The muscles that alter the size or position of the palpebral fissure include the m. orbicularis oculi, the m. levator palpebrae superioris, m. levator anguli oculi medialis, m. retractor anguli oculi lateralis, mm. tarsalis inferior et superior, and the pars palpebralis of the m. sphincter colli profundus.
The m. orbicularis oculi is the most important muscle that acts to close the palpebral fissure. It is composed of two parts: the pars orbitalis and the pars palpebralis. The division between the parts is not distinct in the dog. The pars palpebralis is composed of fibers that run in the substance of the lids themselves. These fibers originate from the medial palpebral ligament, encircle the palpebral fissure, and insert again on the ligament. The muscle is wedge-shaped in transverse section, tapering toward the lid margin. Fibers of the pars palpebralis lie anterior to the tarsus and tarsal glands and closely approach the lid margin; some fibers are found almost to the level of the opening of the tarsal glands. The pars palpebralis is better developed in the superior lid.
The pars orbitalis of the orbicularis oculi surrounds the pars palpebralis. It consists of dorsal and ventral components that originate from the medial palpebral ligament and follow the curve of the orbital margin laterally. At the lateral commissure of the lids, some of the peripheral fibers of the ventral component fan out caudally and dorsally on the superficial surface of the frontalis muscle. Fibers of the dorsal pars orbitalis decussate with the ventral fibers caudal to the lateral commissure, forming the lateral palpebral raphe (raphe palpebralis lateralis). No pars lacrimalis could be identified in the dog.
The orbicularis oculi muscle exerts a sphincterlike action to close the palpebral fissure. The medial and lateral palpebral ligaments and the retractor anguli oculi lateralis muscle stabilize the commissures of the lids and prevent the fissure from becoming circular when the muscle contracts. Constant corneal irritation results in hypertrophy of the orbicularis oculi muscle, which, if pronounced, will actually roll the haired anterior surface of the lid posteriorly against the cornea (spastic entropion). Motor innervation is supplied by the palpebral branches of the auriculopalpebral nerve, a branch of the facial nerve. A prominent sign of facial nerve palsy is therefore the inability to close the palpebral fissure (de Lahunta, 1983). Blood supply to the orbicularis oculi is derived from the malar artery medially and the superior and inferior lateral palpebral arteries laterally.
The m. levator palpebrae superioris is the most important muscle that acts to retract the superior eyelid. The muscle originates deep within the orbit, dorsal to the optic canal between the origins of the dorsal rectus and dorsal oblique muscles. The levator courses rostrally deep to the periorbita on the dorsomedial aspect of the dorsal rectus muscle toward its insertion in the superior eyelid. The muscle becomes progressively wider and flatter anteriorly. Anterior to the equator of the eyeball, a broad aponeurosis continues the muscle into the superior lid, where it inserts on the palpebral connective tissue among the fibers of the orbicularis oculi muscle. The m. levator palpebrae superioris is innervated by the oculomotor nerve. Blood is supplied by the muscular branches of the external ophthalmic artery.
The m. retractor anguli oculi lateralis is a small, flat muscle that arises from the temporal fascia near the temporozygomatic suture. The muscle is parallel and superficial to the lateral palpebral ligament. It passes rostrally superficial to the orbital part of the orbicularis oculi muscle and inserts by blending with fascicles of the palpebral part at the lateral commissure of the lid. The lateral retractor draws the lateral canthus posteriorly and thus has some action in closing the palpebral fissure. The retractor is innervated by the zygomatic branch of the auriculopalpebral nerve, and is supplied by branches of the lateral ventral palpebral artery.
Superior and inferior tarsal muscles, consisting of smooth muscle fibers, are derived from the periorbital orbitalis muscle and help maintain lid position. These muscles are innervated by sympathetic nerves. One of the signs of ocular sympathetic denervation (Horner syndrome) is the narrowing of the palpebral fissure.
The pars palpebralis of the m. sphincter colli profundus acts as a depressor of the inferior lid. It consists of several delicate straps of muscle that originate near the ventral midline. These course dorsally caudal to the angle of the mouth to insert on the inferior tarsus. The ventral portion of these muscular straps is deep to the platysma. The dorsal portion is subcutaneous and closely applied to the skin. The muscle is innervated by the buccal branches of the facial nerve.