748 SPECIAL HISTOLOGY. 



optic nerve, acquire more and more of the character of common nerve- 

 tubules, and follow a more direct course. However interesting this 

 notion may be, according to which the "rods" would be the termina- 

 tions of the optic fibres, the considerable difficulties attending it must 

 not be concealed ; among which not the least is the circumstance, that, 

 although the " rods" and " cones" are certainly fifty times more 

 numerous than the fibres of the optic nerve, yet the radiating fibres 

 arising from the former, on their passage into the optic fibres, subdivide, 

 and, as it must probably be assumed, are continuous with several of 

 them a difficulty, which might indeed be removed on the hypothesis, 

 that a single optic fibre receives or gives off numerous radiating fibres, 

 but is nevertheless of such a kind that I do not consider it advisable to 

 proceed any further upon a basis unsupported by facts. 



In spite of the obscurity which, from what precedes, still hangs over 

 a very important point in the anatomy of the retina, physiology may 

 nevertheless even at present draw some useful conclusions from the facts 

 in our possession. In the first place, since the demonstration by H. 

 Miiller and myself of its connection with the radiating fibre-system and 

 the "granules," the bacillar layer appears in quite a different light from 

 that in which it was previously held, and it is now obviously impossible 

 to regard it, with Briicke, as a catoptric, reflecting apparatus. I look 

 upon the "rods" and " cones," which may also be said to correspond in 

 all chemical characters with the nerve-fibres of the retina, and the,w T hole 

 of the radiating fibre-system of the retina, as true nervous elements ; 

 and venture at the same time to broach the bold supposition, founded 

 upon a less established basis, which has been already thrown out, that 

 the "rods" and "cones" are the true percipients of light, and that they 

 communicate their condition to the fibres of the optic nerve, by means 

 of the direct or indirect connection of their fibrous processes with the 

 former, through which again the impressions are conveyed to the senso- 

 rium. That the optic-fibres in the nervous expansion of the retina, do 

 not perceive light, appears to me to be proved by the circumstance : 1, 

 that the point of the retina, where those fibres alone, and no other ele- 

 ments of the retina, are found, viz. at the entrance of the optic nerve, 

 is not sensitive to light ; 2, that the optic fibres are superimposed upon 

 each other in such numbers, in almost every part of the retina, and 

 above all in the neighborhood of the macula lutea, that it is impossible 

 they should perceive light, inasmuch as each luminous impression, owing 

 to the transparency of the fibres, must in any case always affect many 

 of them, and consequently would of necessity give rise to confused sen- 

 sations ; and 3, because the part of the retina in which there is no con- 

 tinuous layer of nerve-fibres on the inner surface, that is to say, the 

 "yellow spot," is the most sensitive to luminous impressions. Under 

 this notion, the import of the "rods," and their remarkable arrange- 



