THE SENSES 



milk glass shade of a lamp or the blue sky, and moving the slightly 

 separated fingers or a perforated card rapidly before the eye. From 

 the rate of their apparent movement, Vierordt calculated the velocity 

 of the blood in the retinal capillaries at 0*5 to o'g mm. per second. 

 One reason why the shadows of these intra-retinal structures do not 

 appear in ordinary vision seems to be their small size. The retinal 

 vessels are in reality only vascular threads ; the thickest branch of the 

 central vein is not -,^ r - mm. in diameter. The apex of the cone of 

 complete shadow (umbra) cast by a disc of this size, at a distance 

 of 20 mm. from a pupil 4 mm. wide, would lie only i mm. behind the 

 disc rthat is to say. the umbra of the retinal vessels would not 

 reach the layer of the rods and cones at all, and only the penumbra, 

 or region of relative darkness, would fall upon it. 



When the eyes, after being closed for some time, are suddenly 

 opened, the branches of the 

 retinal vessels may be seen for 

 a moment. This is especially 

 the case after sleep ; and a 

 good view of the phenomenon 

 may be obtained by looking 

 at a white pillow or the ceiling 

 immediately on awaking. If 

 the eyes are kept open for 

 a few seconds, the branching 

 pattern fades away ; if they 

 are only allowed to remain 

 open for an instant, it may 

 be seen many times in succes- 

 sion. The maui vessels appear 

 to radiate out from a central 

 point. But their actual junc- 

 tion there is not seen, since it 

 lies in the optic disc or blind 

 spot. 



The Blind Spot. The 

 fibres of the optic nerve are 

 insensible to light ; light 

 only stimulates them 

 through their end-organs. 

 This can be proved by direct- 

 ing by means of an ophthal- 

 moscope a beam of light upon the optic disc, where the true 

 retinal layers do not exist. The person experimented on has no 

 sensation of light when the beam falls entirely upon the disc ; 

 when its direction is shifted so that it impinges upon any other 

 portion of the retina, a sensation of light is at once experienced. 

 The blind spot is not recognised in ordinary vision, for (i) the 

 two optic discs do not correspond. The left disc has its corre- 

 sponding points on a sensitive part of the right retina, and the 

 right disc on a sensitive part of the left retina ; and the con- 

 sequence is that in binocular vision the objects whose images 

 are formed on the corresponding points fill up the blind spots. 



592 



FIG. 409. METHOD OF RENDERING THE 

 BLOODVESSELS OF THE RETINA VISIBLE 

 BY OBLIQUE ILLUMINATION THROUGH 

 THE CORNEA. 



Light from a candle at a illuminates a', 

 and rays proceeding from a' cast a shadow 

 of the bloodvessel, v, at a", which is referred 

 to a'". When a is moved to b, the shadow 

 on the retina moves to b", and the shadow 

 in the visual field of the illuminated eye 

 to &"'. 



