VISUAL TISSUES 



255 



and hold the lens in place (Fig. 224, D). The rear wall of the optic vesi- 

 cle becomes a pigment layer, and the anterior or external wall becomes 

 the visual layer, which is much complicated by the differentiation of com- 

 municatory nerve cells as well as sustentacular cells out of its epithelium. 



The corneal layer become separated from the lens by a space contain- 

 ing a fluid, the aqueous fluid (Fig. 224, E). The space behind the lens 

 and between it and the retina is filled with the vitreous body. This 

 material has its origin in the connective tissue which occupies the space 

 when the optic cup is first formed. It develops blood vessels and a 

 bounding membrane which persists, while the blood vessels and part of 

 the connective tissue atrophy, leaving the space filled with a fluid and a 

 few cells. The cornea is lined outwardly by a clear, stratified epithelium 

 of some thickness and two principal layers. It rests on a thick basal 

 membrane. Under this is found the thick, strong connective tissue 

 which is laminated and composed of fine fibrils of connective tissue 

 with long, flat nuclei. Inwardly is found a somewhat thinner basal 

 membrane on which lies a thin and single-layered epithelium which 

 is in contact with the fluid that fills the anterior chamber of the eye. 



The lids are two 



folds which arise from 

 evaginations and arc 

 analogous to the 

 squid's cornea, while 

 the vertebrate cornea 

 would have to be com- 

 pared superficially, as 

 to its origin, with the 

 squid's iris. Glands 

 are found in the lids, 

 and a large compound 

 tubular gland, the 

 lachrymal gland, is 

 placed near the eye 

 to pour a lubricating, 

 cleansing, and moist- 

 ening fluid out upon 

 its corneal surface. 



The retina is the 

 most interesting layer 

 and is very complex. 

 We shall study it 

 by describing or, more properly, enumerating its various apparent 

 layers in order, and then explaining the meaning of some of them as 



r.&c. 



o. nu. 



H.fi. I. 



in. n. I. 



g.c.L 



nv.f. I. 



FIG. 225. Section to show the apparent layers of the human 

 retina, nv.f.l., nerve fiber layer; g.c.l., ganglion cell layer; 

 in.n.l., inner nuclear layer; H.fi.l., Henle's fibrous layer; 

 o.nu.l., outer nuclear layer; r. & c., rods and cones; pg., single 

 layer of pigment cells ; bl.v., blood vessels. (From "SxbHR's 

 Text-book of Histology" by LEWIS.) 



