Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 30

Three sheets of orbital fascia are separated by orbital fat. Orbital fat fills the dead space in the orbit and acts as a protective cushion for the eye. The amount of orbital fat varies between individuals and to a greater extent between species. The color of orbital fat ranges from white to yellow. Some animals, including birds and many reptiles, have very little orbital fat. When the retractor oculi muscle contracts, orbital fat can displace the glandular tissue associated with the nictitating membrane (NM), resulting in its passive movement over the cornea.

The 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.