MODE IN WHICH LIGHT ACTS ON THE RETINA. 277 



its functional current (that is, the common nervous current) 

 excited or initiated by electrical or other physical or chemical 

 agencies, yet this current can only be initiated or excited, for 

 the special functional purposes for which each nervous fila- 

 ment is provided in the economy, by the structure or tissue 

 with which such filament is connected peripherally. If so, 

 then not only are the individual filaments of the nerves of 

 special sense provided with current-exciting structures at 

 their peripheral extremities, by means of which alone the 

 objects to which they are related can initiate the nerve-cur- 

 rent ; but also centripetal nerve-filaments of whatever kind 

 are provided, in their connection with the textures from which 

 they proceed, with arrangements by means of which alone 

 their functional currents can be initiated. 



From this point of view every particular structure in the 

 organism from which nervous filaments proceed to the nervous 

 centre may be considered, with reference to the nervous 

 system, as a peripheral nervous organ that is, an organ 

 capable of exciting or initiating centripetal nerve-current; 

 which is physiologically converted, or psychically interpreted, 

 at the corresponding central organ, according to the special 

 endowments of that central organ. 



After this preliminary statement, I am in a position from 

 which I can explain the mode in which I understand the 

 structure and actions of the rods of the retina in the simple, 

 and the columns in the compound eye. 



1. In the Simple Eye. A ray of light can only impress an 

 ultimate retinal nervous filament under certain conditions. 

 These conditions are, that it should impinge upon the distal 

 extremity of the filament in, or parallel to, the axis of that 

 filament, or within a certain angle to that axis. 



All rays impinging on the distal extremity of an ultimate 

 retinal nervous filament under the conditions stated I term 

 photogenic rays. Kays impinging upon, or passing through, 



