VISION 



1045 



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 blood- 

 vessels, the only competitors of the rods and cones are the external 

 nuclear layer and the pigmented epithelium. The nuclear layer 

 may be at once excluded as a separate mechanism, since, as we have 

 seen (p. 1015), 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, and men, in 

 whose eyes pigment is absent, can see. In man and most mammals 

 there are cones, but no rods in the yellow spot and fovea centralis; 

 the relative proportion of rods increases as we pass out from the 

 fovea towards the ora serrata. But this does not enable us to 



Fig. 444, Mariotte's Experiment. 



analyze the bacillary layer into sensitive cones and non-sensitive 

 rods, for on the rim of the retina, which is still sensitive to light, 

 there are only rods; in the bat and mole there are said to be. no 

 cones even in the yellow spot, in the rabbit very few. Reptiles 

 possess only cones over the whole retinal surface, and birds, true 

 to their reptilian affinities, have everywhere more cones than rods, 

 as have also fishes. . 



One of the difficulties in the way of understanding how a ray of 

 light can set up an excitation in a rod or cones is the transparency 

 of these structures. An absolutely transparent substance that 

 is, a substance which would allow light to traverse it without the 

 least absorption would, after the passage of a ray, remain in 

 precisely the same state as before; its condition could not be 

 altered by the passage of the light unless some of the energy of the 

 ethereal vibrations was transferred to it. But an absolutely trans- 

 parent body does not exist in Nature; and it is not necessary to 

 suppose that all the energy required to stimulate the end-organs 

 of the optic nerve comes from the luminous vibrations. These 

 may, and probably do, act by setting free energy stored up in the 



