Anatomy of the Eye: Human Eye Anatomy
The eye is the organ responsible for vision. Vision is our window to the outside world.
This article explores the anatomy of the eye looking at the different structures of the human eye and their function. The diagrams below show cross sections of the human eyeball. As we journey through the different structures, refer to the diagrams to quickly digest the content on this page.
Our eyeballs are fairly round organs cushioned by fatty tissues and they sit in two bony sockets inside the skull. This helps to protect our eyes from injury.
Anatomy of the Eye
The sclera is outermost layer of the eyeball. It is the white (and opaque) part of the eyeball. Muscles responsible for moving the eyeball are attached to the eyeball at the sclera.
At the front of the eyeball, the sclera becomes the cornea. The cornea is the transparent dome-shaped part of the eyeball. Light rays from the outside world first pass through the cornea before reaching the lens. Together with the lens, the cornea is responsible focussing light on the retina.
The choroid is the middle layer of the eyeball located between the sclera and the retina. It provides nutrients and oxygen to the outer surface of the retina.
The space between the cornea and the lens is known as the anterior chamber. It is filled with fluid called aqueous humour. The anterior chamber is also known as anterior cavity.
The Aqueous humour is a transparent watery fluid that circulates in the anterior chamber. It provides oxygen and nutrients to the inner eye and exerts fluid pressure that helps maintain the shape of the eye. The aqueous humour is produced by the ciliary body.
The posterior chamber is a larger area than the anterior chamber. It is located opposite to the anterior chamber at the back of the lens. It is filled with a fluid called vitreous humour. The posterior Chamber is also referred to as the Vitreous body as indicated in the diagram below - anatomy of the eye.
The vitreous humour is a transparent jelly-like fluid that fills the posterior chamber. It exerts fluid pressure that keeps the retina layers pressed together to maintain the shape of the eye and to maintain sharp focus of images on the retina.
The choroid continues at the front of the eyeball to form the Iris. The iris is a flat, thin, ring-shaped structure sticking in to the anterior chamber. This is the part that identifies a person’s eye colour. The iris contains circular muscles which go around the pupil and radial muscles that radiate toward the pupil. When the circular muscles contract they make the pupil smaller, when the radial muscles contract, they makes the pupil wider.
The cilliary muscles are located inside the ciliary body. These are the muscles that continuously change the shape of the lens for near and distant vision. See diagram anatomy of the eye above.
The choroid continues at the front of the eyeball to form the ciliary body. It produces the aqueous humour. The ciliary body also contains the ciliary muscles that contract or relax to change the shape of the lens.
The zonule also known as suspensory ligaments is a ring of small fibres that hold the lens suspended in place. It connects the lens to the ciliary body and allows the lens to change shape.
The lens is a biconvex transparent disc made of proteins called crystallines. It is located directly behind the iris and focuses light on to the retina. In humans, the lens changes shape for near and for distant vision.
The pupil is the hole at the center of the iris located in front of the lens. Whenever more light needs to enter the eyeball, the muscles in the iris contract like the diaphragm of a camera to increase or decrease the size of the pupil.
The retina is the innermost layer lining the back of the eyeball. It is the light sensitive part of the eye. The retina contains photo receptors that detect light. These photo receptors are known as cones and rods. Cones enable us to detect color while rods enable us to see in poor light. The retina contains nerve cells that transmit signals from the retina to the brain.
The fovea is a small depression in the retina near the optic disc. The fovea has a high concentration of cones. It is the part of the retina where visual acuity is greatest.
The optic nerve is located at the back of the eyeball. It contains the axons of retina ganglion cell (nerve cells of the retina) and it transmits impulses from the retina to the brain.
Impulses are transmitted to the brain from the back of the eyeball at the optic disc also called the blind spot. It is called the blind spot because it contains no photoreceptors, hence any light that falls on it will not be detected.
Muscles of the eye are very strong and efficient, they work together to move the eyeball in many different directions. The main muscles of the eye are Lateral rectus, Medial rectus, Superior rectus and inferior rectus.
Central Artery and Vein
The central artery and vein runs through the center of the optic nerve. The central artery supplies the retina while the central vein drains the retina. In the diagram above - anatomy of the eye, the artery is shown in red while the vein is shown in blue.
This is a small tube that runs from the eye to the nasal cavity. Tear drains from the eyes in to the nose through the tear duct. This is why a teary eye is usually accompanied by a runny nose.