MODE IN WHICH LIGHT ACTS ON THE RETINA. 275 



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

 with Kolliker in his speculations as to the part of the rod on 

 which the objective light acts, that I have found myself com- 

 pelled, not only from the consideration of the structures them- 

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

 the arrangements of the compound 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 Mlillerian 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 photcesthetic 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 sounds 

 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 corresponding terminal 

 structures in the vestibule. Corresponding terminal structures 

 are in like manner appended to the tactile, olfactory, and 

 gustatory nerves, apparently for a similar purpose, to provide 

 the necessary conditions of the initial excitement of the nervous 

 current by those secondary properties of external bodies to 

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



When the attention of anatomists was directed, a few years 



