THE ORGANS OF SPECI.\L SENSE 



5G5 



cell again becomes narrow (Fig. 374, 5) and in the outer nuclear layer 

 broadens out into a sponge- like reticulum (Fig. 374, 4), which sup- 

 ports the rod and cone bipolars. At the inner margin of the layer of 

 rods and cones the protoplasm of the IMiiller's cells spreads out and 

 unites to form the so-called cuter hmiting membrane (Fig. 374,3), 

 from which delicate fibrils (fibre baskets) pass outward between the 



rods and cones. In addition to the Miil- 

 ler's cells, which are neurogha elements, 

 spider cells also occur in the retina (Fig. 

 373,^4, 18). 



The retina of the macula lutea presents 

 certain peculiarities. Its name is derived 

 from the yellow pigment which is dis- 

 tributed diffusely through the inner layers, 

 extending as far out as the outer mole- 

 cular layer. The ganglion-cell layer and 

 the inner nuclear layer are thicker than in 

 other parts of the retina. In the layer of 

 rods and cones there is a gradual re- 

 duction in the number of rods, while 

 the number of cones is correspondingly 

 increased. 



In the centre of the macula is a depres- 

 sion, the fovea centralis. As the retina 

 approaches this area it becomes greatly 

 thinned, little remaining but the layer of 

 cone cells and the somewhat thickened 

 layer of pigmented epithelium. 



At the ora serrata the nervous elements 

 of the retina cease. The non-nervous ret- 

 inal extension over the ciliary body (pars 

 ciliaris retina) and over the iris {pars 

 iridica retince) have been described in con- 

 nection with the ciHary body and iris. 

 The Optic Nerve. — The optic nerve (Fig. 375, d) is enclosed by 

 two connective-tissue sheaths, both of which are extensions of the 

 brain membranes. The outer or diiral sheath, (Fig. 375, a) is continu- 

 ous with the dura mater of the brain posteriorly, while anteriorly it 

 blends with the sclera. The inner or pial sheath, (Fig. 375, &) is an ex- 

 tension of the pia mater and is separated from the outer sheath by 



Fig. 374. — Two M tiller's 

 Fibres from Retina of Ox 

 showing Relation to Layers of 

 Retina. (Ramon y Cajal.) 

 3, Outer limiting layer; 4, 

 outer nuclear layer; 5, outer 

 molecular layer; 6, inner nu- 

 clear layer; 7, inner molecular 

 layer; 8, layer of nerve cells; 

 9, layer of nerve iibres; 10, in- 

 ner limiting layer; a, nucleus; 

 b, cup-like depression caused 

 bv pressure from surrounding 

 cells. 



