492 



then, I hold that, although the ultimate nervous filament may have 

 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 filament 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 current ; but also cen- 

 tripetal nerve filaments of whatever kind, are provided, in their con- 

 nection with the] textures from which they proceed, with arrange- 

 ments, by means of which alone their functional currents can be 

 initiated. 



From this point of view, every particular structure in the or- 

 ganism from which nervous filaments proceed to the nervous centre, 

 may be considered with reference to the nervous system, as a peri- 

 pheral nervous organ, — that is, an organ capable of exciting or ini- 

 tiating centripetal nerve current ; which is physiologically converted, 

 or psychically interpreted at the corresponding central organ, accord- 

 ing 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 ulti- 

 mate retinal nervous filament under certain conditions. These con- 

 ditions 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. 

 Rays impinging upon, or passing through, the filament in any other 

 direction, may be termed aphotogenic. The distal portion of the 

 ultimate retinal nervous filament, I distinguish as the photcesthetic 

 surface. 



In order that the ultimate retinal nervous filament may be sub- 

 jected to the rays of light under the required conditions of vision, its 

 distal extremity or photsesthetic surface is inclosed in a peculiar 



