368 SEA-SIDE STUDIES. 



ganglionic nerve-cells, in whicli we may suppose the 

 nervous impression to be excited ; this impression is 

 thence transmitted by means of the optic fibres to the 

 optic ganglion, and there it becomes a sensation. This 

 is hypothetical, I admit ; but it is the only hypothesis 

 which can agree with the present condition of our ana- 

 tomical knowledge. Funke has a good illustration. 

 The wave of light, he says, can no more excite the 

 optic nerve directly, than the pressure of a finger on 

 the air, or the walls of the organ-pipes 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^ aro^ument ao;ainst the retina as 

 the receiving screen of images, and in favour of the pig- 

 ment 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 tliis point with great care, and the 

 result is, that, although in crabs and insects, for in- 

 stance, radial fibres in connection with the retina pass 

 through the pigment, and are consequently exposed to 

 the light, yet in every case the vesicular and granular 

 layers and the optic fibres are beneath the pigment. In 

 the eye of the Cephalopoda this position of the pigment 

 has long been a puzzle, and Professor Owen says that 

 it must doubtless be "perforated by the retinal papilla. 



