THE EYE AND THE PHYSIOLOGY OF VISION. 



551 



Eighth, the external limiting membrane, followed by, 

 Ninth, the rod and cone layer, the layer in which the 

 actual nervous stimuli arise. As the name indicates, two 

 kinds of structures are found here, somewhat longer and 

 narrower rods, and scattered between these shorter and 

 thicker cones. These rods and cones project into, 



Tenth, the pigment-cell layer, which lies in contact with 

 the choroid just beneath. 



Fig. 171. DIAGRAMMATIC REPRESENTATION OF THE STRUCTURE OF THE RETINA. (After 



Y. Cajal.) 



a, layer of rods S and cones Z Z\ F, F, fibers of Miiller; A, h, external limiting mem- 

 brane; b, outer nuclear layer; c, outer molecular layer; d, inner nuclear layer; e, inner 

 molecular layer; /, nerve cell layer; g, nerve fiber layer; k, k, internal limiting mem- 

 brane. The rods are composed of a highly refractive end portion and a somewhat more 

 protoplasmic basal portion which then, continued through the cells K in the layer &, end 

 in the layer c in little round buttons. Several such buttons seem in turn invested by the 

 dendrons of a single cell H from the layer d. The cones also have a refractive terminal 

 portion, but a more rounded ellipsoid basal portion of protoplasmic structure, from which 

 a relatively large fiber at once passes through the layer b, and ending in dendrous in the 

 layer c, invests the dendrons of a single cell of the layer d. The layer d contains also 

 cells L, which run outward only, and cells M, which send dendrons inward only. The 

 superposition of neurons is quite evident, there being in the retina itself at least three 

 such superposed units. 



Here and there through the retina may be seen some- 

 what stronger fibers reaching from the internal limiting 

 membrane as far as the rod and cone layer. They serve 

 probably for purposes of support and are called the fibers 

 of Miiller. The meaning of the layers of the retina will be 

 more evident by reference to Figure 171, which is a diagram- 



