The main branch of CN VII exits the stylomastoid foramen just ­behind the styloid pro­cess at the base of the mastoid. The extracranial trunk of the nerve passes between the superficial and deep lobes of the parotid gland, where it divides into 2 trunks: the temporo­ facial superiorly and the smaller cervicofacial inferiorly. ­These usually divide into 5 major sub-­branches:

The temporal and zygomatic branches laterally innervate the orbicularis oculi mus­ cles. Infraorbital and buccal branches may also variably contribute to the inferior orbicu­ laris (see Fig  1-44). Ophthalmologists are sometimes concerned about encountering branches of CN VII, particularly when performing a temporal artery biopsy or during orbital or lacrimal surgery.

The muscles of the eyelids are divided into 2 components: orbital (responsible for forced closure) and palpebral. The palpebral muscles are further separated into pretarsal (pre­ dominantly intermediate fast-­twitch fibers responsible for normal blinks) and preseptal muscles. This separation occurs at the upper eyelid crease, approximately 6–7 mm above the eyelid margin. The upper eyelid crease represents the attachments of the levator apo­ neurosis to the pretarsal orbicularis muscle and skin. Closure of the eyelids is marked by increased activity of the orbicularis muscle and inhibition of the levator palpebrae supe­ rioris muscle. The balance of tonic orbicularis and active levator activity determines the amount of eyelid opening.

For discussion of eyelid abnormalities encountered in neuro-­ophthalmic practice, see Chapter 12. For further discussion of eyelid anatomy, including illustrations, see BCSC Section 7, Oculofacial Plastic and Orbital Surgery.

Lacrimal function is governed by branches of the parasympathetic system, whereas pupil size is controlled by a balance between innervation of the sympathetic fibers to the iris dilator muscles and innervation of the parasympathetic fibers to the iris sphincter mus­ cles. The accessory retractor muscles, including Müller muscle in the upper eyelid, receive sympathetic innervation.

Sympathetic activity originates in the posterolateral region of the hypothalamus, which is influenced by signals in the frontal, sensorimotor, and occipital cortex and in the limbic system (cingulate gyrus). Along their course, sympathetic fibers destined for the orbit are divided into first-­, second-­, and third-­order segments (Fig 1-46). Axons destined for the dilator muscles of the pupil and Müller muscle represent the first-­order segment; along with other sympathetic fibers, they descend superficially in the anteromedial column through the brainstem to the spinal cord. Within the cervical spinal cord, the sympathetic fibers con­ tinue in the intermediolateral column. From cervical level C8 to upper thoracic level T2, the sympathetic fibers destined for the orbit synapse in the ciliospinal center of Budge-­Waller.

The postsynaptic second-­order fibers leave the spinal cord through the ventral rami of C8 and the T1 and T2 levels before joining the paravertebral sympathetic plexus. Ascending rostrally, the sympathetic chain passes in the anterior loop of the ansa subclavia proximate to the innominate artery on the right and the subclavian artery on the left just above the lung apex. ­These fibers pass through the inferior and ­middle cervical ganglia to terminate in the superior cervical ganglion, at the level of the ­angle of the jaw (C2) and the carotid ar­ tery bifurcation. The postganglionic third-­order fibers continue in the wall of the bifurcated carotid. Sympathetic fibers innervating the sweat glands of the lower face follow the ECA.

The oculosympathetic fibers destined for the pupil continue along the ICA, entering the cranium through the carotid canal. Some sympathetic fibers leave the carotid artery as it exits the petrous bone, combining with the greater superficial petrosal nerve to form the vidian nerve. ­These sympathetic fibers parallel the parasympathetic fibers to the lacrimal gland (see Fig 1-45). Within the cavernous sinus, sympathetic fibers destined for the dila­ tor muscles leave the carotid in conjunction with CN VI for a few millimeters. Further anteriorly in the cavernous sinus, the sympathetic fibers join the nasociliary branch of CN V₁. In the orbital apex, the fibers then pass through the ciliary ganglion (without synapsing). Along with the nasociliary branch, the sympathetic fibers reach the globe and travel with the long ciliary nerves to the dilator muscles of the pupil. The dilator muscles lie just superficial to the posterior pigment epithelium of the iris, which continues periph­ erally as the nonpigmented superficial layer of the ciliary body. The myoepithelial cells are approximately 12.5 µm thick, with an apical epithelial portion and a basilar muscular portion that is oriented radially ­toward the pupillary opening. The muscular pro­cesses terminate peripheral to the sphincter muscle. Peripherally at the iris root, ­these cells are continuous with the pigmented epithelium of the ciliary body.

The fibers destined for the Müller muscle travel along the OphA and its subsequent frontal and lacrimal branches. The Müller muscle originates near the origin of the levator aponeurosis and inserts 10–12 mm inferiorly on the superior border of the tarsus. The superior orbital sympathetic fibers also innervate the sweat glands of the forehead. Dis­ ruption of ­these superior orbital sympathetic fibers ­causes the mild ptosis and the frontal anhidrosis associated with distal Horner syndrome.