414 PRACTICAL ANATOMY. 



a larger posterior portion, which forms five-sixths of the 

 eyeball. The eyeball measures twenty-five millimetres 

 antero-posteriorly, twenty-four millimetres transversely, 

 and twenty-three millimetres vertically. It is held in 

 place by the optic nerve, muscles, and palpebral struc- 

 tures. The optic nerve enters the posterior portion of 

 the eyeball a trifle to the nasal side of the centre ; it 

 pierces ^the two outer coats, and expands to form the 

 retinae. The visual axes of the eyeballs are parallel ; the 

 axes of the orbits diverge. The eyeball presents three 

 coats or tunics, the outer, formed by the sclerotic 

 and cornea; the middle, formed by the choroid, ciliary 

 processes, and the iris; and the inner, called the retina. 

 The refracting media are the vitreous and aqueous 

 humors and the crystalline lens. The accessory refract- 

 ing media are the cornea and capsule of the lens. 



THE SCLEROTIC. 



The sclerotic is a dense, white, fibrous structure 

 which invests the posterior five-sixths of the eyeball, and 

 serves to maintain it in form. It is thicker at the posterior 

 part, where it presents numerous foramina for the trans- 

 mission of vessels, and called the lamina cribrosa. It is 

 about one millimetre in thickness, smooth externally, 

 grooved internally by the ciliary nerves. The cornea is 

 the transparent segment which forms the anterior one- 

 sixth of the eye. It is about one millimetre thick, non- 

 vascular, almost circular, and sets in the anterior margin 

 of the sclerotic like the crystal of a watch on its case. 

 It is composed of five layers, the outer, the conjuncti- 

 val mucous membrane, formed of flat, transitional, and 

 columnar cells ; the second, or anterior elastic lamina, 

 composed of dense fibrous tissue, and measures about 

 nicn m thickness ; the central layer, or true cornea, 



