MODE IX WHICH LIGHT ACTS OX THE KETIXA. 277 



its functional current (tliat is, the coninion nervous current) 

 excited or initiated by electrical or other physical or chemical 

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

 the special functional purj)OSes for whicli each nervous fila- 

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

 with which such lihiment is connected peripherally. If so, 

 then not only are the individual hlaments of the nerves of 

 special sense provided with cuiTent-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-Hlaments of whatever kind 

 are provided, in. their connection with the textures from wliich 

 they proceed, with arrangements by means of which alone 

 their functional currents can be initiated. 



From this point of view every particidar 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 nurve-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, T am in a position from 

 which I can explain the mode in which I undei-staud the 

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

 and the columns in the compound eye. 



1. Ill the Simjdc Eye. — A ray of light can only impn.-.s> ;in 

 ultimate retinal nervous filament under certain conditions. 

 These conditions are, that it should impinge \\\)0\\ the distal 

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

 filament, or within a certiiin angle to that axis. 



All rays impinging on the disUd extivmity of an ultimate 

 retinal nervous tilament mider the conditions stated I term 

 photoijfiiir rays. Kays iinpingiug tijion, or passing through. 



