354 DISEASES OF CATTLE. 



bathed by the aqueous humor. The greater part of the posterior sur- 

 face is in contact with the capsule of the lens and glides on it during 

 the movements of the curtain. The circumferential border is at- 

 tached within the junction of the sclerotic cornea. The inner border 

 circumscribes the pupil, which varies in outline according to its size. 

 When much contracted the pupil is a veiy elongated ellipse, the long 

 axis of which is in the line joining the nasal and temporal angles of 

 the eyelids. It contains muscular tissue, which, by contracting or 

 relaxing, lessens or dilates the pupillary opening. 



The choroid coat is a bell-shaped, dark membrane which lines the 

 sclerotic. Its outer surface has a shaggy appearance^ due to the 

 tunica fusca, which unites the two coats. Between the two the ciliary 

 vessels and nerves pass forward. Behind it is pierced by the optic 

 nerve; in front it is continued as the ciliary processes, which form, 

 as it were, the rim of the bell. The ciliary processes form a fringe 

 around the slightly inverted rim of the choroid. 



The retina ia the most delicate of the coats of the eyeball. It is 

 formed by the expansion of the optic nerve on the inner surface of 

 the choroid, and, like that coat, it is bell-shaped. Its inner surface 

 is molded on the vitreous humor. The nervous structures of the 

 retina terminate at a wavy line, the ora serrata, behind the ciliary 

 processes. Ten distinct layers are described as composing the thick- 

 ness of the retina. 



The lens is situated behind the pupil, and is contained within a cap- 

 sule of its own. 



The capsule is a close-fitting, firm^ transparent membrane. The 

 anterior surface forms the posterior boundary of the cavity in which 

 the aqueous humor is contained, and the iris in its movement glides 

 on it. The posterior surface is in contact with the vitreous humor. 



The vitreous humor occupies four-fifths of the interior of the eye- 

 ball. It is globular in forai, with a depression in front for the lodg- 

 ment of the lens. It is colorless, transparent, and of a consistency 

 like thin jelly. It is enveloped by a delicate capsule — the hyaloid 

 membrane — which is connected in front with the suspensory ligament 

 of the lens, and ends by joining the capsule behind the lens. 



The orbital cavity^ situated at the side of the head, is circumscribed 

 by a bony margin ; posteriorly, however, there are no bony walls, and 

 the cavity is confounded with the depression above and behind the 

 orbit — the temporal fossa. A fibrous membrane completes this cavity 

 and keeps it distinct from temporal fossa. This membrane — the 

 ocular sheath or periorbita — is attached posteriorly around the open- 

 ing in the back part of the orbital cavity (the orbital hiatus) and 

 anteriorly to its inner face; then it becomes prolonged beyond the 

 margin to form the fibrous membrane of the eyelids. The orbital 

 cavity has the form, when complete, of a regular hollow cone, open 



