362 HISTOLOGY. 



consists of an outer pigmented and an inner visual layer. The latter 

 includes an outer layer of visual cells, rod cells and cone cells. The 

 bipolar cells of the ganglion retinae receive dendritic fibers which have 

 free endings between the visual cells. They give rise to branching neu- 

 raxons which communicate with the ganglion cells of the optic nerve. 

 The neuraxons of the latter converge at the papilla of the nerve and extend 

 to the brain. The retina also receives fibers from the brain. It contains 

 an ectodermal supporting tissue, blood vessels in its inner layers, and 

 nerve cells perhaps commissural, the significance of which is still obscure. 



Macula lutea and fovea centralis. When vision is centered upon a 

 particular object the eyes are so directed that the image of the object falls 

 upon the macula lutea or yellow spot of the retina, within which there is a 

 depression, the fovea centralis. The macula receives straight slender 

 fibers from the papilla of the optic nerve which is close by on its median 

 side; other coarser optic fibers diverge as they pass the macula, forming 

 an ellipse around it. The retinal layers of the macula are arranged as 

 shown in Fig. 414. At its border the number of rod cells diminishes and 

 within the macula they are entirely absent. The nuclei of the numerous 

 cone cells, which are here somewhat smaller than elsewhere, form an inner 

 nuclear layer of twice the usual thickness. The basal portions of the 

 cone cells make a broad Henle's fiber layer and slope away from the 

 fovea. The bipolar cells of the ganglion retinae are so numerous that 

 their nuclei may form nine rows. The ganglion cells of the optic nerve 

 are also abundant. All of these strata become thin toward the fovea, 

 the deepest part of which contains scarcely more than the cone cells. In 

 some individuals the slope of the sides of the fovea is less steep than in the 

 figure; its depth is variable. The macula and fovea are saturated with a 

 yellow pigment soluble in alcohol. 



Pars ciliaris retinae. The optic nerve fibers and their ganglion cells 

 disappear before reaching the ora serrata. The cone cells extend further 

 than the rods, but the last of them appear to lack outer segments. By 

 the thinning of the reticular layer the nuclear layers become confluent 

 (Fig. 415). Near the ora serrata large clear spaces normally occur in 

 the outer nuclear layer and they may extend into the deeper layers (Fig. 

 415). The radial sustentacular cells form a simple columnar epithelium 

 as the other layers disappear, and they constitute the visual layer of the 

 pars ciliaris. The pigmented epithelium is apparently unmodified as it 

 extends from the optic to the ciliary portion. Along the inner surface of 

 the visual layer of the ciliary retina the cells produce horizontal fibers 

 closely packed, which form a refractive hyaline membrane. 



Zonula ciliaris. Some fibers arising from the pars ciliaris immedi- 



