THE EYE. 251 



bony orbit of the skull, surrounded by protective fat and 

 by motor muscles. The greater part of this eyeball is 

 occupied by a semi-fluid, clear gelatinous substance, the 

 vitreous body (corpus vitreum). The crystalline lens 

 (Fig. 241, I) is embedded in the anterior surface of the 

 vitreous body. It is a lentil-shaped, bi-convex, transparent 

 body — the most important of the light-refracting media of 

 the eye. Among these media is, in addition to the lens 

 and vitreous body, the aqueous humour (humor aqueus, at 

 m, in Fig. 241), in front of the lens. These three pellucid, 

 light-refracting media — the vitreous body, the crystalline 

 lens, and the aqueous humour — by which the rays of light, 

 incident on the eye, are refracted and concentrated, are 

 enclosed in a firm globular capsule consisting of several 

 different membranes, comparable with the concentric layers 

 of an onion. The outer and thickest of these forms the 

 white protective membrane of the eye (sclerotica, a). It 

 consists of firm, compact white connective tissue. In front 

 of the lens a circular, very convex, transparent plate, re- 

 sembling a watch glass, is inserted in the white protective 

 membrane; this is the horny membrane (cornea, b). On 

 its outer surface the horny membrane is covered by a very 

 thin coating of outer skin (epidermis) ; this coating is 

 called the connecting membrane (conjunctiva); it extends 

 from the horny membrane over the inner surface of both 

 eyelids — the upper and lower folds of skin which on closing 

 the eyes are drawn together over them. At the inner 

 corner of our eye there is, as a sort of rudimentary organ, 

 the remnant of a third (inner) eyelid, which, as the " nic- 

 titating membrane," is highly developed in the lower 

 Vertebrates (vol. i. p. 110). Below the upper eyelid are lodged 



