746 THE SENSE ORGANS AND SKIN OF THE HORSE 



The transverse diameter of the papilhi is about ^^ to 7 mm., and tlie vertical about 4 to 5 mm. 

 It is commonly situated a little l)elo\v tlie margin of tlie tapetum, but the latter may extend down 

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

 tion of the fundus with the ophtiialmoscope numerous fine branches of the arteria centralis retina> 

 are seen radiating from the periphery 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 sui)ported 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 si)ecialized neniro-ejiithelium, 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 rtms in the axis of the nerve. 

 The artery divides 2 to 8 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 accom])any the arteries except in the capillary plexuses; their walls consist 

 merely of a la}cr 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 behind by the iris and lens (Figs. 552, 555, 557). It communicates 

 through the pupil with the posterior chamber (Camera oculi posterior); this is a 

 small annular space, triangular in cros.s-section, which is bounded in front by the 

 iris, behind })y the peripheral part of the lens and its ligament, 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 

 sodiinn chlorid and traces of all)umin and extractives. It is carried oft" chiefly 

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

 lens into the plexus venosus sclerae. 



REFRACTIVE MEDIA OF THE EYEBALL 



The vitreous body (("()r]Mis 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 the ]:>osteri()r surface of the lens. It 

 consists of a framework of delicate fibrils, the stroma vitreum, the meshes of which 

 are filled by the fluid humor vitreus. Tlie sm-face is covered by a condensation of 

 the stroma known as tlir iiieml)rana hyaloidea. 



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

 in front of the vitreous Ixxly and in iiartial contact with the posterior surface of the 

 iris. Its periphery, the aequator lentis, is almost circular and is closely surrounded 

 by the ciliary processes. Tiic anterior surface (Facies anterior) is convex; it is 

 bathed by the aqueous Inmior and i.s in contact with the iris to an extent which 

 varies with the state of t he ptiiiil. The posterior surface (Facies posterior) is much 



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



