350 SEA-SIDE STUDIES. 



ill the pigment becomes a luminous sensation will not be 

 difficult, if, remembering that the luminous sensation is one 

 not depending on the specific stimulus of light, but on the 

 specific nature of the optic centre, we follow this change in 

 its passage from the pigment to the rods and cones of Jacob's 

 membrane, which it first affects ; these are in direct connec- 

 tion with the ganglionic nerve-cells, in which we may suppose 

 the nervous impression to be excited ; this impression is 

 thence transmitted by means of the optic fibres to the optic 

 ganglion, aiul there it becomes a sensation. This is hyjjo- 

 thetical, I admit ; but it is the only hypothesis which can 

 agi'ee with the present condition of our anatomical knowledge. 

 Funke has a good illustration. The wave of light, he says, 

 can no more excite the optic neiTC directly, than the pressure 

 of a finger on the air, or the walls of the organ-pijies can 

 excite musical notes. The finger produces a tone by pressing 

 on the keys ; each particular key that is pressed brings forth 

 a corresponding tone as the air enters the pipe. In this 

 illustration the optic fibres are as the organ-pipes, the rods 

 and cones of Jacob's membrane as the keys, and the wave of 

 light as the wave of air. 



The most convincing argument against the retina as the 

 receiving screen of images, and in favour of the pigment layer, 

 is, in my opinion, to be found in the eyes of the Invertebrata, 

 where the pigment is in front of the retina, instead of behind 

 it, as in the Vertebrata. I have examined this point with 

 great care, and the result is, that, although in crabs and 

 insects, for instance, radial fibres in connection with the 

 retina pass through the pigment, and are consequently 



