278 MODE IN WHICH LIGHT ACTS ON THE KETINA. 



the filament in any other direction, may be termed aphotogenic. 

 The distal portion of the ultimate retinal nervous filament I 

 distinguish as the photccsthetic sm^faec. 



In order that the ultimate retinal nervous filament may 

 be subjected to the rays of light under the required conditions 

 of vision, its distal extremity or photsesthetic surface is inclosed 

 in a peculiar structure, consisting of a so-called rod or cone 

 (which I distinguish as the crystalline column), and its ap- 

 pended Miillerian filament, with its nuclear enlargements. 

 This structure constitutes a specific kind of peripheral nervous 

 organ, which, from its function, I term a 2^^iotcesthetic body. 



A photsesthetic body consists of a distal segment, or 

 dioptric portion, elongated, cylindrical, or club-shaped, homo- 

 geneous, transparent, and highly refractive, usually termed 

 the rod or cone ; and a proximal segment or peduncle, with 

 its nuclear enlargements, into which the ultimate nervous 

 filament passes, and within which it apparently terminates, 

 probably at its outer end. 



The entire aspect and arrangement of these photsesthetic 

 bodies, their predominance over the other parts of the retina 

 at the axial spot of the eye, and the direct continuity of their 

 stems with the nerve-filaments at that spot, appear to me to 

 indicate not only the nature of their functions, but also the 

 general features of the mode in which it is effected. It 

 appears to me that the rays which act upon the nervous 

 filaments must be such rays as the arrangement permits to 

 pass from behind forwards in the axes of the photsesthetic 

 bodies. It has now been ascertained, that the quantity of 

 light reflected, and consequently irregularly dispersed within 

 the eyeball from the choroid and bacillary layer, etc., is very 

 considerable ; and it consequently becomes a very important 

 question, to determme in what manner this reflected and 

 irregularly -dispersed light is prevented from affecting the 

 retina. The vipw which I have already given of the structure 



