Course Content
Deepen your understanding of ocular anatomy as you delve further into the complex structures and functions of the eye. This lesson covers the eye's main parts and regions while developing your understanding of the principles of refraction and accommodation. You will also learn about the six extraocular muscles and gain valuable insight into how the eye moves.
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Refraction and Accommodation

Chapter 2 of 5


There are 2 main refractive bodies in the eye: the cornea and the crystalline lens. Most of the refractive power in the eye comes from the cornea, due to the differences in the indices of refraction between the air (refractive index of about 1.00) and the aqueous humor, which has an index of refraction of 1.34.


Accommodation is the ability of the lens to change shape and automatically alter focus from distant to near objects. The ability to focus deteriorates over time. Typically, the first signs of deterioration become noticeable around the age of 40 due to a gradual hardening of the crystalline lens, leading to presbyopia.

Vitreous Humor

A clear, colorless, transparent jelly-like substance that fills the eyeball behind the lens. The vitreous humor lets light through without refraction, maintains the shape of the eye, and suspends the delicate lens.

Crystalline Lens

The crystalline lens is primarily responsible for changing the eye’s focal point. The lens is made up almost entirely of proteins, a higher protein concentration than any other bodily tissue. The tissue is transparent, which allows light to easily enter the eye. It’s also flexible, so it can change shape and bend the light to focus properly on the retina.

Aqueous Humor

This watery fluid fills the space behind the cornea and in front of the iris and pupil. Aqueous humor is constantly produced and flows between the lens and the iris and through the pupil into the anterior chamber. The intraocular pressure is checked during an eye exam. Too much pressure is a risk factor for open-angle glaucoma.


The cornea is the primary refractive element in the eye (75%). The tear film maintains both the health and optics of the cornea. The refractive power lies in the cornea’s index of refraction along with its sharply curved surface. The anterior chamber is the space between the cornea and the iris. It is filled with the aqueous humor (watery fluid).

Zonular Fibers

Contraction of the ciliary muscle causes the zonular fibers of the lens to relax which in turn allows the lens of the eye to change its shape.

Ciliary Muscles

Active contraction of the ciliary muscle allows for change in focusing power.

Crystalline Lens

As a child and young adult, your lens flexes during accommodation. The lens becomes less flexible as you age, decreasing your ability to focus on near objects. This is called presbyopia, which literally means “old sight.”