644 THE SENSE OF SIGHT. 



another law in optics that rays of light impinging upon a 

 convex transparent surface, are refracted towards the 

 centre, those being most refracted which are farthest from 

 the centre of the convex surface. 



Behind the cornea is a space containing a thin watery 

 fluid, the aqueous humour, holding in solution a small quan- 

 tity of chloride of sodium and extractive matter. The 

 space containing the aqueous humour is divided into an 

 anterior and posterior chamber by a membranous partition, 

 the iris, to be presently again mentioned. The effect pro- 

 duced by the aqueous humour on the rays of light travers- 

 ing it, is not yet fully ascertained. Its chief use, probably, 

 is to assist in filling the eyeball, so as to maintain its proper 

 convexity, and at the same time to furnish a medium in 

 which the movements of the iris can take place. 



Behind the aqueous humour and the iris, and imbedded 

 F . 6 # in the anterior part of the me- 



dium next to be described, viz., 

 the vitreous humour, is seated a 

 doubly- convex body, the crystal- 

 line lens, which is the most im- 

 portant refracting structure of 

 the eye. The structure of the 

 lens is very complex. It consists 

 essentially of fibres united side 

 by side to each other, and 

 arranged together in very numerous laminae, which are so 

 placed upon one another, that when hardened in spirit 

 the lens splits into three portions, in the form of sectors, 

 each of which is composed of superimposed concentric 

 laminse. The lens increases in density and, consequently, 

 in power of refraction, from without inwards; the central 



* Fig. 176. Laminated structure of the crystalline lens (from Arnold). 

 \. The laminse are split up after hardening in alcohol. I, the denser 

 central part or nucleus ; 2, the successive external layers. 



