380 SPECIAL HISTOLOGY. 



also be said to correspond in all chemical characters with the 

 nerve-fibres of the retina, and the whole 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 connexion of 

 their fibrous processes with the former, through which again 

 the impressions are conveyed to the sensorium. 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 elements 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 neigh- 

 bourhood 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 sensations ; and 3. because the part of the retina 

 in which there is no continuous 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 im- 

 port of the " rods," and their remarkable arrangement, would 

 be intelligible, and the almost inexplicable correspondence in the 

 size of the images of the smallest distinguishable interspaces be- 

 tween two objects, with the diameter of the "rods" and "cones," 

 be placed in its true light. I consider it impossible to say 

 anything with respect to the import of the other elements of 

 the retina, for although the w granules" may be compared to 

 minute bipolar ganglion-corpuscles, and their continuity with 

 the radiating fibres is known, this affords as little ground for 

 discussion as to their function, as is given by the knowledge 

 of the fact that the large nerve-cells of the inner layers have 

 numerous processes and probably free terminations. 



The vessels of the retina are derived from the art. centralis 

 retinae, which enters the eye inclosed in the optic nerve, and 

 begins to ramify from the centre of the colliculus nervi optici 



