Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 31

The eyelids, or palpebrae, are thin folds of skin continuous with the facial skin (Figures 1.10 and 1.11). The upper (superior) and lower (inferior) eyelids meet to form the lateral and medial canthi (singular canthus). The opening formed by the upper and lower eyelids is the palpebral fissure. This fissure is prevented from assuming a circular shape by the medial (nasal) and lateral (temporal) palpebral ligaments that attach each canthus to the respective orbital wall. The medial ligament inserts into the periosteum of the nasal bones, whereas the lateral ligament inserts into the temporal fascia and bones associated with the lateral orbit. In the dog, the lateral ligament is essentially replaced by the retractor anguli oculi muscle and its tendon; this in large breeds of dogs results often in entropion. Closure of the eyelids is achieved by contraction of the orbicularis oculi muscle located deep in the eyelids. Opening the eyelids is accomplished by relaxation of the orbicularis oculi muscle and contraction of the levator palpebrae superioris muscle, which inserts into the upper tarsus.

Figure 1.10 Canine eye. Medial canthus (A), lateral canthus (B), cilia (C), NM (D), ciliary zone of iris (E), pupillary zone of iris (F), and collarette (G). Inset: Arrows indicate meibomian gland openings.

Figure 1.11 Equine eye. (a) Medial canthus (A), lateral canthus (B), cilia (C), NM (D), lacrimal caruncle (E), ciliary zone of iris (F), pupillary zone of iris (G), and granula iridica (H). (b) Arrows indicate vibrissae.

The upper eyelid has two to four rows of eyelashes (i.e., cilia) that usually begin near the medial quarter or third and either extend across to the lateral canthus or end shortly before the canthus (Figure 1.12). The lower eyelid has no cilia and has a hairless region approximately 2 mm wide adjacent to the eyelid margin extending the length of the lower eyelid and around the lateral canthus. The medial canthus, unlike the lateral canthus, has variable amounts of facial hair.

In the cat, neither lid has cilia, but the leading row of hair from the medial third laterally on the upper eyelid is distinct enough in most cats to be considered cilia (accessory cilia or eyelashes).

In the horse, a protuberance of variable size and pigmentation (i.e., the lacrimal caruncle) is present at the medial canthus. The lateral canthus is more rounded than that of the dog, and small amounts of bulbar conjunctiva and sclera are visible both medially and laterally. The exposed lateral conjunctiva is often pigmented. The cilia are well developed on the upper eyelid but absent on the lower eyelid. The facial hair is sparse adjacent to the lower eyelid margins at both the medial and 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.