812 A MANUAL OF PHYSIOLOGY 



of the blood-corpuscles in the retinal vessels may be rendered visible 

 by looking at a bright and uniformly illuminated ground, like the 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 0*9 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 j the thickest branch of 

 the central vein is not -^ 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 ~ mm. behind 

 the disc that 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 

 succession. The main vessels appear to radiate out from a central 

 point. But their actual junction 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 directing by means of an ophthalmo- 

 scope 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 corre- 

 spond. The left disc has its corresponding points on a 

 sensitive part of the right retina, and the right disc on a 

 sensitive part of the left retina ; and the consequence is that 

 in binocular vision the objects whose images are formed on 

 the corresponding points fill up the blind spots. (2) The 

 optic disc does not lie in the line of direct, and therefore 

 distinct, vision. The eye is constantly moving so as to bring 

 the surrounding objects successively on the fovea centralis ; 

 and the gap which the blind spot makes in the visual field 



