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Structure and Function of the Eye in Dogs

By: Dr. Bari Spielman

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What Is the Eye?

The eyes are the receptors for the special sense of sight.

Where Is the Eye Located?

There are two eyes, located on the left and right sides of the face. Considerable variation exists among species of animals relative to the position of the eyes, the size of the orbit (the bony cavity that contains the eyeball), and the size and shape of the palpebral fissure (the opening between the eyelids). In the dog there is also a wide variety in these structures among breeds. Due to hundreds of years of selective breeding and the production by man of numerous types of dog breeds, dogs have the greatest variation of eye and orbit structure of any species. For example, such breeds as the brachycephalic dogs (those with short, wide heads) have eyes that appear to be more prominent, e.g. Pekingese, Boston terrier and pug. In other breeds (such as collie, Doberman pinscher and setters) the eyes are more recessed into the skull.

What Is the General Structure of the Eye?

The eye is formed by three concentric (circular) tunics or layers of tissue. These are the outer fibrous tunic, the middle vascular tunic and the inner nervous tunic. The fibrous tunic is the outermost layer of the eye and is made up of the sclera (the white of the eye) and the cornea (the transparent covering of the front of the eye). The vascular tunic represents the thick middle layer of the eye and is commonly referred to as the uveal tract. It is made up of the choroid (the thin, dark, blood-vessel containing layer behind the retina), the ciliary body (that makes the fluid in the front chamber of the eye and helps to support the lens) and the iris (the tissue that makes up the pupil). The pupil is the opening (or black dot) in the center of the iris that regulates the amount of light received by the retina. The inner nervous tunic of the back of the eye is the retina. The retina acts like the film in a camera and transmits electrical images through the optic nerves to the brain.

The interior of the eye is divided into three chambers or compartments referred to as the anterior (front) chamber, posterior (back) chamber and vitreous chamber. The anterior chamber is located between the cornea and the iris and contains aqueous humor (a clear fluid produced by the ciliary body). The posterior chamber is located between the iris and lens and also contains aqueous humor. The vitreous chamber is behind the lens and in front of the retina. It is occupied by vitreous, a gel-like substance that helps to maintain the round shape of the eye.

Other important structures of the eye include the following:

  • The orbit is the bony cavity that contains and protects the eyeball.

  • The eyelids are extensions of the skin of the face, and they are designed to protect the eye. The outer surface of the eyelid is covered with skin and sometimes contains the cilia (eyelashes). The inside is lined with a pink-white colored conjunctival membrane.

  • The nictitans or third eyelid arises from the inside corner of the eye and contains a strong cartilage support and a tear gland. It is also designed as an extra protective mechanism for the eye.

  • The conjunctiva is a thin, nearly transparent, vascularized (containing blood vessels) tissue that covers the white of the eye and lines the eyelids.

  • The lens is a soft, transparent, spherical structure that is suspended within the eye just behind the pupil. The lens is responsible for focusing light coming in through the pupil onto the retina in the back of the eye.

  • The lacrimal system, which includes the lacrimal (major tear) gland and the gland of the third eyelid, is responsible for tear production and drainage of tears away from the eye.

    What Are the Functions of the Eye?

    The function of the eye is to allow the animal to see or have vision. The ability to see is dependent on the actions of several structures in and around the eyeball. When you look at an object, light rays are reflected from the object to the cornea. The light rays are bent (refracted) by the cornea and directed through the pupil to the lens, and then through the vitreous to the retina.

    The lens' job is to make sure the light rays come into focus sharply on the retina. The resulting image on the retina is upside-down and it is the brain's responsibility to turn the image so that you see the image correctly. The retina contains millions of light receptors called rods and cones. Rods are sensitive to dim light and cones are sensitive to bright light and colors. The retina converts light energy into electrical signals and sends them to the brain via the optic nerve, which is the nerve that runs from the eye to the brain. In the brain the electrical signals are translated into an image that is perceived in an upright position.

    What Are Common Diseases of the Eye?

    Despite the relatively small size of the eyes, any disorder that might impact sight is considered serious. A surprisingly great number of diseases affect the various structures and portions of the eye. The common diseases include injury and trauma, conjunctivitis (inflammation of the conjunctiva or "pink eye"), corneal ulcers, corneal inflammation (keratitis), inflammation of the uveal layer of the eye (uveitis), glaucoma (high pressure within the eye), and certain diseases of the retina and orbit. A variety of tumors can also affect the eyelids, internal structures of the eye, and the orbit.

    What Types of Diagnostic Tests Are Used to Evaluate the Eye?

    Veterinarians and veterinary ophthalmologists use a number of different tests to evaluate the eye. Specific tests are designed to evaluate selected segments and functions of the eye. A thorough ophthalmic examination using an external light and ophthalmoscope is very important. In addition, the following tests are important disgnostic tools:

  • Fluorescein staining of the cornea is used to detect ulcers.
  • A Schirmer tear test is used to measure tear production.
  • Slit lamp biomicroscopy is used to examine the front 1/3 of the eye including the lens.
  • Tonometry is used to measure intra-ocular pressure.
  • Ocular ultrasound is used to evaluate the inner eye and structures of the orbit.
  • Electroretinography (ERG) is used to test the retina.
  • X-rays or specialized imaging (CAT scan, MRI) may be needed to evaluate the orbit, the nerves leading back to the brain, and the bones that surround the eye.

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