THE EYE AS AN OPTICAL INSTRUMENT. 513 



The Microscopic Structure of the Retina. A simpli- 

 fied stratum, continuous with the proper retina, and formed 

 of a layer of nucleated columnar cells, is continued over the 

 ciliary processes; elsewhere the membrane has a very com- 

 plex structure, and a section taken, except at the yellow spot 

 or the optic mound, shows ten layers, partly sensory appa- 

 ratuses and nerve-tissues, and partly accessory structures. 



Beginning (Fig. 144) on the front side we find, first, the 

 internal limiting membrane, 1, a thin structureless layer. 

 Next comes the nerve-fibre, layer, 2, formed by radiating 

 fibres of the optic nerve; third, the nerve-cell layer, 3; fourth, 



FIG. 143. The right retina as it would be seen if the front part of tho eyeball 

 with the lens and vitreous humor were removed. 



the inner molecular layer, 4, consisting partly of very fine 

 nerve-fibrils, and largely of connective tissue; fifth, the 

 inner nuclear layer, 5, composed of nucleated cells, with a 

 small amount of protoplasm at each end, and a nucleolus. 

 These cells, or at any rate the majority of them, have an 

 inner process running to the inner molecular layer and an 

 outer running to, 6, the outer molecular layvr, which is 

 thinner than the inner. Then comes, seventh, the rod and 

 cone fibre layer, 7, or outer nuclear layer; composed of thick 

 and thin fibres in each of which is a conspicuous nucleus 

 with a nucleolus. Next is the thin external limiting mem- 

 brane, 8, perforated by apertures through which the rods and 

 cones, 9, of the ninth layer join the fibres of the seventh. 

 Outside of all, next the choroid, is the pigmentary layer, 10; 



