Eyeball: Definition, Structure and Functions of This Component of the Human Eye

It is a bilateral and spherical organ, which houses the structures responsible for vision.

The eyes are wonderful sense organs. They help people learn about the world they live in. The eyes see all kinds of things, big or small, near or far, smooth or textured, colors and dimensions.

The eyes have many parts, all of which must work in order to see properly.

The eyeball is located in a bony cavity within the facial skeleton, known as the bony orbit.

Division and structure of the eyeball

Anatomically, the eyeball can be divided into three layered parts:

  1. Fibrous.
  2. Vascular.
  3. Internal

These layers have different structures and functions. Now we will look at these layers in more detail.

Fibrous layer

The fibrous layer of the eye is the outermost layer. It is formed by the sclera and the cornea, which are continuous with each other. Its main functions are to shape the eye and support the deeper structures.

The sclera comprises the majority of the fibrous layer (approximately 85%). Provides attachment to the extraocular muscles, which are responsible for eye movement. It is visible as the white of the eye.

The cornea is transparent and is positioned centrally in the front of the eye. Light that enters the eye is refracted by the cornea.

Vascular layer

The vascular layer of the eye is located under the fibrous layer. It consists of the choroid, the ciliary body, and the iris.

  • Choroid : layer of connective tissue and blood vessels. Provides nutrition to the outer layers of the retina.
  • Ciliary body : composed of two parts; the ciliary muscle and the ciliary processes. The ciliary muscle consists of a collection of smooth muscle fibers. These are attached to the lens of the eye by the ciliary processes. The ciliary body controls the shape of the lens and contributes to the formation of aqueous humor.
  • Iris : circular structure, with an opening in the center (the pupil). The diameter of the pupil is altered by smooth muscle fibers within the iris, which are innervated by the autonomic nervous system. It is located between the lens and the cornea.

inner cover

The inner layer of the eye consists of the retina, the part that detects light from the eye. The retina itself is made up of two cell layers:

  1. The neural layer : it is the innermost layer of the retina. It is made up of photoreceptors; The light that detects the cells of the retina. It is located posteriorly and laterally in the eye.
  2. The pigmented layer : the outer layer of the retina. It is attached to the choroidal layer and acts to support the neural layer. It continues around the entire inner surface of the eye.

Previously, the pigmented layer continues but the neural layer does not, this is known as the non-visual retina. Posteriorly and laterally, both layers of the retina are present. This is the optical part of the retina.

The optical part of the retina can be seen during ophthalmoscopy. The center of the retina is marked by an area known as the macula. It is yellowish in color and highly pigmented.

The macula contains a depression called the fovea, which has a high concentration of cells that detect light. It is the area responsible for high acuity vision.

The area where the optic nerve enters the retina is known as the optic disc: it contains no cells that detect light.

Other structures in the eyeball

Inside the eyeball, there are structures that are not located in all three layers. These are the lenses and cameras of the eye.

The lens

The lens of the eye is located anteriorly, between the vitreous humor and the pupil. The shape of the lens is altered by the ciliary body, altering its refractive power.

In old age, the lens can become cloudy, a condition known as cataract.

The anterior and posterior chambers

There are two fluid-filled areas in the eye, known as the anterior and posterior chambers. The anterior chamber is located between the cornea and the iris, and the posterior chamber is located between the iris and the ciliary processes.

The chambers are filled with aqueous humor, a clear plasma-like liquid that nourishes and protects the eye.

Aqueous humor is constantly produced and drains through the trabecular meshwork, an area of ​​tissue at the base of the cornea, near the anterior chamber.

If the drainage of the aqueous humor is blocked, a condition known as glaucoma can occur .

Vasculature of the eyeball

The eyeball receives arterial blood mainly through the ophthalmic artery. This is a branch of the internal carotid artery, which arises immediately distal to the cavernous sinus.

The ophthalmic artery gives rise to many branches, which supply different components of the eye. The central retinal artery is the most important branch, supplying the inner surface of the retina.

Occlusion of this artery will quickly result in blindness.

Venous drainage of the eyeball is carried out by the superior and inferior ophthalmic veins. These drain into the cavernous sinus, a dural venous sinus very close to the eye.