lateral canthi and often at the medial upper eyelid. Horizontal folds are present in both the upper and lower eyelids. Vibrissae (long, specialized tactile hairs) are present on the base of the lower eyelid and on the medial aspect of the upper eyelid.
Figure 1.12 Photomicrograph of the eyelid of a dog. Hair follicle (HF), cilia follicle (CF), palpebral conjunctiva (PC), tarsal gland (TG), skin (S), and orbicularis oculi muscle fibers (O).
The eyelids protect the eyes from light, produce part of the tear film, spread the tear film across the cornea, and remove debris from the cornea and conjunctival surfaces. Through closure in a “zipper‐like” fashion from lateral to medial, the eyelids also direct the preocular tear film toward the nasolacrimal drainage system.
Histologically, the eyelids consist of four parts: (i) the outermost layer contiguous with adjacent skin, (ii) the subjacent orbicularis oculi muscle layer, (iii) followed internally by a tarsus and stromal layer, and lastly (iv) the innermost layer, the palpebral conjunctiva (see Figure 1.12).
The outer layer of the eyelid is skin covered by a dense coat of hairs with associated sebaceous and tubular glands. In dogs and cats, the hair follicles might be compound. Tactile hairs (pili supraorbitales), similar to the eyebrows of humans, may be present on or near the upper eyelids. Bundles of smooth muscle fibers, arrectores ciliorum, extend from the follicles of the eyelashes toward the tarsus. These muscle bundles are absent in carnivores and humans, but they are common in ruminants. The roots of the large cilia are in close association with prominent sebaceous glands (glands of Zeis) and modified apocrine sweat glands (glands of Moll, ciliary glands). These apocrine glands may provide host defense at the margin of the eyelids and possibly in the tears.
Deep to the eyelid skin, there is dense collagenous stroma and bundles of striated muscle fibers that comprise the orbicularis oculi muscle. The orbicularis oculi muscle is arranged in parallel rows that extend nearly the full length of each eyelid. In the upper eyelid, the levator palpebrae superioris muscle, which originates from the orbital apex, fans out along the dorsal half of the mid‐stroma. The muscle extends toward the inner connective tissue boundary of the orbicularis oculi muscle ending in individual small tendons. The eyelid muscles are separated from the posterior epithelial lining of the eyelids (i.e., the palpebral conjunctiva) by a narrow layer of dense connective tissue. In most veterinary species, it is less developed (fibrous rather than cartilaginous tissue) and referred to as the tarsus.
The meibomian (tarsal) glands are located in the distal portion of the tarsus near the eyelid margins and contribute to the outer, oily component of the preocular tear film. There are typically 20–40 glands present in each eyelid in the dog, and they are usually more developed in the upper eyelid, especially in cats. These holocrine, modified sebaceous glands form parallel rows of lobules, which have their duct openings on the eyelid margins. The nerve fibers, which are largely parasympathetic in origin, closely appose the basement membrane of each acinus.
In addition to the meibomian glands, there are accessory lacrimal glands associated with the eyelids. In humans, they are referred to as the glands of Krause and Wolfring. In domestic species, these accessory glands are most commonly located in the conjunctiva and have been referred to as conjunctival glands. Their contribution to the volume of tear film in cats is negligible.
Conjunctiva
The conjunctiva is a thin mucous membrane that lines the inner aspect of the eyelids, the anterior and posterior surfaces of the NM, and the exposed sclera. The conjunctiva consists of a thin layer of loose connective tissue beneath a simple to stratified epithelium that becomes consistently stratified squamous toward the eyelid margin, and provides the primary surgical source of tissues to cover deep and progressing corneal ulcerations (Figure 1.13). The palpebral conjunctiva lines the inner aspect of the eyelids and the anterior portion of the NM. As the conjunctiva reflects onto the globe, it is called the bulbar conjunctiva and becomes continuous with the limbal and corneal epithelium. The bulbar conjunctiva also lines the posterior portion of the NM. The junction between the palpebral and bulbar conjunctiva is the conjunctival fornix, and the epithelial lining in this region varies according to species, ranging from pseudostratified columnar to stratified cuboidal.
Ventrally, an additional fold is formed by reflection of the conjunctiva over the NM. The reflections at the conjunctival fornix and NM form the conjunctival sac. All parts of the conjunctiva are continuous, but for descriptive purposes, it is divided into the palpebral, bulbar, and fornix conjunctiva and further referenced to specific eyelids. The distribution of goblet cells in the conjunctiva is heterogeneous in the dog. The highest densities occur along the lower nasal and middle fornix, and the lower tarsal portion of the palpebral conjunctiva; this information is important when performing conjunctival biopsies. In cats, the conjunctival goblet cell density varies widely by region but is highest in the anterior surface of the NM and the conjunctival fornices. Additionally, in most domestic species, the bulbar conjunctiva has been reported to either essentially lack goblet cells or have a much lower population of these mucus‐forming cells. The substantia propria of the conjunctiva is composed of two layers: a superficial adenoid layer, which in the dog and cat contains a variable presence of lymphatic follicles and glands; and a deep, fibrous layer that contains the conjunctival nerves and vessels. The arteries of the conjunctiva arise from the anterior ciliary arteries, which are branches of the external ophthalmic artery, and from branches of the superior and inferior palpebral and malar arteries.
Figure 1.13 Bulbar conjunctiva of a porcine eyelid is externally lined by a stratified to pseudostratified columnar epithelium possessing numerous goblet cells (GC) near the fornix.
The lymphatics of the conjunctiva, called the conjunctiva‐associated lymphatic tissue (CALT), are arranged in two plexuses: a superficial and a deep system. CALT is generally diffuse with intermittent nodules or follicles. Often, the diffuse component of CALT infiltrates and is adjacent to tear‐secreting glands, especially those associated with the NM. Variations in the size and distribution of nodules occur between the upper and lower eyelids and are influenced by exposure to various foreign substances, including potentially infectious microorganisms. The conjunctiva at the fornix is very thin and translucent, and it lies loosely on the underlying connective tissue. In the domestic carnivore, approximately 3 mm from the limbus, the bulbar conjunctiva, Tenon's capsule, and sclera become closely united. The connective tissue is much more abundant in this location in the dog than in humans and other species. The primary functions of the conjunctiva are to prevent desiccation of the cornea, to allow mobility of the eyelids and the globe, and to provide a physical and physiological barrier against microorganisms and foreign bodies.
Nictitating Membrane
The NM (membrana nictitans, third eyelid, or plica semilunaris) protrudes from the medial canthus in the ventromedial anterior orbit. It contains a cartilaginous, T‐shaped plate, the horizontal part of which is parallel to the free or leading edge of the membrane (Figures 1.14 and 1.15). In many species, its free edge is pigmented. The stroma consists of loose to dense connective tissue that supports glandular and lymphoid tissue. The distal portion of the
