746 THE SENSE ORGANS AND SKIN OF THE HORSE 



The transverse diameter of the papilla is about (i to 7 mm., and the vertical about 4 to 5 mm. 

 It is commonly situated a little l)elow the margin of the tapetum, but the latter may extend down 

 somewhat on either side of the papilla. The lower margin is often indented a little. In inspec- 

 tion of the fundus with the ophthalmoscope numerous fine branches of the arteria centralis retinae 

 are seen radiating from the peripliery of the papilla. 



The optic nerve fibers converge from all parts of the pars optica to the papilla, 

 where they collect into bundles which traverse the lamina cribrosa of the chorioidea 

 and sclera, and constitute the optic nerve. The area centralis retinae is a round 

 spot, 2 to 3 mm. in diameter, situated above and external to the optic papilla; it 

 corresponds 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 retina^). 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 retina? and 

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

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

 The artery divides 2 to 3 mm. 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. The anterior part of the retina is non-vascular. 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 inclosed in front by 

 the cornea and liehind by the iris and lens (Figs. 552, 555, 557). It communicates 

 through the pupil with the posterior chamber (Cam(>ra oculi posterior); this is a 

 small anntilar space, triangular in cross-section, which is l)ounded in front by the 

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

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

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

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

 through the spaces (of Fontana) 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 receives tlie postericjr surface of the lens. It 

 consists of a framework of delicate flbrils, the stroma vitrevmi, the meshes of which 

 are filled by the fluid humor vitreus. The surface is covered by a condensation of 

 the stroma known as the membrana hyaloidea. 



The crystalline lens (Lens crystallina) is a biconvc^x, transparent body, situated 

 in front of the vitreous Ixxly and in ])artial contact with the posterior surface of the 

 iris. Its periphery, the aequator 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 Innnor and is in contact with the iris to an extent wliich 

 varies with the state of tlu^ jnipil. The posterior surface (Facies posterior) is much 



' For the minute structun; of the I'etina reference must be made to histological works. 



