The eyelids (palpebrae) are mobile folds of skin that can be drawn over the anterior aspect of the eyeball to occlude light and protect the cornea.
The opening, palpebral fissure, between the lids (rima palpebrarum) is variable in size. This fissure also varies considerably in orientation and is located more frontally in smaller breeds and more laterally in larger breeds. The width of the opening is controlled by opposing groups of muscles. The superior lid (palpebra superior) is slightly greater in extent and somewhat more mobile than the inferior (palpebra inferior). The m. orbicularis oculi acts to close the palpebral fissure. The levator palpebrae superioris, the pars palpebralis of the m. sphincter colli profundus, and smooth muscles derived from the periorbita (mm. tarsales) widen the fissure. The dog blinks approximately 14 times per minute with two-thirds of the blink excursions being incomplete. It has been shown that dogs with a higher complete blink rate frequently have a thinner lipid layer of the precorneal tear film.
The superior and inferior lids join at the medial and lateral commissures (commissura palpebrarum medialis et lateralis). The angles formed by the lids at the commissures are the medial and lateral angles (canthi) of the eye (anguli oculi medialis et lateralis). The lateral is slightly more acute. A triangular prominence, the lacrimal caruncle, lies in the medial angle. Small, fine hairs project from the lacrimal caruncle, and sebaceous glands, similar to the tarsal glands but smaller, are present (glandulae carunculae lacrimalis). The caruncle may or may not be pigmented. The superior and inferior lacrimal puncta, through which the tear film drains, open onto the bulbar surfaces of the lid margins 2 to 5 mm from the medial commissure.
The skin of the face continues onto the anterior surface of the lids with little alteration. The typical hair and glandular structure of the skin can be identified in sections of the lids.
Long hairs (cilia) project from the superior lid margin. Cilia are not present on the inferior lid in dogs. At the level of the dorsal medial orbital margin, there is a tuft of long tactile hairs (pili supraorbitales) that corresponds to the eyebrows in humans. In many dogs, the region of the pili supraorbitales contrasts in color to the rest of the face. Such dogs achieve particularly animated facial expressions.
Although similar to glands found elsewhere in the skin, the glands of the lid margin have received special designations. Sebaceous glands (glandulae sebaceae) open into the follicles of the cilia on the superior lid. Ciliary glands (glandulae ciliares) are coiled, tubular, apocrine sweat glands that secrete into hair follicles or sebaceous glands or directly onto the lid margin. At the palpebral margin, the epidermis changes abruptly from pigmented, keratinized, stratified squamous epithelium typical of skin to the unpigmented, nonkeratinized, stratified squamous epithelium of the conjunctiva (see section on conjunctiva).
Specially modified compound sebaceous glands, the tarsal glands (Meibomian glands) (glandulae tarsales), are present in both eyelids. The openings of the ducts of the tarsal glands lie in a shallow furrow immediately posterior to the mucocutaneous junction of the palpebral margin of each lid. They are easily visualized when the lid is everted slightly. The glands themselves are usually visible through the conjunctiva as white or yellow columnar structures (3 mm long) that run at right angles to the palpebral margin. There are 20 to 40 glands in each eyelid. They are usually better developed on the superior lid. The oily superficial layer of the tear film is produced by the tarsal glands. It is common to find very fine hairs originating from some of these glands. This condition (distichiasis), if severe, may lead to corneal irritation and ulceration.
Surrounding the tarsal glands in the human is a thickening of the palpebral fibrous tissue, the tarsus, which helps to stiffen the lid margin. In contrast to the human and some other domestic animals, the tarsus of the dog is much less developed or essentially absent.
The commissures of the lids are stabilized by the medial and lateral palpebral ligaments. The lateral palpebral ligament (ligamentum palpebrale laterale) is a poorly defined thickening of the orbital septum deep to the retractor anguli oculi lateralis muscle. The lateral ligament originates from the zygomatic arch and ventral end of the orbital ligament and inserts by blending with the fibers of the superior and inferior tarsi.
The medial commissure is much more firmly anchored than the lateral. The medial palpebral ligament is a distinct fibrous band originating from the periosteum of the frontal bone near the nasomaxillary suture. A small oval area of roughening at the site of origin is observed on most skulls. From its origin, the ligament passes laterally deep to the angular vein of the eye, then superficial to the origin of the levator nasolabialis to blend with the tarsi at the medial commissure. The orbicularis oculi muscle both originates and inserts on the medial palpebral ligament (see section on muscles). The palpebral ligaments and retractor anguli oculi lateralis muscle prevent the palpebral fissure from becoming circular when the sphincterlike orbicularis oculi muscle contracts.
The inner aspect of the eyelids is lined by a special mucous membrane, the palpebral conjunctiva. At the level of the orbital rim, the palpebral conjunctiva reflects onto the surface of the globe to become the bulbar conjunctiva. The point of reflection is the conjunctival fornix. The conjunctival sac is the potential space between the lid and the eyeball that normally contains a thread of mucus and fluid tears.
