362 SEA-SIDE STUDIES. 



which the optic centre is alone capable, and these will 

 be transmitted* 



When, a little while ago, I said that the formation 

 of an image was a primary requisite in vision, I meant 

 that unless the rays from an object converged into an 

 image on a proper surface, no distinct perception of 

 that object could result. The reader will not, there- 

 fore, suppose that, in throwing doubt on the notion of 

 images being formed on the retina, as they are formed 

 on the camera-obscm^a, any attempt will be made to over- 

 throw the optical principles minutely established by phi- 

 losophers. A brief description of the retina and its con- 

 nections will enable us to argue this point at our ease.-|- 



The retina is not, as commonly supposed, simply an 

 expansion of the optic nerve — if by that be meant a 

 purely fibrous layer ; it is more accurately described as 

 a membraniform ganglion. After entering the eye, 

 the nerve expands, and lines the inner surface with a 

 layer of fibres ; but beneath this layer is one of cells, 

 not distinguishable from those of the brain, and 

 beneath that, one of granules ; beneath this layer, 

 again, is another of perpendicular rods and cones, 

 known as the "membrane of Jacob." So that we 

 have four distinct layers, very dissimilar in structure, 

 and of course very diff"erent from the optic nerve, 

 which is simply fibrous. Instead of regarding the 



* " Light and colour are actions of the retina, and of its nei-vous pro- 

 longations to the brain." — Mueller : Physiology, Eng. Trans., p. 1162. 



+ The student should carefully read KoLLIKER, Hcuidhiich der 

 Gewebelehre, and H. Muller, Anatomisch-physiologische Untersudmnfjen 

 uher die Retina lei Menschen u. Wirhelthieren. 1856. 



