Anatomy of the Eye


The clear front part of the eye that covers the iris, pupil and anterior chamber. It is covered with a transparent epithelial (or skin) layer.

Corneal Ulcer

An erosion or scrape of the cornea in which some of the epithelium is lost.


Colored part of the eye. Brown irises have pigment. Blue irises have much less pigment. The iris controls the amount of light that enters the eye, by varying the size of the pupil opening.

Aqueous Humor

Clear fluid inside the eye, which provides nutrition for the lens and cornea.

Iridocorneal Angle or Drainage Angle

The eye contains a clear fluid called aqueous humor. The aqueous humor is produced in the ciliary body and drains, via the angle, out of the eye into the bloodstream.


Elevated intraocular pressure that usually causes pain and vision loss. Glaucoma occurs when the drainage angle is blocked with inflammatory debris, or if the drainage angle is abnormally formed from birth, or of the normal flow of fluid inside the eye is blocked in some other way (by a tumor for example).


The hole in the iris. The pupil gets smaller in bright light, larger in dim light conditions.


The vascular tissues of the eye. This includes the iris, ciliary body and choroid.


Inflammation of the uvea, which can be anterior or posterior.


Clear and as thick as a stack of 5 dimes. It is suspended behind the iris by hundreds of microscopic fibers. The lens is biconvex (ie it is shaped like a lentil bean). The lens helps to bring rays of light to a focus on the retina.


An opacification (cloudiness) of the normally clear lens.


Irritation, redness, swelling of any tissue. Inflamed eyes may appear red. A pet may rub or scratch their eye when inflammation is present. If painful, animals may also squint.

Lens Capsule

A cellophane-like covering of the lens (only much thinner!). During cataract surgery the anterior lens capsule is partially opened so that the abnormal lens material can be removed.

Nictitating Membrane Third Eyelid

The Nictitating Membrane is a thin piece of tissue, supported by cartilage, which moves across the eyeball like a windshield wiper, to give the cornea additional protection. It is often called a third eyelid or haw. In cats and dogs, the nictitating membrane is not usually visible, and its appearance is often a sign of poor health or a painful eye.

Vitreous Humor

Clear gel behind the lens which fills the rear 2/3 of the eyeball (aqueous humor fills the anterior 1/3) and helps keep the retina attached.


A thin layer of light sensitive nerve tissue in the back of the eye that allows us to see, by conversion of optical images to electrical impulses that are sent by the optic nerve to the brain.


The tapetum is a reflective structure that lies beneath the retina. It acts like a mirror; reflecting light back through the retina. Animals that are active at night have a tapetum. Dogs, Cats, Horses, and Cows all have tapetums. It causes the yellow or green glow you see when light hits an animal’s eyes. Humans do not have a tapetum.