STEUCTUEE OF THE EETINA. 273 



fibres and nuclei of the rod-and-cone elements. At the bases of the 

 rods and cones, this sustentacular tissue ceases, being here bounded by 

 a distinct margin which has been called the external limiting membrane 

 (fig, 304, m.e.L), but delicate sheaths pass from it round the bases of 

 the rods and cones. Each Miillerian fibre, as it passes, through the 

 inner nuclear layer, has a nucleated enlargement (n), indicating the 

 original cell-nature of the fibre. 



There are two parts of the retina which call for special description. 



The macula lutea (yellow spot, fig. 305), with its central fovea, lies 

 in the visual axis, and is the part of the retina which is most immedi- 

 ately concerned in direct vision. It is characterised firstly by its 



FlG. 306. A SMALL PORTION OF THE 

 CILIARY PART OF THE RETINA. 



(Kolliker.) (350 diameters. ) 



1, pigment-cells ; 2, columnar cells. % 





greater thickness (except at the middle of the fovea), secondly by the 

 large number of ganglion-cells, which are all distinctly bipolar (2), and 

 thirdly by the large number of cones it contains as compared with the 

 rods. In the central fovea itself there are no rods, and the cones are 

 very long and slender ; 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 appear- 

 ance 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 retinae, which commences at the ora serrata, where 

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

 (fig. 306), and has no nervous structures. Of the two layers, the 

 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 perivascular 

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

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



choroid. 



s 



