The aqueous humour flows through the pupil from the posterior chamber (a small space between the iris and the lens) to the anterior chamber and out of the eye through the trabecular meshwork and Schlemm’s canal, which encircles the peripheral iris. The iris is a doughnut-shaped, muscular curtain that opens and closes to regulate the amount of light entering the eye through the pupil, the opening at the iris’s centre. It is filled with a watery fluid called the aqueous humour. Behind the cornea is the anterior chamber, which extends posteriorly to the plane of the iris and pupil. Defects in corneal curvature cause a distortion of vision known as astigmatism. The cornea, where the focusing process begins, is curved to a much greater extent than the rest of the eyeball. How deep is your body of knowledge about the inner workings of humans? Test it with this quiz.
The choroid is a highly vascular tissue layer that provides blood to the outer layers of the retina that lie over it. The other components of the uvea are the iris and the choroid. One component of the uvea is the ciliary body, a muscular structure located behind the iris that alters the shape of the lens during focusing and produces the aqueous humour that bathes the anterior chamber. Immediately beneath the sclera is an underlying vascular layer, called the uvea, that supplies nutrients to many parts of the eye.
Much of the eyeball is filled with a transparent gel-like material, called the vitreous humour, that helps to maintain the spheroidal shape. Other important structures in the eyeball include the iris and the lens. However, unlike the lens, the shape and focusing power of the cornea are not adjustable. Working in concert with the aqueous humour behind it, the cornea provides the greatest focusing power of the eye. At the front of the eye, the tear film covers the transparent cornea, the “window” through which light passes into the eye. The sclera is itself covered anteriorly by the conjunctiva, a transparent mucous membrane that prevents the eye from drying out. The sclera, the tough protective outer shell of the eyeball, is composed of dense fibrous tissue that covers four-fifths of the eyeball and provides attachments for the muscles that move the eye. The eyeball houses the retina-an extremely metabolically active layer of nerve tissue made up of millions of light receptors (photoreceptors)-and all of the structures needed to focus light onto it.
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