Anatomy of the Eyeball

Study Notes

The eyeball is a globe-shaped structure occupying the anterior part of the orbit, with an outward projection representing about one-sixth of its total area, which is the transparent cornea.
The eyeball consists of several structures, including the anterior chamber, iris and pupil, posterior chamber, lens, vitreous chamber, and retina.
The anterior chamber is the area directly posterior to the cornea and anterior to the iris, while the posterior chamber is smaller and lies posterior to the iris and anterior to the lens.
The anterior and posterior chambers are continuous with each other through the pupil and are filled with a fluid called aqueous humor, which supplies nutrients to the avascular cornea and lens and maintains intra-ocular pressure.
The increase of intra-ocular pressure is known as glaucoma.
The lens is a transparent, biconvex elastic disc that separates the anterior one-fifth of the eyeball from the posterior four-fifths, and is attached circumferentially to ciliary muscles and zonules.
The lens has the ability to change its refractive ability to maintain visual acuity due to its lateral attachment.
The posterior four-fifths of the eyeball is occupied by the vitreous chamber, which is filled with a transparent, gelatinous substance called the vitreous body.
The walls of the eyeball consist of three layers: the outer fibrous layer, the middle vascular layer, and the inner layer.
The outer fibrous layer consists of the sclera posteriorly and the cornea anteriorly, with the sclera covering about five-sixths of the surface of the eyeball.
The sclera is an opaque layer of dense connective tissue that can be seen anteriorly through its conjunctival covering as the "white of the eye."
The cornea is transparent and continuous with the sclera anteriorly, covering the anterior one-sixth of the surface of the eyeball and allowing light to enter.
The vascular layer consists of the choroid, the ciliary body, and the iris from posterior to anterior, with the choroid being a thin, highly vascular and pigmented layer.
The ciliary body extends from the anterior border of the choroid and is triangular in shape, forming a complete ring around the eyeball.
The ciliary muscle consists of smooth muscle fibers arranged longitudinally, circularly, and radially, which decrease the size of the ring formed by the ciliary body on contraction.
The ciliary processes are longitudinal ridges projecting from the inner surface of the ciliary body, extending from which are zonular fibers attached to the lens of the eyeball.
Contraction of the ciliary muscle reduces tension on the suspensory ligament of the lens, allowing the lens to become more rounded (relaxed) for near vision.
The iris completes the vascular layer of the eyeball anteriorly, with a central opening (the pupil) and smooth muscle fibers that control the size of the pupil.
The inner layer of the eyeball consists of the optic part of the retina, which is sensitive to light, and the nonvisual part, which covers the internal surface of the ciliary body and iris.
The optic part of the retina consists of two layers: an outer pigmented layer and an inner neural layer.
The optic disc is where the optic nerve leaves the retina, and is referred to as a blind spot in the retina due to the lack of light-sensitive receptor cells.
The macula lutea is a small area with a hint of yellowish coloration lateral to the optic disc, with its central depression, the fovea centralis, being the thinnest area of the retina.
The arterial supply to the eyeball is from several sources, including the short posterior ciliary arteries, long posterior ciliary arteries, anterior ciliary arteries, and others.

Description

Test your knowledge of the structure and components of the human eyeball, including the cornea, iris, pupil, and retina. Learn about the anterior and posterior chambers, and the functions of each part.