If the eye is to function as an effective optical instrument, it is clear that the intraocular pressure must be maintained at a constant level. At the same time, an active circulation of fluid through the globe is essential if the structures within it are to receive adequate nourishment. The cornea and sclera form a tough fibrous and unyielding envelope and within this an even pressure is maintained by a balance between the production and drainage of aqueous fluid.
Aqueous is produced by the ciliary epithelium by active secretion and ultrafiltration. A continuous flow is maintained through the pupil, where it reaches the angle of the anterior chamber.
On reaching the angle of the anterior chamber, aqueous passes through a grill known as the trabecular meshwork and then reaches a circular canal embedded in the sclera known as Schlemm's canal. This canal runs as a ring around the limbus (corneoscleral junction) and from it, minute channels radiate outwards through the sclera to reach the episcleral circulation. These channels are known as aqueous veins and they transmit clear aqueous to the episcleral veins, which lie in the connective tissue underlying the conjunctiva. In actual fact, the proof of the route of drainage of aqueous can be verified by any medical student - it simply entails examining the white of the eye around the cornea with extreme care, using the high power of the slit-lamp microscope. After a time, one can sometimes detect that some of the deeper veins convey parallel halves of blood and aqueous in the region beyond the junction of aqueous and episcleral vein.
The relative parts played by ciliary epithelium and trabecular meshwork in maintaining what is a remarkably constant intraocular pressure throughout life are not fully understood. It would appear that the production of aqueous is an active secretion, whereas the drainage is more passive, although changing the tone of the ciliary muscle can alter the rate of drainage. In normal subjects, the intraocular pressure does not differ in the two eyes by more than about 3 mmHg. Wider differences can lead one to suspect early glaucoma, especially if there is a family history of the disease. The normal intraocular pressure undergoes a diurnal variation, being highest in the early morning and gradually falling during the first half of the day. This diurnal change could become exaggerated as the first sign of glaucoma.
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