. The structure and life of birds . he human eye,at the lowest estimate. On these millions of sensitivepoints the image is formed. They extend overthe back of the eyeball, but there is a central mark,called the Macula Lutea (ML) or yellow spot, which The entrance of is the region of clearest visionthe nerve is rather towardsthe nose (ON), and at thispoint the eye is blind, asa simple experiment willshow (BL). Hold the book at armslength, close the left eye, andfix the right upon the crossmark, the image of whichwill fall upon the MaculaLutea. The dot will be alsovisible. Now move the bookslowl
. The structure and life of birds . he human eye,at the lowest estimate. On these millions of sensitivepoints the image is formed. They extend overthe back of the eyeball, but there is a central mark,called the Macula Lutea (ML) or yellow spot, which The entrance of is the region of clearest visionthe nerve is rather towardsthe nose (ON), and at thispoint the eye is blind, asa simple experiment willshow (BL). Hold the book at armslength, close the left eye, andfix the right upon the crossmark, the image of whichwill fall upon the MaculaLutea. The dot will be alsovisible. Now move the bookslowly towards you, and theimage of the dot must atlength fall upon the pointwhere the nerve enters theeye. At this moment the +dot will disappear, then again at a nearer distancecome again into view, as the image of it gets oncemore clear of the blind spot. The rods and conescover the whole of the back of the eye, except this onepoint. Another experiment shows that it is the hind-most layer of the retina that is sensitive. Let a candle. Fig. 32.—Section of Retina of duck(after Cajal); C, Cone; R, Rod;S, cells of supporting tissues. 126 THE STRUCTURE AND LIFE OF BIRDS chap. be the only light in the room. Get some one to holdit by the side of your eye, and with the helpof a lens to focus the rays upon it. Look at the wall,which must be uniformly coloured. The shadows ofthe blood vessels which ramify in the retina in front ofthe rods and cones will be distinctly visible. Not onlyis it the hindmost layer on which light makes itselffelt, but the rods and cones look backward. In theinvertebrate eye the retina looks forward, and its frontsurface is the sensitive one. In this important pointthe pineal body is the eye of an Behindthe retina is a deep layer of dark pigment, called theChoroid (CH); in this the rays after passing the sensitivecells are absorbed. Were they reflected from one partof the retina to another, any clearness of vision wouldbe impossible. But it would
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