THE SENSE OF SIGHT. 



These structures may be now examined rather more in 

 detail. 



The sclerotic coat is composed of connective tissue, 

 arranged in variously disposed and intercommunicating 

 layers. It is strong, tough, and opaque, and not very 

 elastic. 



The cornea (fig. 172) is, like the sclerotic, with which it 



Fig. 172* 



is continuous, chiefly of a fibrous 

 structure, but the fibres are so 

 modified and arranged as to form 

 a transparent membrane for the 

 passage of light. Both in front of 

 and behind the fibrous tissue of 

 the cornea is a structureless elastic 

 membrane with epithelium. 



The choroid, which is the next 

 tunic of the eye within the sclerotic 

 and immediately outside the retina, 

 consists of a thin and highly vascu- 

 lar membrane, of which the inter- 

 nal surface is covered by a layer 

 of black pigment-cells. The prin- 

 cipal use of the choroid is to absorb, 

 by means of its pigment, those rays 

 of light which pass through the 

 transparent retina, and thus to 

 prevent their being thrown again 

 upon the retina, so as to interfere 



* Fig. 172. Structures of the cornea (after Bowman). A 8 , B & C, ^-. 

 A, Small portion of a vertical section of the cornea in the adult ; a, con- 

 junctival epithelium ; I, anterior elastic lamina ; e to d, fibrous laminae 

 with nuclear bodies interspersed between them ; c, fibres shooting 

 through some of these layers from the external elastic lamina ; d, pos- 

 terior elastic lamina ; e, internal epithelium. B, epithelium of the mem- 

 brane of Demours, as seen looking towards its surface. C, the same 

 seen in section. 



