Parts of the Eye and Their Functions

I want to look at the parts of the eye and their function because the anatomy and physiology of the eye is a subject every high school A&P student must understand.

It can be important for consumers considering laser eye surgery. Vision is complex and our dominant sense, but the basic parts and structure of the eye can be diagrammed and defined with little effort.

But before we define the human eye structure, lets look at a visual illustration of the eyeball and a brief overview of this window to our soul.

The eye in an adult human is about 2.5 centimeters and is spherical. The bulk of the eye is not visible, is enclosed by fat and nestled into the skulls bony orbit. Most noticeable, though, are external accessory structures, including the eyebrows, eyelids, and eyelashes.

Parts of the eye diagram

There are many parts of the human eye, and its best to dissect them by layers. The three layers that form the eyeball wall are the fibrous layer, vascular layer, and inner layer.

Fibrous layer

The fibrous layer, which is the outermost portion of the eye, is made of avascular connective tissues. The outer art of the eye consists of two different structures: the sclera and cornea.

Sclera: The sclera is the white of the eye. It protects and shapes the eyeball, forming the posterior. The sclera is also an anchoring site for the extrinsic eye muscles.

Cornea: The cornea, which lets light enter the eye, is the most exposed portion of it, making it most vulnerable to damage. The cornea bulges anteriorly from its junction with the sclera.

Vascular layer

The vascular layer, which is pigmented and also called the uvea, is the middle layer of the eyeball. It is composed of three different regions: choroid, ciliary body, and iris.

Choroid: The choroid layer is rich in blood vessels and provides nutrition to the entire vascular layer. The choroid is a dark brown membrane layer that forms 83 percent of the posterior portion of the vascular layer.

Ciliary body: The ciliary body is anterior of the choroid and merges with it. The ciliary body, which is a thickened ring of tissue that encircles the lens, has ciliary muscles, which control the shape of the lens.

Also related to controlling shape is the suspensory ligaments also known as the ciliary zonule. The suspensory ligament helps keep the lens upright.

Iris: When we think of the anatomy of eyes, we think of the iris the colored portion of the eye. This is the most anterior portion of the vascular layer and is made up of two muscles that allow for contraction and dilation of the pupil.

The pupil is the opening that allows light to enter the eye.

Inner layer

The inner part of the eye consists of the retina.

Retina: This layer is fragile and is composed of a pigmented layer and neural layer. The neural layer contains the photoreceptors.

The photoreceptors, which number about a quarter-billion, come in two types: rods and cones.

Rods and cones: The rods are more numerous and are used for dim light and peripheral vision. The are really receptive to light but dont work for color or bright light thats the job of the cones.

Also part of the inner layer is the optic disc/blind spot, fovea centralis, and the hyaloid canal.

Optic disc: Weve all heard of our blind spot. This is the portion of the inner layer where the optic nerve enters the eye. Its often called a blind spot because it does not have any photoreceptors, which means any light focused on it we cant see. (Our brain does fill in the gaps in our vision for us, though.)

Fovea: The fovea, which is only about pin-head size, is located posterior in the eye and lateral to the optic disc. The fovea is a small pit that contains cones and is responsible for visual acuity i.e., seeing details. (Your fovea is responsible for helping you focus and see the text on this page.)

Internal segments, chambers and fluids

The three layers of the eye house an anterior and posterior segment, which contains different types of fluid.

The posterior segment contains the vitreous humor, which is a clear gel. This gel binds to water and serves three purposes. First, it transmits light. Second, it provides a support system, pushing the retina against the pigmented layer. Lastly, the vitreous humor helps push against extrinsic eye muscle force, helping keep intraoccular pressure normalized.

The anterior segment is subdivided into an anterior and posterior chamber.

Anterior chamber: The anterior chamber is the portion between the cornea and the iris.

Posterior chamber: The posterior chamber is between the iris and the lens.

Both chambers are filled with aqueous humor, a clear fluid which is similar to plasma in consistency. The aqueous humor has many functions, including help maintain intraoccular pressure.

Lens: The lens is similar to the cornea; both are avascular. But the lens is biconvex and flexible, changing shape to allow light to precisely target the retina.

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