204 HAND-BOOK OF PHYSIOLOGY. 



An apparatus consisting of certain refractory media, cornea, crystalline 

 lens, aqueous and vitreous humor, the function of which is to collect 

 together into one point the different divergent rays emitted by each point 

 of every external body and of giving them such directions that they are 

 exactly focussed upon the retina, and thus produce an exact image of 

 the object from which they proceed. For as light radiates from a lu- 

 minous body in all directions, when the media offer no impediment to 

 its transmission, a luminous point will necessarily illuminate all parts 

 of a surface, such as the retina opposed to it, and not merely one single 

 point. A retina, therefore, without any optical apparatus placed in 

 front of it to separate the light of different objects, would not allow of 

 distinct vision, but would merely transmit such a general impression of 

 daylight as would distinguish it from the night; (3) A contractile dia- 

 phragm (iris) with a central aperture for regulating the quantity of light 

 admitted into the eye; and (4) a contractile structure (ciliary muscle), 

 an arrangement by which the chief refracting medium (crystalline lens) 

 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) . 



KEFRACTING MEDIA. 



Of the refracting media the cornea is in a twofold 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 per- 

 pendicular, they are bent out of their former direction toward 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 transparent 

 surface, are refracted toward 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 aque- 

 ous humor, holding in solution a small quantity of sodium chloride and 

 extractive matter. The space containing the aqueous humor is divided 

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

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

 ous humor on the rays of light traversing it, is not yet fully ascertained. 

 Its chief use, probably, is to assist in filling the eyeball, so as to main- 

 tain its proper convexity, and^t 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 important 



