226 



THE ESSENTIALS OF HISTOLOGY 



central fovea itself there are no rods, and the cones are very long and 

 slender ; moreover, all the other layers become gradually thinned 

 down almost to complete disappearance, so that the middle of the central 

 fovea is the thinnest part of the retina. Since there are few rods, the 

 outer nuclear layer (6) loses in great measure its appearance of being 

 composed of closely packed nuclei, and the cone-fibres are very distinct. 

 The direction of all the fibres is very oblique in this part of the retina. 

 The pars ciliaris retina which commences at the ora serrata, where 

 the retina proper abruptly ends, is composed of two epithelial layers 

 (fig. 263), and has 110 nervous structures. Of the two layers, the 



FIG. 263. A SMALL PORTION OF 



THE CILIARY PART OF THE RE- 

 TINA. (350 diameters.) 



1, pigment-cells ; 2, columnar cells. 



external is a thick stratum of pigmented epithelium formed of rounded 

 cells and continuous with the pigmentary layer of the retina on the 

 one hand, and with the uvea of the iris on the other ; the inner is a layer 

 of columnar cells each containing an oval nucleus. 



The retina contains but few blood-vessels. The artery enters and 

 the vein leaves it in the middle of the optic nerve. The larger vessels 

 ramify in the nerve-fibre layer, and there are capillary networks in 

 this layer and in the inner nuclear layer. There are peri-vascular 

 lymphatic spaces around the veins and capillaries. The neural epithe- 

 lium receives no blood-vessels, but is nourished from the vessels of the 

 choroid. 



Structure of the lens. The lens is a laminated fibrous body en- 

 closed by a transparent elastic capsule into which, around the circurn- 



FIG. 264. SECTION THROUGH THE MARGIN OF THE RABBIT'S LENS, SHOWING THE 

 TRANSITION OF THE EPITHELIUM INTO THE LENS-FIBRES. 



ference, the fibres of the suspensory ligament are inserted. Immedi- 

 ately within the capsule, in front and at the sides, there is a layer of 

 cubical epithelium termed the epithelium of the capsule, but at the 

 margin of the lens the cells become longer and pass by a gradual 

 transition into the lens-fibres (fig. 264). The fibres which compose the 

 lens are long and riband- shaped, with finely serrated edges (fig. 265, A) ; 



