594: HANDBOOK OF PHYSIOLOGY. 



a general impression of daylight as would distinguish it from the night; 

 (3) A contractile diaphragm (iris) with a central aperture for regulating 

 the quantity of light admitted into the eye; and (4) an arrangement by 

 which the chief refracting medium shall be so controlled as to enable 

 objects to be seen at various distances, causing convergence of the rays 

 of light that fall upon and traverse it (accommodation). Of the re- 

 fracting media the cornea is in a two-fold manner capable of refracting 

 and causing convergence of the rays of light that fall upon and traverse 

 it. It thus affects them first, by its density; for it is a law in optics 

 that when rays of light pass from a rarer into a denser medium, if they 

 impinge upon the surface in a direction removed from the perpendicu- 

 lar, they are bent out of their former direction towards that of a line 

 perpendicular to the surface of the denser medium, and, secondly, by 

 its convexity; since rays of light impinging upon a convex transpa- 

 rent 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 humor, holding in solution a small quantity of sodium chloride 

 and extractive matter. The space containing the aqueous humor is di- 

 vided into an anterior and posterior chamber by a membranous partition, 

 the iris, to be presently again mentioned. The effect produced by the 

 aqueous humor on the rays of light traversing it, is not yet fully ascer- 

 tained. 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 humor and the iris, and imbedded in the anterior 

 part of the medium next to be described, viz., the vitreous humor, is 

 seated a doubly-convex body, the crystalline 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 laminaB, 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 

 part, usually termed the nucleus, being the most dense. 



The vitreous humor constitutes nearly four-fifths of the whole globe 

 of the eye. It fills up the space between the retina and the lens, and its 

 soft jelly-like substance consists essentially of numerous layers, formed 

 of delicate, simple membrane, the spaces between which are filled with 

 a watery, pellucid fluid. Its principal use appears to be that of giving 

 the proper distention to the globe of the eye, and of keeping the surface 

 of the retina at a proper distance from the lens. 



Action of the Iris. The iris is a vertically-placed membranous 



