EYE ANATOMY AND DISEASE MASTER CLASS™
Vision is one of the five senses along with taste, smell, hearing, and touch. The eyes are direct extensions of the brain, receiving visual information and processing it to give a stereoscopic view of the world. For the purposes of this discussion we will concentrate on diseases and conditions that affect the major portion of the eye as seen in clinical practice.
The eyelids serve to protect the eyes – there is an upper and lower eyelid. In dogs and cats, and many other animals there is also a third eyelid or nictitating membrane as well. The eyelids are lined with lashes and each lash is associated with a gland that produces oil that becomes part of the tear film.
Tears are produced by the tear glands and serve to feed and lubricate the clear part of the eye or cornea. Because the cornea must remain clear for normal vision, it cannot have any blood vessels in it. The tears therefore, provide the cornea with its only source of nutrition. A single layer of cells that is called the corneal epithelium covers the cornea. This layer of cells is very thin but the underlying stroma of the cornea is relatively thick.
The circumference of the cornea is bordered by the sclera, which is the “white of the eye” and is of similar composition as the cornea except it has blood vessels.
The iris works to limit light into the eye in bright conditions and to allow more light into the eye in dim conditions. The iris is the colored part of the eye which opens and closes creating a larger and smaller pupil. Just inside iris lies the lens.
The lens is suspended in the center of the eye by fibers that relax and contract to change the shape of the lens. This action allows the animal to focus. The area between the lens and the iris at the periphery of the eye is the ciliary body.
The ciliary body produces the fluid that resides in the anterior chamber of the eye. This fluid is constantly being produced by the ciliary body and circulated through the pupil into the anterior chamber. The pressure that is created by this fluid is what maintains the integrity of the eye. This fluid drains out of the eye at the same rate it is produced by the ciliary body. This drainage occurs at the filtration angle which is at the internal junction of the cornea and sclera.
The back part of the eye is the retina. Filling the space between the lens and the back part of the globe is a gelatinous mass called the vitreous humor. A layer of retinal vessels that are very sensitive to disease feeds the retina and examination of the vessels is common during an opthalmic examination. The retina covers the inside of the back part of the eye. The retina is composed of a carpet of specialized nerves. The top half of the retina has specialized cells that reflect light and are called the tapetum lucidum (“light carpet”) and are responsible for the green glow of most animals’ eyes in the dark. The retina is composed of two type of “seeing” cells, rods and cones. The rods are specialized nerves for seeing light and the cones are specialized for color perception.
Disease of the eyes will be discussed from the outer eyelids progressing inward. The objective is to give you an understanding of the most common dog eye diseases and how they are handled from a customer contact standpoint. This discussion is not exhaustive in depth of coverage, yet you will have a conversational knowledge of eye disease when you have completed this course.
The eyelids have two major causes for disease: Problems with the eyelashes and problems with the glands of the eyelashes.
Entropion is a common problem in which the eyelid is turned under so that the lashes rub the cornea, which results in corneal irritation and ulceration. This condition is common in many breeds including Rottweilers, Chow Chows, Shar pei and English bulldogs. The condition may be corrected by a surgery that results in an more normal relationship of the eyelid to the cornea.
Abnormally placed eyelashes are also a common problem seen in clinical practice –
Distichiasis is the term for fine hairs that grows at the eyelid margin which occur mostly in cocker spaniels. These fine and wispy eyelash type hairs should not be removed unless you are certain they are causing discomfort.
Eyelashes sometimes grow on the inner portion of the eyelids causing corneal ulcers – these are called ectopic cilia. These cilia can be removed surgically.
Ectropion is the outward rolling of the eyelid. This condition is common in Basset hounds as well as cocker spaniels and other breeds. This condition causes the eyes to gather too much dust and debris resulting in chronic inflammation in the eyes. Surgical correction can be done if necessary.