THE ACTION OF THE RETINA. 449 



thus excite the sensation of a uniform visual field, having definite 

 points of locality, which would be impossible if the light directly 

 stimulated the plexiform nerve-fibres ; for, in that case, the same fibre 

 would receive numerous luminous impressions along successive points 

 of its course, and so would be excited, without an exact localization 

 of the sensory impressions. The position of the rods and cones per- 

 pendicularly, or nearly so, to the retinal surface, evidently adapts 

 them for the distinct reception of local points of light, for which pur- 

 pose the lateral surfaces of the linearly-expanded optic fibres are ill 

 suited. Beyond the rods and cones, is placed the dark ehoroid coat, 

 the pigment of which is supposed incessantly to absorb the light which 

 passes through, so as to prevent confusion from successive impressions. 

 In Albinos, in whom this black pigment is wanting, vision is imper- 

 fect, especially in strong lights, which may even cause pain. 



[Professor Draper,* of New York city, has- recently revived and de- 

 veloped the theory held by some early optical writers, but long since 

 abandoned, that the black pigment of the choroid coat is the receiving 

 screen for the image. His arguments, based upo-n optical and anatom- 

 ical grounds, are well founded, and apparently unanswerable. In the 

 first place, the retina, during life, is a perfectly transparent medium, 

 and therefore as incapable of receiving an image as is a sheet of trans- 

 parent glass or the atmosphere itself. But the black pigment, by 

 completely absorbing the rays, not only prevents indistinctness, by 

 preventing reflection, but also converts the rays into heat. It is well 

 known that an essential condition of perfect vision requires that im- 

 ages should form on a mathematical superficies, and not in the midst 

 of a transparent medium; a condition supplied by the black pigment 

 and not by the retina. 



By considering the retina in radial section, as recommended by H. 

 Miiller, it appears that the four strata composing it 1st, Jacob's layer 

 of rods and cones; 2d, The granular layer; 3d, The vesicular layer; 

 4th, The fibres of the optic nerve are really connected in such a way 

 that, passing in a radial direction, as respects the globe of the eye, 

 these different elements are successively combined to constitute the 

 "radiated fibre system." That is, from each of the proper fibres of 

 the optic nerve, going from within outwards, passes a thread-like 

 body, including, successively, a vesicle, a granule, a co-ne, and termi- 

 nating in a rod; keeping up, in this way, through the thickness of the 

 retina, a continuous nervous communication from the extremity of the 

 rod to the fibres of the optic nerve, so that the true termination of 

 each fibre is therefore a rod. According to Miiller and Kolliker, the 

 rods and cones of Jacob's membrane are the true percipients of light, 

 communicating their condition to the fibres of the optic nerve, by 

 means of the connection which they thus maintain with it ; or, per- 

 haps, as stated by Dr. Draper, "the rods and cones are conductors of 

 the luminous impression to the nerve-cells of the retina, which consti- 



* Human Physiology, by J. W. Draper, M.D. 



29 



