Muscles of Protraction
The orbicularis oculi muscle is the main protractor of the eyelid. Innervated by CN VII, contraction of this muscle narrows the palpebral fissure. Specific portions of this muscle also constitute the lacrimal pump. The orbicularis oculi muscle is divided into pretarsal, preseptal, and orbital parts (Fig 9-12; see also Fig 9-7). The palpebral (pretarsal and preseptal) parts are integral to involuntary eyelid movements (blinking), whereas the orbital portion is primarily involved in forced eyelid closure.
The pretarsal orbicularis muscle arises from deep origins at the posterior lacrimal crest and superficial origins at the anterior limb of the medial canthal tendon (Fig 9-13). Near the common canaliculus, the deep heads of the pretarsal orbicularis fuse to form a prominent bundle of fibers known as the Horner muscle, which continues just behind the posterior arm of the medial canthal tendon to insert onto the posterior lacrimal crest. The upper and lower eyelid segments of the pretarsal orbicularis fuse in the lateral canthal area to become the lateral canthal tendon.
Figure 9-12 Segments of the orbicularis oculi muscle.
(Illustration courtesy of Mark Miller.)
Figure 9-13 Lacrimal drainage system. A, Medial attachments of the orbicularis oculi muscle. B, Deep head of the orbicularis oculi muscle.
(Illustrations courtesy of Mark Miller.)
Figure 9-14 Eyelid margin anatomy.
(Illustration by Christine Gralapp.)
The preseptal orbicularis arises from the upper and lower borders of the medial canthal tendon, or as a single head from the common tendon. In the upper eyelid, the preseptal muscle has an anterior head from the common tendon and a posterior head from both the superior and posterior arms of the tendon. Laterally, the preseptal muscles form the lateral palpebral raphe.
The orbital portions of the orbicularis muscle arise from the anterior limb of the medial canthal tendon, the orbital process of the frontal bone, and the frontal process of the maxillary bone in front of the anterior lacrimal crest. Its fibers form a continuous ellipse and insert just below the point of origin. At the eyelid margin, a specialized bundle of striated muscle fibers, the muscle of Riolan, lies more posterior than the main portion of the orbicularis and creates the gray line (Fig 9-14). The muscle of Riolan may play a role in meibomian glandular discharge, blinking, and eyelash position.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.