 it demonstrated, that the optic nerve was insensible to light, he drew 

 the inference, that the retina is so likewise ; and as vision was effected 

 in every part of the interior of the eye, except at the base of the optic 

 nerve, where the choroid is alone absent, he inferred that the choroid 

 must be the true seat of vision. The controversy, at one time main- 

 tained on this subject, has died away, and it is not our intention to 

 disturb its ashes, farther than to remark, that De La Hire, 1 who en- 

 gaged in it, entertained the opinion, that the retina receives the impres- 

 sion of the light in a secondary way, and through the choroid coat as 

 an intermediate organ ; and that by the light striking the choroid, the 

 membrane is agitated, and the agitation communicated from it to the 

 retina. The views of De La Hire are embraced by Sir David Brewster, 2 

 as well as by numerous other philosophers. 



The opinions of Mariotte have now few supporters. The remarks 

 already made regarding the optic nerve ; the effect of disease of the 

 retina, of the nerve itself, and of its thalami, compel us to regard its 

 expansion as the seat of vision ; and if we were even to admit, with 

 Mariotte, that the insensible portion is really a part of the medullary 

 matter of the nerve, and not a bloodvessel existing there, we could 

 still satisfactorily account for the phenomenon by the anomalous cir- 

 cumstances in which the nervous part of the organ is there placed. 

 The choroid coat, of great importance in the function, as well as the 

 pigmentum nigrum, is absent ; and hence we ought not to be surprised, 

 that the function is imperfectly executed : we say imperfectly, for the 

 experiment with the candles exhibits, that the part is not really insen- 

 sible to light, or is so in a very small portion of its surface only. It 

 may seem at first sight, that the fact of this defect existing only in the 

 centre of the optic nerve, or at the porus options as it has been termed, 

 where the central artery of the retina enters, and the corresponding 

 vein leaves the organ, militates against the idea of its being caused by 

 the rays impinging upon these vessels ; as, if so, we ought to have 

 similar defects in every part of the retina, where the ramifications of 

 these vessels exist. Circumstances are not here, however, identical. 

 When the ray falls upon the porus opticus, it strikes the vessels in the 

 direction of their length ; but, in the other cases, it falls transversely 

 upon them, pierces them, and impresses the retina beneath ; so that, 

 under ordinary circumstances, little or no difference is perceived between 

 the parts of the retina over which the vessels creep, and others. We 

 can, however, by an experiment of Purkinje, described by J. Gr. Stein- 

 buch, 3 exhibit, that under particular circumstances such difference really 

 does exist, and renders the bloodvessels of the organ perceptible to its 

 own vision. If, without closing the eyelids, the left eye be covered 

 with the hand, or some other body, and a candle or lamp be held in the 

 right hand, within two or three inches of the right eye, but rather 



1 Mem. de I'Academie, torn. ix. 



3 Treatise on Optics, Amer. edit., by A. D. Bache, p. 243, Philad., 1833. 

 3 Beitrag zur Physiologic der Sinne, Niirnberg, 1811. J. Miiller, Elements of Physiology, 

 by Baly, ii. 1163, Lond., 1839. 



