Chapter 3 



THE VERTEBRATE RETINA 



(A) Histology and Physiology 



The sensory retina of any vertebrate consists essentially of four layers 

 of cells. One of these, the pigment epithelium, is not immediately con- 

 cerned with the process of photoreception. The other three layers com- 

 prise the retina proper, which lies against the pigment epithelium but is 

 rarely connected with the latter by any continuity of material. 



The Pigment Epithelium — The pigment epithelium of the retina 

 (Fig. 19) is firmly joined to the inner surface of the chorioid coat. 

 Each cell in the epithelium is like a six-sided tile and the cells are set 

 in a regular mosaic with only a thin layer of cement between their 

 contiguous sides. The base of the cell, toward the chorioid, is also 

 covered by cement which the cell secretes, so that an unbroken layer 

 of this cement lies between the pigment epithelium and the chorioid. 

 The innermost layer of the chorioid is an extremely thin elastic sheet 

 which, together with the cuticular cement layer between it and the bodies 

 of the pigment cells, comprises the 'glass membrane' (lamina vitrea). 

 The whole of the thickness of this really double membrane is often 

 assigned to the chorioid — or, by some, even to the pigment epithelium, 

 which clings much more tightly to the chorioid than to the retina proper 

 when an attempt is made to peel the layers of the eyeball wall apart. 

 The pigment epithelium belongs to the retina physiologically and embry- 

 ologically, however, if not anatomically. It is nowhere continuous with 

 the chorioid, whereas, as we have seen (see Fig. 7g, p. 15) it is contin- 

 uous at the pupil margin with the anterior prolongation of the sensory 

 retina. 



The free surface of the pigment cell usually bears a number of pro- 

 cesses which may be few and heavy (even single) or numerous and 

 filamentous, like a tuft of microscopic hairs (Fig. 20). The granules of 

 pigment, which consist of a colorless matrix impregnated with a light 

 brown form of melanin called 'fuscin', are of two sorts — round ones 

 tending to occupy the cuboidal base of the cell around the nucleus, and 

 spindle-shaped ones filling the processes and often migrating in and out 



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