MODE IN WHICH LICiHT ACTS ON THE RETINA. 27 



Si ■! 



rods of Corti, etc. ; and, morcover, so far am I from coinciding 

 with Kollikt-r in his specnlati(jns as to the part of the rod on 

 whicli the objective light acts, that I have fonnd myself com- 

 pelled, not only from the consideration of tlie sti-uctures them- 

 selves, hut also from the development of the eye itself, and 

 the arrangements of the compoimd 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 reflection from the choroid. 



The general aspect of the rods, and more especially of 

 those portions termed ^Ilillerian filaments, where they col- 

 lectively 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 photccsthctic bodies. 



That special structures are required for the initiation of 

 action in the filaments of the optic nerve by objective light, 

 appears to be established by the facts, that the nervous fila- 

 ments of the retina, and the cut extremities of these filaments 

 on the stump of the optic nerve, are not affected by it, although 

 irritation of the same filaments by electrical or other means 

 produces subjective luminous phenomena. Subjective sminds 

 may be produced by various modes of irritation ; but actual 

 sonant vibrations can only excite the acoustic filaments through 

 the medium of the rods of Corti, or the coiTCsponding teraiinal 

 structures in the vestibule. Corresponding terminal structures 

 are in like manner appended to the tactile, olfactorj', and 

 gustatory nerves, apparently for a similar purpose, to provide 

 the necessary conditions of the initial excitement of the ner\'ous 

 current by those secgndaiy properties of external bodies to 

 which the organs of touch, taste, and smell, are related. 



When the attention of anatrimists was directed, a few yeai-s 



