SPtClAL ANATOMY. 149 



layers of the retina are connected together by areolar tissue, 

 which is continued over the posterior surface of the ciliary 

 processes, forming the zona ciliaris or zone of Zinn, which 

 extends as far as the capsule of the lens. At the inner 

 and upper part of the choroid we distinguish a part pre- 

 senting a peculiar greenish metallic lustre, tapetum 

 lucidum, which by reflecting instead of absorbing the rays 

 of light, enables the animal to " see in the darJc.'' 



The chambers of the eye are three in number. The 

 anterior chamber is situated between the cornea and the 

 iris. It communicates with the posterior chamber, which is 

 between the iris, ciliary processes, and the capsule of the 

 lens, through the pupillary opening (this communication 

 seems to be closed in the foetus by a vascular membrane, 

 memhrana pupillarisy which, however, is merely the vascular 

 anterior part of the capsule of the lens to which the iris is 

 attached). These two chambers are lined by the mem- 

 brane of Demeurs or Descemet, which, therefore, covers the 

 posterior surface of the cornea, invests both surfaces of 

 the iris, and is continued over the anterior surface of the 

 ciliary processes and the capsule of the lens. It secretes the 

 aqueous humour, a fluid consisting of water and a small 

 quantity of saline matter and albumen dissolved in it. It 

 serves to afford free play to the iris, to preserve the due 

 convexity of the cornea, and to (slightly) influence the 

 direction of the rays of light. 



The dark chamber is the largest of the three, anteriorly 

 bounded by the capsule of the lens, m all other parts it is 

 circumscribed by the retina. The membrane lining it, the 

 hyaloid membrane, also sends trabeculse inwards, by which 

 the cavity is divided into numerous cells, in which we find 

 a fluid analogous to the aqueous humour, but which in 

 consequence of this arrangement does not flow freely when 

 the dark chamber is punctured ; this is the vitreous humour, 

 through its centre in the foetus, from behind forwards to 

 the lens runs a branch of arteria centralis retinae. Between 

 the aqueous and vitreous humours is situated the crystal- 

 line lens (or third humour of the eye) . It is enclosed in 

 a capsule, the outer circumferent margin of which is 

 attached to the free extremity of the ciliary processes and 

 of the zone of Zinn. Its anterior surface is covered by the 

 membrane of Demeurs, its posterior by the hyaloid mem- 

 brane, and the connective continuation of the retina is 



