Physiological Pharmaceutics

Ocular drug delivery 253chamber. It flows as a gentle stream through the pupil into the anterior chamber, from whichit is drained by he canal of Schlemm.The vitreous body is a jelly made up of a form of collagen, vitrosin, and themucopolysaccharide, hyaluronic acid. Its composition is similar to that of the cornea, butthe proportion of water is much greater, about 98 percent or more, compared with about75 percent for the cornea. The vitreous body serves to keep the underlying retina pressedagainst the choroid.The eyelidsThe eyelids are movable folds of modified fleshy skin consisting of orbital and palpebralportions positioned in front of the eyeball (Figure 11.3). They have an obvious protectivefunction and play an important role in the maintenance of the tear film and lacrimaldrainage. Fibrous tarsal plates provide the framework for the lids.Between the bulbar and the palpebral conjunctiva there are two loose, redundantportions forming recesses that project back. These recesses are called the upper and lowerfornices, or conjunctival sacs. The looseness of the conjunctiva in this region makesmovements of lids and eyeball possible.Figure 11.3 Vertical section through the eyelids and conjunctivaThe precorneal tear filmThe maintenance of a clear, healthy cornea requires that the anterior surface of its epitheliallayer be kept moist. The precorneal tear film is a very thin fluid layer continuously bathingthe corneal epithelium, the conjunctiva and walls of the conjunctival cul-desac. Normalsecretion of tears by the lacrimal system is necessary for:1. nutrition of the cornea,2. protection against bacterial infection,3. removal of cellular debris and foreign matter4. formation of a stable, continuous fluid film over the cornea producing a highquality optical surface.5. lubrication for the movement of the eyelids.