The normal conjunctiva of the dog has been described by Riis (1976). Near the palpebral margin the conjunctival epithelium is stratified squamous. Toward the fornices, the epithelium thins, and the surface cells become cuboidal. Moore et al. (1987) have described the distribution and density of the goblet cells in the dog. They found the greatest density of goblet cells in the inferior medial and middle fornix, and inferior medial palpebral regions. Goblet cells are essentially absent from the bulbar conjunctival epithelium. The bulbar conjunctival epithelium is very thin and is continuous with the limbal epithelium. Goblet cells are absent in the perilimbal region, where the epithelium again becomes squamous. There are no perilimbal glands (glands of Manz) in the dog. With regard to the conjunctival lining of the third eyelid, numerous goblet cells reside on the palpebral side, being much less populated on its bulbar side.
The conjunctival epithelium rests on a loose connective tissue stroma rich in fibrocytes, mast cells, plasma cells, lymphocytes, and macrophages. It is very mobile, permitting extensive excursions of the eyeball and eyelids. Multiple small folds of conjunctiva are formed in the fornix when the lids are open.
The conjunctiva is richly vascular. It is supplied by branches of the dorsal and ventral palpebral and malar arteries as well as by terminal small branches from the anterior ciliary arteries. Deep and superficial components of the conjunctival vasculature have been recognized. These react differently to inflammation of various segments of the eyeball and may be distinguished by biomicroscopy or topically applied vasoconstrictor pharmaceuticals. The conjunctiva is well innervated by branches of the long ciliary, zygomaticofacial, zygomaticotemporal, infratrochlear, and frontal nerves (see section on innervation). Corneal or conjunctival irritation results in reflex lacrimation.
Lymphatic tissue within the conjunctiva is both diffuse and nodular. Numerous lymphatic nodules are found throughout the conjunctiva. Their number, size, and location vary with the age of the dog and the degree of antigenic stimulation. They are especially prominent on the bulbar surface of the third eyelid, where they may greatly enlarge in chronically infected or irritated eyes. When enlarged, they may protrude from posterior to the third eyelid into the palpebral fissure. These lymphatic nodules are distinct from, and more superficial than, the superficial gland of the third eyelid, with which they have often been confused. The lymphatic drainage from the conjunctiva in the dog empties into the parotid lymph nodes.
The third eyelid (palpebra tertia), or semilunar fold of the conjunctiva (plica semilunaris conjunctivae), is well developed in the dog. The third eyelid arises as a fold from the ventromedial aspect of the conjunctiva. The free edge of the fold is concave and faces superiolaterally to accommodate the shape of the eyeball and is usually darkly pigmented in contrast to the rest of the conjunctiva. The third eyelid is highly mobile and sufficient in extent to cover the entire anterior face of the cornea. When the eye is in its normal position in the live dog, the bulk of the third eyelid is hidden within the orbit; only the free edge is visible in the ventromedial aspect of the palpebral fissure being covered in part by the lacrimal caruncle.
The body of the third eyelid of the dog is reinforced by a T-shaped hyaline cartilage plate (cartilago plica semilunaris conjunctivae). The column of the T curves around the inferiomedial aspect of the globe. The concave crossbar of the T stiffens the free edge of the fold. The length of the cartilaginous column is surrounded by the superficial gland of the third eyelid. The gland is a pink, tear-drop-shaped, mixed seromucous gland. Numerous microscopic ducts empty the secretion of the gland into the inferior conjunctival fornix. The gland contributes significantly to the production of the tear film (see section on lacrimal apparatus). Consequently, surgical removal of the third eyelid results in morphologic changes in the corneal epithelium including decreased bright cells (as assessed by scanning electron microscopy), cell exfoliation, intercellular detachment of superficial cell layers, and hemidesmosome detachment of basal cell layers.
The superficial and deep muscular fasciae of the orbit (see section on orbital fasciae) are associated with the base of the third eyelid. A portion of the middle muscular fasciae inserts at the lateral angle of the third eyelid, forming a supportive ligament.
When the eyeball is retracted into the orbit, the column of the cartilage, surrounded by its bulky gland, is displaced anteriorly, and the third eyelid sweeps across the cornea from inferiomedial to superiolateral. The motion of the third eyelid is passive, the result of displacement by the globe’s being pulled into the orbit by the retractor bulbi and rectus muscles. Consequently, the third eyelid may be exposed for examination by manual displacement of the globe into the orbit. The dog has no specific muscle that draws the third eyelid across the globe such as is present in the cat. Smooth muscle cells derived from the periorbital orbitalis muscle attach to the fascia of the base of the third eyelid and help to maintain its retracted position. Loss of sympathetic innervation to the orbitalis muscle results in partial protrusion of the third eyelid.