8i4 A MANUAL OF PHYSIOLOGY 



rabbhs 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 analyze 

 the baciilary 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 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 most serious difficulties in the way of under- 

 standing how a ray of light can set up an excitation in a rod 

 or cone 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 

 transparent 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 retina, just as the touch of a 

 child's hand could be made to fire a mine, or launch a ship, 

 or flood a province. Some have looked upon the transverse 

 lamellae into which the outer members of the rods and cones 

 can be made to split as an arrangement for reflecting back 

 the light to the inner members, and have compared them to 

 a pile of plates of glass, which, transparent as it is, is a most 

 efficient reflector. It is even possible, although here we are 

 already treading the thin air of pure speculation, that the 

 light may be polarized in the process of reflection, and that 

 the rods and cones may be less transparent to light polarized 

 in certain planes than to unpolarized light. 



As to the nature of the transformation undergone by the 

 ethereal vibrations in the rods and cones, various theories 



