CRYSTALLINE LENS. 



685 



the macula lutea has no blood-vessels, it is the spot of all others in the retina 

 which is most abundantly supplied with minute vascular branches. These 

 vessels can be dis- 

 tinctly seen even 

 with the ophthal- 

 moscope ; and mi- 

 croscopical exam- 

 ination shows that 

 the capillary plex- 

 us in the macula 

 lutea is closer and 

 richer than in any 

 other part of the 

 retina." 



The arteries of 

 the retina send 

 branches to the 

 periphery, where 

 they supply a wide 

 plexus of very 

 small capillaries in 

 the ora serrata- 

 These capillaries 

 empty into an in- 

 complete venous 



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

 centralis. 



Crystalline Lens. The crystalline is a double-convex lens, which is per- 

 fectly transparent and very elastic. Its action in the refraction of the rays 

 of light is analogous to that of convex lenses in optical instruments. It 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 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. The diameters of the lens in the 

 adult are about of an inch (8'5 mm.) transversely and \ of an inch (6'4 mm.) 

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

 anterior surface. In foetal life the convexities of the lens are greater than 

 in the adult and its structure is much softer. In old age the convexities 

 are diminished and the lens becomes harder and less elastic. The substance 

 of the lens is made up of layers of fibres of different degrees of density, and 

 the whole is enveloped in a delicate membrane, called the capsule. 



45 



FIG. 249. Blood-vessels of the retina ; magnified 74 diameters (Loring). 



