The muscle fibers encountered next in an anteroposterior direction through the eyelid belong to the palpebral part of the orbicularis oculi (Fig. 8.77). This muscle is part of the larger orbicularis oculi muscle, which consists primarily of two parts—an orbital part, which surrounds the orbit, and the palpebral part, which is in the eyelids. The orbicularis oculi is innervated by the facial nerve [VII] and closes the eyelids.
The palpebral part is thin and anchored medially by the medial palpebral ligament (Fig. 8.78), which attaches to the anterior lacrimal crest and laterally blends with fibers from the muscle in the lower eyelid at the lateral palpebral ligament (Fig. 8.78).
A third part of the orbicularis oculi muscle that can be identified consists of fibers on the medial border, which pass deeply to attach to the posterior lacrimal crest. These fibers form the lacrimal part of the orbicularis oculi, which may be involved in the drainage of tears.
Deep to the palpebral part of the orbicularis oculi is an extension of periosteum into both the upper and lower eyelids from the margin of the orbit (Fig. 8.79). This is the orbital septum, which extends downward into the upper eyelid and upward into the lower eyelid and is continuous with the periosteum outside and inside the orbit (Fig. 8.79). The orbital septum attaches to the tendon of the levator palpebrae superioris muscle in the upper eyelid and attaches to the tarsus in the lower eyelid.
Providing major support for each eyelid is the tarsus (Fig. 8.80). There is a large superior tarsus in the upper eyelid and a smaller inferior tarsus in the lower eyelid (Fig. 8.80). These plates of dense connective tissue are attached medially to the anterior lacrimal crest of the maxilla by the medial palpebral ligament and laterally to the orbital tubercle on the zygomatic bone by the lateral palpebral ligament.
Although the tarsal plates in the upper and lower eyelids are generally similar in structure and function, there is one unique difference. Associated with the tarsus in the upper eyelid is the levator palpebrae superioris muscle (Fig. 8.80), which raises the eyelid. Its origin is from the posterior part of the roof of the orbit, just superior to the optic foramen, and it inserts into the anterior surface of the superior tarsus, with the possibility of a few fibers attaching to the skin of the upper eyelid. It is innervated by the oculomotor nerve [III].
In companion with the levator palpebrae superioris muscle is a collection of smooth muscle fibers passing from the inferior surface of the levator to the upper edge of the superior tarsus (see Fig. 8.77). Innervated by postganglionic sympathetic fibers from the superior cervical ganglion, this muscle is the superior tarsal muscle.
Loss of function of either the levator palpebrae superioris muscle or the superior tarsal muscle results in a ptosis or drooping of the upper eyelid.
The structure of the eyelid is completed by a thin membrane (the conjunctiva), which covers the posterior surface of each eyelid (see Fig. 8.77). This membrane covers the full extent of the posterior surface of each eyelid before reflecting onto the outer surface (sclera) of the eyeball. It attaches to the eyeball at the junction between the sclera and the cornea. With this membrane in place, a conjunctival sac is formed when the eyelids are closed, and the upper and lower extensions of this sac are the superior and inferior conjunctival fornices (Fig. 8.77).