Anatomy of the Human Eye

Anatomy of the Human Eye
Cross-section view

Choroid – A thin membrane that consists largely of blood vessels that nourishes the outer part of the retina. It is the most posterior part of the vascular coat (sheet) of the eye and is located between the sclera and the retina. It is difficult to estimate the thickness of the choroid because of the numerous vessels. It is thickest in the macular area, posteriorly (about 0.22 mm) and thinnest anteriorly (about 0.1 mm) near the ora serrata where it ends along with the retina.

Iris – A thin circular disc that gives our eyes their “color” and acts like the diaphragm of a camera. It is perforated near its center by a circular aperture called the pupil. The pupil varies greatly in size under different levels of light. It regulates the amount of light which reaches the retina by constricting in bright light and dilating in dim light. It also constricts during accommodation (focusing for near vision) to sharpen the focus by diminishing spherical aberration. Eye color is controlled by the amount of pigment deposited in the layers of the iris. Blue eyes have less pigment deposited and brown eyes have more pigment in the iris. It is true that most babies of the white race are born with blue eyes. As time goes on in an infant’s eyes the pigment can change.

Pupil – The center opening or circular aperture of the iris that appears black when viewing one’s eyes.

Central Retinal Vein – The main blood vessel that carries blood away from the eye. Veins appear darker and wider when viewed by special ophthalmic instruments looking into the eyes.

Conjunctiva – The thin transparent tissue overlying the sclera which continues over the posterior part of the upper and lower lids. Containing the most immunocytes of all the anterior eye structures, it serves as a protective layer against allergens and infective agents. The conjunctiva can be broken down into its three structurally separate parts: the palpebral conjunctiva lines the internal eyelids, the bulbar conjunctiva lines the sclera, and the fornix connects the latter two forming a cul de sac portion.  An inflammation or infection of the conjunctiva is called conjunctivitis.

Ciliary Body – Muscle which functions to control accommodation of the lens. It is connected to the lens by zonular fibers. When focusing fine detail up close, the ciliary body contracts, the zonular fibers relax causing the lens to become more convex. This creates a built-in zoom lens allowing the eyes to change focal power from far to near. The epithelium of this structure secretes aqueous fluid which is filtered from the blood circulation.

Aqueous – Fluid that bathes structures in the anterior 1/3 of the eye. This humor is derived from the blood vessels in the ciliary body processes and through a circulatory recycling process flows from the posterior chamber into the anterior chamber. Eventually it returns to the blood flow by draining out through structures including the Canal of Schlemm.

Canal of Schlemm – A vascular structure which spans the circumference of the anterior chamber and functions to return aqueous humor back to the blood circulation.

Anterior Chamber – A term used to describe the area outlined anteriorly by the corneal endothelium and posteriorly by the crystalline lens.

Posterior Chamber – A term used to describe the area outline anteriorly by the crystalline lens and posteriorly by the vitreous gel.

Cornea – The anterior clear part of the eye. It is the first clear “window” through which the light enters the eye. The transparency of the cornea is important in order for vision to be clear. Though it has many functions, one of the most important is to bend (refract) light entering the eye to the lens, which in turn focuses the light to the retina. The cornea also has a protective function.

Lens – This is a biconvex structure located between the pupil and vitreous. It acts as the second “clear” window of the eye. The lens differs from the cornea in that it has the ability to change shape in order to focus images at varying distances (accommodate) becoming shorter and fatter for near vision (more convex) and thin and long for far vision.  A cloudiness or opacity of the lens is called a cataract.

Vitreous - The gel-like material of the posterior eye. As we age the vitreous becomes more liquid-like and shrinks. It consists of water, collagen, hyaluronic acid and various proteins. During early eye development, the vitreous forms attachments with surrounding ocular structures: optic nerve, the fovea, the posterior lens surface, and the ora serrata. The strongest attachment is at the ora serrata.

Ora Serrata – The peripheral edge of retinal attachment to the ciliary body. This area marks the outer limits of perceptible retina. The ora is less developed and thinner than the central portion of the retina. It is also the location of the strongest attachment of vitreous base to retina and therefore is a common site for retinal detachments.

Sclera – The “white” part of the eye is actually a fibrous tunic which comprises the 5/6 of the eye’s outer coat. It functions to protect the intra-ocular structures and sustain the shape of the eye. Due to its fragile nature in younger children, it may have a “bluish” color. The sclera is penetrated by blood vessels and nerves, but the most important penetration is the optic nerve.

Retina – A thin multi-layered sensory tissue that lines the back of the eye. The retina contains two types of photoreceptors called rods and cones. Visual information received by the photoreceptors is translated to neural signals by the nerves called ganglion cells. The rods are more abundant in the peripheral retina and work to detect light/dark changes as well as shape and movement. Cones detect color and distinguish fine detail which is why they are denser in the central retina, especially in the fovea.

Macula – This is the central part of the retina where cones are densely packed. The fovea is the center part of the macula and is the part of our eyes that fixates when looking directly at an object.  Macular Degeneration is the most common complication that occurs in the macula and is one of the leading causes of blindness.

Optic Nerve – Is the collective input of information from ganglion cells which are carrying information from the retinal photoreceptors. The optic nerve carries this information to the brain where the signals are interpreted allowing visual perception. Since it is devoid of any photoreceptors, the optic nerve corresponds with our natural blind spot.

Central Retinal Artery – The artery which supplies blood to most of the cells of the neural retina.

Rectus Medialis – One of the six extraocular muscles. The rectus medialis is located on the medial side of the eye and works to adduct the eyes.