500 



THE SENSE OF SIGHT. 



FIG. 179. 



of the sclerotic are covered by mucous membrane, the con- 

 junctiva; that which covers the front of the cornea being little 



more than squamous epithelium. 

 Immediately within the sclerotic 

 is the choroid coat, and within 

 the choroid is the retina. The 

 interior of the eyeball is well- 

 nigh filled by the aqueous and 

 vitreous humors and the crystal- 

 line lens; but also, there is sus- 

 pended in the interior a contrac- 

 tile and perforated curtain, 

 the iris, for regulating the ad- 

 mission of light, and behind the 

 junction of the sclerotic and cor- 

 nea is the ciliary muscles, the 

 function of which is to adapt the 

 eye for seeing objects at various 

 distances. 



These structures may be now 

 examined rather more in detail. 



The sclerotic coat is composed 

 of connective tissue, arranged 

 in variously disposed and inter- 

 communicating .layers. It is 

 strong, tough, and opaque, and 

 not very elastic. 



The cornea (Fig. 179) is, like 

 the sclerotic, with which it 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 fib- 

 __ rous tissue of the cornea is a 



structure of the cornea (after BOW- structureless elastic membrane 



man). A 80 ; B and O soo. A, small w ith epithelium. 



pwrtionofaTerti f al8ecttoIlof i ^ '' The choroid, which is the 

 ^ a r in . f a ' a> conjunc " next tunic of the eye within the 

 d/fibrouViaminrwUh'rucir^bod'ies sclerotic and immediately out- 

 interspersed between them ; d, posterior side the retina, consists of a thin 



elastic lamina or membrane of De- an( J highly vascular membrane, 



mours; e, internal epithelium of d. B, of which the i uterna l surface is 

 epithelium of the membrane of De- -, , i f 11 i 



mours, as seen looking towards its sur- C ?red by a layer of black 



face. O, the same seen in section. pigment-Cells. I he principal 



