Structures Associated with the Parotid Gland

• Motor branches of CN VII. CN VII exits the stylomastoid foramen, enters the posterior surface of the parotid gland, and courses through the gland en route to innervate the muscles of facial expression. CN VII also provides visceral motor innervation to all glands of the head, with the only exception being the parotid gland, the gland that you would think CN VII would innervate (Figure 20-3A and B).
• Retromandibular vein. The superficial temporal vein and the maxillary vein join to form the retromandibular vein within the parotid gland and exit its inferior border, where the retromandibular vein bifurcates into anterior and posterior divisions. The anterior division joins with the facial vein to become the common facial vein, which enters the internal jugular vein. The posterior division joins with the posterior auricular vein, forming the external jugular vein.
• External carotid artery. The common carotid artery bifurcates at the level of the thyroid cartilage into the external and internal carotid arteries. The external carotid artery enters the inferior border of the parotid gland and within the gland divides into the superficial temporal artery and the maxillary artery. Both arteries emerge from the anterosuperior border and provide vascular supply to the temporal fossa and scalp (superficial temporal artery) and the deep face (maxillary artery).

Innervation of the Parotid Gland

The parotid gland is innervated by visceral motor neurons from CN IX (Figure 20-3C).

• Preganglionic parasympathetic neurons from CN IX originate in the inferior salivatory nucleus of the medulla and exit the jugular foramen along with the vagus nerve (CN X) and the spinal accessory nerve (CN XI).
• A branch of CN IX, the tympanic nerve, re-enters the skull via the tympanic canaliculus, enters the middle ear, and forms the tympanic plexus.
• CN IX gives rise to the lesser petrosal nerve, which exits the middle ear and the skull via the foramen ovale to synapse in the otic ganglion.
• Postganglionic parasympathetic neurons exit the otic ganglion and then “hitch-hike” along the auriculotemporal nerve en route to the parotid gland.

Mumps is a viral infection characterized by inflammation and swelling of the parotid gland, resulting in pain within the tight parotid fascia that covers the gland. Symptoms include discomfort in swallowing and chewing. The disease will usually run its course, with analgesics given to treat the pain and fever that is associated with the infection.

Vessels of the Face

The face has a very rich blood supply provided primarily from the following vessels (Figure 20-1C):

• Facial artery. Branches off the external carotid artery and courses deep to the submandibular gland; winds around the inferior border of the mandible, anterior to the masseter muscle, to enter the face. As it ascends in the face, the facial artery supplies most of the face by way of the inferior labial, superior labial, lateral nasal, and angular arteries.
• Supraorbital and supratrochlear arteries. Terminal branches of the ophthalmic artery, a branch of the internal carotid artery, which supply the anterior portion of the scalp.
• Superficial temporal artery. A terminal branch of the external carotid artery provides arterial supply to the lateral surface of the face and scalp.
• Facial vein. Formed by the union of the supraorbital and supratrochlear veins. The facial vein descends in the face and receives tributaries corresponding to the branches of the facial artery. The facial vein drains into the internal jugular vein.

In the skull, the facial vein connects with the cavernous sinus via the superior ophthalmic vein. Unlike other systemic veins, the facial and superior ophthalmic veins lack valves, thus providing a potential pathway for the spread of infection from the face to the cavernous sinus.

Muscles of the face.

Occipitofrontalis Muscle

The occipitofrontalis muscle, which lies in the scalp, extends from the superior nuchal line in the back to the skin of the eyebrows in the front. It allows for the movement of the scalp against the periosteum of the skull and also serves to raise the eyebrows.

Orbicularis Oculi Muscle

The orbicularis oculi muscle lies in the eyelids and also encircles the eyes. It helps to close the eye in the gentle movements of blinking or in more forceful movements, such as squinting. These movements help express tears and move them across the conjunctival sac to keep the cornea moist.

Orbicularis Oris Muscle

The orbicularis oris muscle encircles the opening of the mouth and helps to bring the lips together to keep the mouth closed.

Buccinator Muscle

The buccinator muscle arises from the pterygomandibular raphe in the back and courses forward in the cheek to blend into the orbicularis oris muscle in the lips. It helps to compress the cheek against the teeth and thus empties food from the vestibule of the mouth during chewing. In addition, it is used while playing musical instruments and performing other actions that require the controlled expression of air from the mouth.

Platysma Muscle

The platysma muscle extends from the skin over the mandible through the superficial fascia of the neck into the skin of the upper chest, helping to tighten this skin and also to depress the angles of the mouth. Although lying primarily in the neck, it is grouped with the muscles of facial expression.

The corneal blink reflex is tested by touching the cornea with a piece of cotton, which should cause bilateral contraction of the orbicularis occuli muscles. The afferent limb is the nasociliary nerve of CN V-1, and the efferent limb of the reflex arch is CN VII.

Motor Nerve Supply to the Face

CN VII provides motor innervation to the muscles of facial expression (Figure 20-2B). The facial nerve exits the skull through the stylomastoid foramen and immediately gives off the posterior auricular nerve and other branches that supply the occipitalis, stylohyoid, and posterior digastricus muscles and the posterior auricular muscle. CN VII courses superficial to the external carotid artery and the retromandibular vein, enters the parotid gland, and divides into the following five terminal branches: temporal, zygomatic, buccal, mandibular, and cervical nerves, which in turn supply the muscles of facial expression. Other muscles of the face include muscles of mastication (temporalis, masseter, and the medial pterygoid and lateral pterygoid muscles), which are innervated by the motor division of CN V-3.

One of the most common problems involving CN VII occurs in the facial canal, just above the stylomastoid foramen. Here, an inflammatory disease of unknown etiology causes a condition known as Bell's palsy, where all of the facial muscles on one side of the face are paralyzed. Bell's palsy is characterized by facial drooping on the affected side, typified by the inability to close the eye, a sagging lower eyelid, and tearing. In addition, the patient has difficulty smiling, and saliva may dribble from the corner of the mouth. If the inflammation spreads, the chorda tympani and nerve to the stapedius muscle may be involved.