342 DISEASES OF CATTLE. 



"When much contracted the pupil is a very 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 ves- 

 sels 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 is 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 proc- 

 esses. Ten distinct layers are described as composing the thickness 

 of the retina. 



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

 sale 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 form, 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 the 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 ante- 

 riorly 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 at its base and 

 closed at the apex. The opening of this cone is directed forward, 

 downward, and outward. Independently of the globe of the eye, this 

 cavity lodges the muscles that move it, the membrana nictitans, and 

 the lachrymal gland. 



The muscles of the eye are seven in number— one retractor, four 

 Straight, and two oblique. The retractor oculi envelops the optic 



