EYE 449 



fibers to 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. 456. 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 retinas. The optic nerve fibers and their ganglion cells 

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

 toward the ora 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. 457). Near the ora serrata large clear spaces 

 normally occur in the outer nuclear layer, and they may extend into the 

 deeper layers (Fig. 457). 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 appar- 

 ently unmodified as it extends from the optic to the ciliary portion. Along 

 the inner surface of the ciliary part of the retina, the cells of the visual 

 layer produce closely packed horizontal fibers, which form a refractive 

 hyaline membrane. 



Zonula ciliaris. Some of the fine homogeneous fibers arising from the 

 pars ciliaris immediately in front of the ora serjrata enter the vitreous body, 

 but a much larger number pass between the ciliary processes to the lens. 

 They are attached to the borders of its capsule, overlapping slightly its 

 anterior and posterior surfaces. Thus they form the zonula ciliaris (sus- 

 pensory ligament) which holds the lens in place (Fig. 452). The zonula 

 is not a continuous layer, nor does it consist of two laminae, one to the an- 

 terior and the other to the posterior surface of the lens, with a space be- 

 tween them. It consists rather of numerous bundles, between which and 

 the vitreous body, and among the bundles themselves, there are zonular 

 spaces (canals of Petit) which communicate with the posterior chamber. 



OPTIC NERVE, 



In its intraorbital portion the optic nerve is surrounded by prolonga- 

 tions of the meninges. On the outside is the dural sheath, consisting of 

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