EOMs suspend the globe in the orbit and provide ocular motility (Table 1.6). There are four rectus muscles: the dorsal, ventral, medial, and lateral recti. They originate from the orbital apex (i.e., annulus of Zinn) and insert, in the dog, approximately 5 mm posterior to the limbus medially, 6 mm ventrally, 7 mm dorsally, and 9 mm laterally (Figures 1.8 and 1.9). They move the eye in the direction of their names. The dorsal (superior) oblique originates from the medial orbital apex, continuing forward dorsomedially to pass through a trochlea located near the medial canthus and pulls the dorsal aspect of the globe medially and ventrally (intorsion). The ventral (inferior) oblique originates from the anterolateral margin of the palatine bone on the medial orbital wall and passes beneath the eye, crossing the ventral rectus tendon. The muscle divides as it reaches the lateral rectus, with the anterior portion covering the insertion of the lateral rectus and the posterior portion inserting beneath the rectus. The ventral oblique moves the globe medially and dorsally (extorsion).
Table 1.6 Muscles of the eye and eyelids.
| Muscle | Function | Nerve supply |
|---|---|---|
| Dorsal (superior) rectus | Rotates globe upward | Oculomotor |
| Ventral (inferior) rectus | Rotates globe downward | Oculomotor |
| Medial rectus | Rotates globe medially | Oculomotor |
| Lateral rectus | Rotates globe laterally | Abducens |
| Dorsal (superior) oblique | Rotates dorsal part of globe medially and ventrally | Trochlear |
| Ventral (inferior) oblique | Rotates ventral part of globe medially and dorsally | Oculomotor |
| Retractor oculi (bulbi) | Retracts globe | Abducens |
| Levator palpebrae superioris | Raises upper eyelid | Oculomotor |
| Orbicularis oculi | Closes palpebral fissure | Facial |
| Retractor anguli oculi | Lengthens lateral palpebral fissure | Facial |
The retractor oculi (retractor bulbi) muscle originates at the orbital apex and continues forward to form a cone surrounding the optic nerve, and inserting posterior and deep to the recti muscles. The retractor oculi muscle retracts the globe into the orbit. The retractor oculi muscle is ubiquitous among mammals, but it is absent in various nonmammalian groups, including birds and snakes. The dorsal, ventral, and medial recti as well as the ventral oblique muscles are innervated by the oculomotor nerve (CN III), whereas the lateral rectus and retractor oculi muscles are innervated by the abducens nerve (CN VI), and the dorsal oblique muscle is innervated by the trochlear nerve (CN IV).
Figure 1.8 Arrangement of the orbital muscles of domestic animals. Annulus of Zinn, ventral oblique muscle, ventral rectus muscle, lateral rectus muscle, retractor bulbi muscle tendon attachments, medial rectus muscle, dorsal oblique muscle, and dorsal rectus muscle.
Figure 1.9 Orbital apex of the dog, illustrating structures passing through the optic foramen and orbital fissure as well as the EOM attachments.
Eyelids
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
