How does the eye work?
Light enters the eye through the transparent anterior part of the eye, the cornea. The iris is a highly mobile diaphragm situated behind the cornea that determines the colour of the eyes. By modifying the size of its central aperture, the pupil, the iris will determine the amount of light entering the posterior part of the eye. Light rays are focused mainly by the cornea and to a lesser extent by the lens, a biconvex structure suspended in the eye immediately behind the iris. The focused light rays will form an image on the retina, a thin layer of nervous tissue that covers the back of the eye ball. The retina contains photosensitive cells, called the photoreceptors, that capture the light signal. Photoreceptors come in 2 types: rods and cones. The cones play a major role in daylight and are responsible for colour vision and central visual acuity; they are the only photoreceptors in the central zone of the retina (the macula) and they are present there in high concentration to enable discrimination of fine detail . Rods are more sensitive than cones: they detect weak light sources but are unable to distinguish colours: they are responsible for night vision. Both rods and cones transmit the information elicited by the light signals to another layer of nerve cells in the retina, the ganglion cell layer. These ganglion cells have a very long nerve fibre. The nerve fibres of the ganglion cells form a layer of the retina, the nerve fibre layer, according to a strict pattern. The nerve fibres converge at the back of the eye to form the optic nerve. The optic nerve then leaves the eye through an opening in the back of the eye (optic nerve head) and allows the information to travel to the brain where an elaborate processing of the information takes place.
In glaucoma patients there will be a gradual loss of retinal ganglion cells with thinning of the nerve fibre layer and alterations of the optic nerve head. Both the thinning of the nerve fibre layer and the alterations of the optic disc can be observed, measured and followed over time by the ophthalmologist.
Fluid in the eye determines eye pressure (intraocular pressure)
The eye is filled with fluid under pressure to maintain the spherical shape of the eyeball. There is a constant renewal of the intraocular fluid. Fluid is produced by the ciliary body, a ringlike structure located behind the base of the iris. Fresh fluid will migrate between lens and iris through the pupil to the anterior part of the eye. Fluid can leave the eyes via 2 routes. The principal one is via the trabecular meshwork, a structure located at the transition between the iris root and the cornea. It is a mesh-like system of microscopic drainage channels that surrounds the iris broot over 360°. The alternative route, less important, is called the uveoscleral route. Both routes transport the fluid from the eye into the venous blood circulation. Intraocular fluid never appears outside at the surface of the eye and has nothing to do with tears. The intraocular pressure is dependent on the equilibrium between production and evacuation. The normal intraocular pressure, expressed in mm mercury (Hg) ranges from 11 to 21 mm Hg in the general population.