LITMIR.BIZ

Figure 1.7 Divisions of orbital fascia: muscle fascia, periorbita, orbital septum, and Tenon's capsule.

Tenon's capsule (fascia bulbi) is connective tissue on the outer aspect of the sclera. Tenon's capsule is separated from the sclera by a narrow, cleft‐like space filled with loose connective tissue, Tenon's space. Tenon's capsule is attached to the sclera near the corneoscleral junction (i.e., limbus), and it becomes continuous with the fascia surrounding the EOMs. The fascial sheaths of the EOMs are dense, fibrous membranes loosely attached to the muscles with fine trabeculae of connective tissue. These sheaths are continuous with, or reflections of, Tenon's capsule, but they are not always considered part of it.

      Extraocular Muscles and Orbital Fat

      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.

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

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