Some aspects of aqueous humour drainage.

It is now 50 years since the discovery of the aqueous veins by Karl Ascher. His finding had a great impact on ophthalmology since it showed that the aqueous humour is not a stagnant fluid; there had to be a continuous formation of aqueous humour by the ciliary processes, flow from the posterior chamber into the anterior chamber and outflow in the chamber angle. Since that time it has become clear that there is also some drainage of aqueous humour via uveoscleral routes. Furthermore the flow through the inner wall of Schlemm's canal has been shown to take place through about 20,000 pores each with diameter of around 0.1-3 microns. Recent measurements of the pressures in the outflow routes indicate that in normal monkey eyes the main outflow resistance is located close to the inner wall of the canal. Most of the aqueous humour leaving Schlemm's canal via the collector channels mixes with blood within the sclera. This is a consequence of the embryology of Schlemm's canal; it develops by the merging of blind extensions from intrascleral veins. Despite its specialisation the endothelium of Schlemm's canal retains the properties of a blood vessel. Platelets are likely to play in role in the integrity of Schlemm's canal as they do in blood vessels and may in fact control the size of the pores by occluding pores larger than 3 microns. It seems likely that in vitro perfusion of glaucomatous eyes with enzymes may be useful in the search for new methods of treatment for glaucoma.