DISEASES OF THE EYE AND ITS APPENDAGES. 355 



the pupil, which varies m outline according to its size. "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 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 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 processes. Ten dis- 

 tinct layers are described as composing the thickness 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, iirin, 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 vitreows humor occupies four-fifths of the interior of the eyeball. 

 It is globular in form, with a depression in front for the lodgment 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 carity, 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 beliind the 

 orbit the temporal fossa. A fibrous membrane completes thin cavity 

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

 ocular sheath or periorbita IK attached jMisteriorly around the owning 

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

 to its inner face; then it becomes prolonged leyond the margin to 

 form the fibrous membrane of the eyelids. The orbital cavity has the 

 form when complete of u regular hollow coue, open at its base and 

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

 downward and outward. Independently of the gloln* of the eye, this 

 cavity lodges the imiHcles that move it, the incnihrami nictitaiis. and 

 the lachrymal gland. 



The MI uncles of the eye are seven in number one retractor, four 

 straight, and two oblique. The retruetor oculi envelops the optic nerve 



