CHAPTER II 

 THE RETINA 



The retina arises as an evagination of the forebrain. 

 It subsequently becomes invaginated to form a double- 

 layered cup (optic cup). The outer layer forms the pigment 

 epithelium (epithelial pigment layer) and is non-nervous. 

 The inner layer forms the nervous portion of the retina 

 (pars optica retinae). 



Pigment Epithelium. The pigment epithelium comprises a 

 single layer of regularly prismatic (usually hexagonal) cells. 

 The outer portion of each cell (choroidal) rests upon the 

 innermost layer of the choroid (lamina vitrea) and contains a 

 spherical nucleus and a moderate quantity of pigment 

 granules. The inner portion is filled with dark needle- 

 shaped pigment granules which extend into processes 

 reaching down between the outer segments of the visual 

 cells. The pigment epithelial cell in humans varies in diam- 

 eter from 12 to 18 /x. In the region of the macula the cells 

 become tall, more slender and very deeply pigmented. 

 Towards the ora they are broader and lower. 



The pigment is a melanin termed fuscin and varies greatly 

 in amount in different animals. It is absent in albinos, in 

 the eyes of elasmobranchs, in the fundus of nocturnal 

 lemurs, and in the fundus of those mammals which possess 

 a choroidal tapetum (Figure 7). It can easily be seen that 

 the presence of epithelial pigment would be of great dis- 

 advantage to those animals which possess a choroidal tape- 

 tum for the reflection of light. Whereas many nocturnal 

 animals are devoid of this pigment in the fundus of the eye, 

 other nocturnal forms are heavily pigmented in this region 

 (e.g. geckos). The epithelial pigment is abundant in bony 

 fishes, anuran amphibians, and birds. It is especially heavy 

 in birds, in fact to such an extent that visual cells cannot be 

 seen in fixed preparations of light-adapted eyes without 



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