ANATOMY OF THE EYEBALL. 61 



light analogous to the action of convex lenses in optical in- 

 struments. When we come to study its exact structure, how- 

 ever, there are 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 treat- 

 ing of the anatomy of the lens, we shall simply describe the 

 most prominent and the well-determined points in its struct- 

 ure. A complete account of the arrangement of its compo- 

 nent parts would necessitate very full and minute descrip- 

 tions, 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 vitreous humor, which is exactly moulded 

 to its posterior convexity. In the foetus, the capsule of the 

 lens receives a branch from the arteria centralis ; but it is non- 

 vascular in the adult. ' The anterior convexity is just behind 

 the iris, and the 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 measure- 

 ments will be discussed fully in connection with the physi- 

 ology of the lens. 



The diameters of the lens in the adult are about -J of an 

 inch transversely and J of an inch antero-posteriorly. The con- 

 vexity is greater on its posterior than on its anterior surface. 

 The form is sufficiently well indicated in Fig. 5 (p. 67) accom- 

 panying our summary of the anatomy of the eye. In f cetal 

 life, the convexities of the lens are much greater than in the 

 adult, and its structure is much softer. In old age, the con- 

 vexities are diminished and the lens becomes harder and 

 quite inelastic, which accounts for the progressive diminu 

 tion in the power of accommodation. 



The important physiological points in the structure of the 

 lens are that it presents an investing membrane, the capsule, 



