STRUCTURE OF THE RETINA. 345 



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 different from the optic nerve, which is simply fibrous. 

 Instead of regarding the retina as composed of layers, how- 

 ever, modern investigators, following Kolliker and Heinrich 

 Miiller, are generally agreed in considering that the fibres of 

 the optic nerve pass radially through the retina : thus from 

 the fibres a thread passes downwards till it meets a cell of 

 the vesicular layer, which in turn is in connection with a 

 granule of the granular layer, which terminates in a cone and 

 rod ; these latter forming the real termination of the optic 

 fibre in the j^ignient layer of the choroid coat. (See Plate 

 III., fig. 4.) It is now universally held that the rods and 

 cones are the percipients of light, which they communicate 

 to the cells of the vesicular layer, thence to the optic fibres, 

 and thence to the optic ganglion. The point to be borne in 

 mind in this description is, that the sensitive part of the 

 retina is not the surface on which the light immediately 

 falls^ hut the surface which is in contact with the black 

 pigment. 



In a parenthesis I may add, that one of the Dorpat school * 



♦ Blessig : De Retina Structara : 1855. See an abstract in Canstatt's 

 JahreshencJd : 1S55. If surprise be felt at the possibility of all the great 



