248 



THE EYE IN EVOLUTION 



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THE GENERAL STRUCTURE OF THE VERTEBRATE EYE 



We have seen that the eyes of Vertebrates are very true to type 

 and (apart from a few degenerate forms ^) consist essentially of a 

 retina derived from neural ectoderm, a lens derived from the surface 

 ectoderm, a uvea wit/h a nutritive function, a protective tunic the 

 anterior segment of which is transparent, and a dark chamber filled 

 with the vitreous body, the entire organ being encased in the orbital 

 cavity and moved by a group of extra-ocular muscles. All the varia- 

 tions in structure — and they are marked and of great interest — seen in 

 the major classes within the phylum are incidental in nature and have 

 evolved essentially as adaptations to differences in habitat or function. 



Of all the ocular tissues the retina is, of course, the most important 

 and undoubtedly the most interesting. Apart from the abundance 

 and motility of its pigment, its occasional assumption of a tapetal 

 function and the rare presence of oil-droplets, the pigmented epithelium 

 shows little fundamental variation. The retina proper (the pars optica 

 retinae) regularly comprises three layers of cells arranged in the following 

 strata (Fig. 263) : 



NEURONE I 



(percipient elements) 



NEURONE II 



(conductive and associa- 

 tive elements) 



NEURONE III 



(conductive elements) 



'1. Layer of rods and cones. 



2. Outer nuclear layer. 

 >3. Outer plexiform layer. 



4. Inner nuclear layer (bipolar, horizontal 



and amacrine cells). 

 >5. Inner plexiform layer. 



6. Ganglion cell layer. 



7. Nerve fibre layer. 



The sustentacular functions of the glial cells of the central nervous 

 system are taken over by large fibres of Mtiller, the nuclei of which lie 

 in the inner nuclear layer, while their extremities combine to form an 



1 p. 721. 



