932 A MANUAL OF PHYSIOLOGY 



(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 

 of a single eye is thus more easily neglected. In any case we 

 ought not to see it as a dark spot, for darkness is only associated 

 with the absence of excitation in parts of the retina capable of 

 being excited by light. There is no more reason why the optic 

 discs should appear dark than there is for our having a sensation 

 of darkness behind us when we are looking straight in front. 

 And since the experience of our other senses the sense of touch, 

 for example tells us that the objects we look at do not in general 

 have a gap in the position corresponding to the part of the 

 image that falls on the blind spot, we see, so to speak, across 

 the spot. 



By Mariotte's experiment, however, the existence of the blind 

 spot can not only be demonstrated, but its size determined and its 

 boundaries mapped out. Let the left eye be closed, and fix with the 



FIG. 410. MARIOTTE'S EXPERIMENT. 



right the small cross ; then, if the eye be moved towards or away 

 from the paper, keeping the cross fixed all the time, a position will 

 be found in which the white disc disappears altogether. In this 

 position its image falls on the blind spot. 



Relation of the Rods and Cones to Vision. We have more 

 than once referred to the rods and cones as the sensitive layer 

 of the retina. It is now necessary to develop a little more the 

 evidence in favour of this statement. And at the outset, since 

 the sensitive layer has been shown to lie behind the plane of 

 the retinal bloodvessels, the only competitors of the rods and 

 cones are the external nuclear layer and the pigmented epi- 

 thelium. The nuclear layer may be at once excluded as a 

 separate mechanism, since, as we have seen (p. 900), the portions 

 of the rod and cone elements in it are continuous with the 

 portions in the layer of the rods and cones proper. In the fovea 

 centralis, where vision is most distinct, the nuclear layer becomes 

 very thin and inconspicuous. 



The layer of pigmented hexagonal cells, or at least their 

 pigment, cannot be essential to vision, for albino rats, rabbits, 



