THE CRYSTALLINE LENS. 841 



the lens. In the adult there may be six or more of these, but in the foetus they 

 are only three in number and diverge from each other at angles of 120 (Fig. 451). 

 On the anterior surface one line ascends vertically and the other two diverge 

 downward and outward. On the posterior surface one ray descends vertically 



FIG. 451. Diagram to show the direction and arrangement of the radiating lines on the front and back of the 

 foetal lens. (A) from the front. (B) from the back. 



and the other two diverge upward. They correspond with the free edges of an 

 equal number of septa in the lens, along which the ends of the lens fibres come 

 into apposition and are joined by transparent amorphous substance. The fibres 

 run in a curved manner from the septa on the anterior surface to those on the 

 posterior surface. No fibres pass from pole to pole, but they are arranged in such 

 a way that fibres which commence near the pole on the one aspect of the lens 

 terminate near the peripheral extremity of the plane on the other, and vice versd. 

 The fibres of the outer layers of the lens each contain a nucleus, which together 

 form a layer (nuclear layer) on the surface of the lens, most distinct toward its 

 circumference. 



The changes produced in the lens by age are the following : 



In the foetus its form is nearly spherical, its color of a slightly reddish tint, it 

 is not perfectly transparent, and is so soft as to break down readily on the slightest 

 pressure. 



In the adult the posterior surface is more convex than the anterior ; it is color- 

 less, transparent, and firm in texture. 



In old age it becomes flattened on both surfaces, slightly opaque, of an amber 

 tint, and increases in density. 



The arteries of the globe of the eye are the short, long, and anterior ciliary 

 arteries and the arteria centralis retinae. They have been already described (see 

 page 509). 



The ciliary veins are seen on the outer surface of the choroid, and are named, 

 from their arrangement, the venae vorticosos. They converge to four or five 

 equidistant trunks, which pierce the sclerotic midway between the margin of the 

 cornea and the entrance of the optic nerve. Another set of veins accompany the 

 anterior ciliary arteries and open into the ophthalmic vein. 



The ciliary nerves are derived from the nasal branch of the ophthalmic and 

 from the ciliary or ophthalmic ganglion. 



Surgical Anatomy. From a surgical point of view the cornea may be regarded as consist- 

 ing of three layers : (1) of an external epithelial layer, developed from the epiblast, and con- 

 tinuous with the external epithelial covering of the rest of the body, and therefore in its lesions 

 resembling those of the epidermis ; (2) of the cornea proper, derived from the mesoblast, and 

 associated in its diseases with the fibro-vascular structures of the body; and (3) the posterior 

 elastic layer with its endothelium, also derived from the mesoblast and having the characters of 

 a serous membrane, so that inflammation of it resembles inflammation of the other serous and 

 synovial membranes of the body. 



The cornea contains no blood-vessels, except at its periphery, where numerous delicate 

 loops, derived from the anterior ciliary arteries, may be demonstrated on the anterior surface of 

 the cornea. The rest of the cornea is nourished by lymph, which gains access to the proper sub- 

 stance of the cornea and the posterior layer through the spaces of Fpntana. This lack of a 

 direct blood-supply renders the cornea very apt to inflame in the cachectic and ill-nourished. In 

 cases of granular lids there is a peculiar affection of the cornea, called pannus, in which the 

 anterior layers of the cornea become vascularized, and a rich network of blood-vessels may be 

 seen on the cornea; and in interstitial keratitis new vessels extend into the cornea, giving it a 

 pinkish hue, to which the term " salmon patch " is applied. The cornea is richly supplied with 



