PHYSIOLOGICAL ANATOMY OF THE EYEBALL. 



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incomplete venous circle, branches from which pass back by the sides of the arteries to 

 the vena centralis. The macula lutea is provided with a rich plexus of minute capillaries. 



Crystalline Lens. The anatomy of the crystalline lens, as far as it bears upon the 

 physiology of vision, is very. simple. It is a double-convex lens, transparent, and exceed- 

 ingly elastic. It has a function in the refraction of the rays of light analogous to the 

 action of convex lenses in optical instruments. When we come to study its exact struct- 

 ure, however, we shall find many points that are still undetermined and somewhat 

 obscure ; but, fortunately, these are not, as far as we now know, of much physiological 

 importance. In treating of the anatomy of the lens, we shall simply describe the most 

 prominent and the well-determined points in its structure. A complete account of the 

 arrangement of its component parts would necessitate very full and minute descriptions, 

 which could only be elucidated by numerous illustrative figures. 



The lens is situated behind the pupil, in what is called the hyaloid fossa of the vitre- 

 ous humor, which is exactly moulded to its posterior convexity. In the foetus, the cap- 

 sule of the lens receives a branch from the arteria centralis, but it is non-vascular in the 

 adult. The anterior convexity of the lens is just behind the iris, and its borders are in 

 relation with what is known as the suspensory ligament. The convexities do not present 

 regular curves, and they are so subject to variations after death that the measurements, 

 post-mortem, are of little value. During life, however, they have been measured very 

 exactly in the various conditions of accommodation. These measurements will be dis- 

 cussed fully in connection with the physiology of the lens. 



FIG. ^^.Crystalline lens, anterior view. (Babuchin.) 



The diameters of the lens in the adult are about of an inch transversely and of an 

 inch antero-posteriorly. The convexity is greater on its posterior than on its anterior 

 surface. In foetal life, the convexities of the lens are much greater than in the adult and 

 its structure is much softer. In old age, the convexities are diminished and the lens 

 becomes harder and quite inelastic, which accounts for the progressive diminution in the 

 power of accommodation. 



The important physiological points in the structure of the lens are that it presents an 

 investing membrane, the capsule, the lens itself being composed of layers of fibres of dif- 

 ferent degrees of density. 



