REFRACTIVE MEDIA OF THE EYEBALL 869 



spends to the macula lutea of man, which is histologically more highly differen- 

 tiated than the rest of the retina and is the area of most acute vision. 



The structure of the retina is very complex. It consists of nervous elements 

 which are supported in a peculiar sustentacular tissue, and are covered externally 

 by a layer of pigmented epithelium (Stratum pigmenti retinae). The nervous ele- 

 ments comprise a highly specialized neuro-epithelium, the rods and cones; ganglion- 

 cells, the axones of which form the optic nerve; and intermediate neurones. Ten 

 layers may be recognized in sections microscopically. ' 



The arteries of the retina are derived from the arteria centralis retinae and 

 anastomotic branches from the short ciliary arteries. The arteria centralis enters 

 the optic nerve a short distance behind the eyeball and runs in the axis of the nerve. 

 It divides two or three millimeters before reaching the papilla, and gives off thirty to 

 forty branches which radiate in the posterior part of the retina and divide dichoto- 

 mously into end-arteries in the layer of nerve fibers. The veins accompany the 

 arteries except in the capillary plexuses; their walls consist merely of a layer of 

 endothelial cells, around which are a lymph-channel and sheath.^ 



CHAMBERS OF THE EYE AND AQUEOUS HUMOR 



The anterior chamber of the eye (Camera oculi anterior) is enclosed in front by 

 the cornea and behind by the iris and lens (Figs. 689, 692, 694). It communicates 

 through the pupil with the posterior chamber of the eye (Camera oculi posterior) ; 

 this is a small annular space, triangular in cross-section, which is bounded in front 

 by the iris, behind by the peripheral part of the lens and its ligaments, and externally 

 by the ciliary processes. The chambers are filled by the aqueous humor (Humor 

 aqueus), a clear fluid which consists of about 98 per cent, of water, with a little 

 sodium chlorid and traces of albumin and extractives. It is carried off chiefly 

 through the spaces in the zonula ciliaris (or suspensory ligament of the lens) into 

 the plexus venosus sclerse. 



REFRACTIVE MEDIA OF THE EYEBALL 



The vitreous body (Corpus vitreum) is a semifluid, transparent substance 

 situated between the crystalline lens and the retina. In front it presents a deep 

 cavity, the fossa hyaloidea, which fits the posterior surface of the lens. It 

 consists of a framework of delicate fibrils, the vitreous stroma (Stroma vitreum), 

 the meshes of which are filled by the fluid vitreous humor (Humor vitreus). The 

 surface is covered by a condensation of the stroma known as the hyaloid membrane 

 (Membrana hyaloidea). 



The crystalline lens (Lens crystallina) is a biconvex, transparent body which 

 is situated in front of the vitreous body and in partial contact with the posterior 

 surface of the iris. Its periphery, the equator of the lens (Equator lentis), is 

 almost circular and is closely surrounded by the ciliary processes. The anterior 

 surface (Facies anterior) is convex; it is bathed by the aqueous humor and is in 

 contact with the iris to an extent which varies with the state of the pupil. The 

 posterior surface (Facies posterior) is much more strongly curved than the anterior. 

 It rests in the fossa of the vitreous body (Fossa hyaloidea). The central points of 

 the surfaces are the anterior and posterior poles (Polus anterior et posterior lentis), 

 and the line which connects them is the axis of the lens (Axis lentis). 



The transverse diameter of the lens is about 2 cm., the vertical diameter is sUghtly smaller, 

 and the axis measures about 13 mm. The radius of curvature of the anterior surface is 13.5 mm., 



1 For the minute structure of the retina reference must be made to histological works. 



2 Martin states that in the horse a capillary plexus does not exist, but that the arteries com- 

 municate with the veins by closely wound loops. 



