Anatomy of the Eye in Dogs and Cats

The anatomical structures of the canine and feline eye serve to protect the organ of vision and enable its complex visual function.

• Orbit. The orbit is a cone-shaped bony cavity that provides mechanical support for the eye and is lined by the periorbita. The bones of the orbit contain canals through which nerves, arteries, and veins reach the eye. The orbital cavity contains the globe, muscles, adipose tissue, lacrimal glands, blood vessels, and nerves. The orbit of horses and cattle has a complete bony rim, whereas in dogs and cats the rim is incomplete; therefore, the lateral orbital margin lacks a bony base and is formed by the orbital ligament.
• Globe. The globe is located within the orbit, and its size varies according to species and breed. Its size is not directly proportional to the size of the animal. The globe is spherical and, because of numerous well-innervated muscles, has pronounced mobility within the orbit.
• Eyelids. The eyelids are composed of skin, muscles, tarsus, ligaments, and conjunctiva. They contain numerous nerves, blood vessels, glands (sebaceous, sweat, and lacrimal), openings of the lacrimal ducts, and eyelashes. In dogs and cats, the third eyelid is easily visible and contains T-shaped cartilage, the lacrimal gland of the third eyelid, and conjunctiva that completely covers it. Anatomically and functionally normal eyelids are extremely important for visual quality and ocular health in dogs and cats. In addition to protecting the eye through blinking, they contain glands important for tear-film quality, participate in even distribution of the tear film across the cornea, and protect the cornea from excessive drying. They also direct the tear film toward the opening of the lacrimal drainage system.
• Outer coat of the eye. The anterior outer coat is formed by the cornea, whereas the posterior portion consists of episcleral and scleral tissue. The transition zone between sclera and cornea is called the limbus. The limbus contains progenitor (stem) cells that produce corneal epithelial cells. Limbal progenitor cells also play an important role in controlling the immune response and preventing excessive inflammation of sensitive corneal tissue. The presence of progenitor cells in the limbus is probably also a reason why many neoplastic processes of corneal and scleral tissues originate in the limbal region. Corneal dimensions in dogs range from 13-17 mm horizontally and 12-16 mm vertically, whereas in cats they are approximately 17 mm horizontally and 16 mm vertically. The cornea protects the eye and, because of its transparency, permits transmission of light. It is composed of stratified squamous epithelium with a basement membrane, stroma, Descemet's membrane, and endothelium.
• Uvea. The uvea is the vascular tunic of the eye. Its anterior portion consists of the iris and ciliary body, and its posterior portion is the choroid, which supplies the retina with blood vessels. Iris color determines eye color. The iris and ciliary body support the lens and are important for pupil shape; they also participate in accommodation. Between the cornea anteriorly and the iris and lens posteriorly is the anterior chamber, which is filled with aqueous humor. Progenitor (stem) cells are also present at the base of the ciliary body. These cells are inactive in mammals, but in other animal species they can completely regenerate damaged retina. The presence of progenitor cells at the base of the ciliary body is probably one reason why many primary ocular tumors begin to grow in this region.
• Lens. The lens is a transparent biconvex structure located behind the iris and attached to the ciliary body by zonular ligaments. It consists of the lens capsule, lens epithelium, and lens fibers. The lens transmits and refracts light rays on their way to the retina and participates in accommodation. Because of its function, it must remain transparent. With age or certain metabolic disorders, the lens may change color and lose some transparency (nuclear sclerosis), which generally does not affect visual quality. In some cases, senile ocular changes (nuclear sclerosis) can affect night vision and depth perception in older animals. In contrast, true lens opacity (cataract) is one of the most common causes of blindness in dogs.
• Vitreous. The vitreous is the largest structure of the eye and occupies the space from the lens anteriorly to the retina posteriorly. It consists of water (99%); the remaining 1% is composed of collagen fibers, hyalocytes, hyaluronic acid (which gives the vitreous its viscosity), and other glycosaminoglycans. Degenerative changes in the vitreous can cause retinal detachment, especially in dogs with genetic vitreous degeneration that habitually shake toys aggressively.
• Retina. The retina is located in the posterior part of the eye. Its role is to receive light stimuli and convert them into electrical impulses that are then transmitted to and processed by the brain. It consists of multiple layers and is rich in blood vessels. Retinal photoreceptors, rods and cones, are responsible for night and daytime vision and color recognition. Retinal nerve cells converge to form the optic nerve, which exits the globe posteriorly, continues to the optic chiasm, and reaches the visual center in the occipital lobe of the brain. Specialized cells associated with the optic nerve that contain the pigment melanopsin can perceive light signals even in the presence of complete degeneration of rods and cones; therefore, the presence of a pupillary light reflex following white-light illumination does not necessarily mean that vision is preserved.

Adapted from the Atlas of Ocular Diseases in Dogs and Cats by Dr. Siniša D. Grozdanić and colleagues. © Animal Eye Consultants of Iowa. The text and clinical images on this page are original content from the authors' own clinical practice and research, and may not be reproduced, distributed, or repurposed without permission.

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