230 



ANATOMY FOR NURSES. [CHAP. XIX. 



the eye-chamber; in a dim light the pupil involuntarily dilates 

 to admit as much light as possible. The posterior surface of 

 the iris is covered by a thick layer of pigment-cells designed 

 to darken the curtain and prevent the entrance of light. The 



anterior surface of the iris 

 is also covered with pig- 

 ment cells, and it is chiefly 

 these latter which cause the 

 beautiful colours seen in the 

 iris. The different colours 

 of eyes, however, are mainly 

 due to the amount, and not 

 to the colour, of the pig- 

 ment deposited. 



The retina, the innermost 

 coat of the eyeball, is the 

 most essential part of the 

 organ of sight, since it is 

 the only one directly sensi- 

 tive to light. The sclerotic 

 is the protective, the choroid 

 the vascular or nutritive, 

 and the retina is the visual 

 or perceptive, layer of the 

 eyeball. It forms a nearly 

 transparent membrane situ- 

 ated between the inner sur- 

 face of the choroid and the 

 outer surface of the vitre- 

 ous humour, and extending 



FIG 132. -DIAGRAMMATIC SECTION OF f rom the entrance of the 

 THE HUMAN KETINA. (M. Schultze.) a, 



inner surface; b, internal limiting mem- OptlC nerve to the COm- 



brane; 1, layer of nerve-fibres; 2, layer of mencement of the ciliary 



processes, where it termi- 

 nates by an indented border, 

 the ora serrata. It is composed of eight layers and two limiting 

 membranes, as shown in the accompanying figure (Fig. 132). 

 The most essential of these layers are the first and second, 

 the layer of nerve-fibres and the layer of nerve-cells, and the 

 seventh layer, the layer of the rods and cones. The layer of 



nerve-cells ; c, external limiting membrane ; 

 7, layer of rods and cones ; 8, pigment cells ; 

 d, outer surface. 



