490 



termed Jacob's membrane. The ultimate filaments of the optic 

 nerve, after being connected in a plexiform arrangement in the gan- 

 glionic layer of the retina, terminate each independently, in the 

 more perfect portion of the retinal field, by passing into, or becoming 

 continuous with, the inner end or side of a rod. Kblliker considers 

 these nodes as nervous structures, that is, as terminal portions of 

 the nerve-filaments themselves, and holds that they constitute the 

 parts of the nervous structure of the eye on which objective light 

 primarily acts. 



Having myself carefully examined the structures to which I have 

 now alluded, I have been able to verify the more important anato- 

 mical details, as described by their discoverers, and agree with Kol- 

 liker in considering the rods as the primary optic apparatus. I 

 cannot, however, coincide with this distinguished observer in holding 

 these rods as modified nerve filaments. I hold them to be special 

 structures appended to the extremities of the ultimate nerve fila- 

 ments, and referable to the same category as the Pacinian bodies, 

 touch-corpuscles, rods of Corti, &c. ; and moreover, so far am I 

 from coinciding with Kolliker in his speculations as to the part of the 

 rod on which the objective light acts, that I have found myself com- 

 pelled, not only from the consideration of the structures themselves, 

 but also from the development of the eye itself, and the arrange- 

 ments of the compound eye, to conceive the rays of light as acting 

 upon the retina, not as they impinge upon it, or pass through it from 

 before, but as they pass backward again out of the eye after re- 

 flection from the choroid. 



The general aspect of the rods, and more especially of those por- 

 tions termed Miillerian filaments, where they collectively amalgamate 

 in the limitary membrane of the retina, indicate, as I believe will be 

 generally admitted, that they consist of a modification of connective 

 tissue, enveloping and supporting the extremities of the ultimate 

 nerve filaments in such a manner as to form special structures, which, 

 from their functions, may be termed photcesthetic bodies. 



That special structures are required for the initiation of action in 

 the filaments of the optic nerve by objective light, appears to be es- 

 tablished by the facts, that the nervous filaments of the retina, and 

 the cut extremities of these filaments on the stump of the optic 

 nerve, are not aff'ected by it, although irritation of the same fila- 

 ments by electrical or other means produces subjective luminous phe- 



