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Parotid Gland

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									Parotid Gland
The parotid gland lies on the lateral side of the face in a depression below the external acoustic meatus, behind the mandible, and in front of the sternocleidomastoid muscle. The parotid gland on each side is entirely outside the boundaries of the oral cavity in a shallow triangular-shaped trench formed by:
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the sternocleidomastoid muscle behind; the ramus of mandible in front; superiorly, the base of the trench is formed by the external acoustic meatus and the posterior aspect of the zygomatic arch.

It is roughly triangular on cross section. It has a lateral or superficial surface, an anteromedial surface, and a posteromedial surface. At its upper end it has a small superior surface. Its lower end is rounded and is called the apex. The superficial surface of the parotid gland extends upwards to the zygomatic arch. Its lower end (apex) lies behind and below the angle of the mandible. Anteriorly, it is prolonged forwards superficial to the masseter. Posteriorly, it overlaps the anterior margin of the sternodeidomastoid. This surface is covered by skin and fascia. Lymph nodes of the superficial parotid group lie over it. The superior surface of the parotid is concave and fits under the external acoustic meatus. It is also in contact with the posterior surface of the temporomandibular joint. The anteromedial surface is in contact with the posterior border of the ramus of the mandible. It will be recalled that lateral to the ramus there is the masseter muscle, and deep to it there is the medial pterygoid. The anteromedial surface is related to all these structures. The posteromedial surface is in contact, in its upper part with the mastoid process, and lower down with the sternodeidomastoid (superficially) and the posterior belly of the digastric (deep). The deepest part of the gland reaches the styloid process and the structures attached to it. These separate the gland from the internal carotid artery and the internal jugular vein.

Structures entering or leaving the gland The parotid gland encloses the external carotid artery, the retromandibular vein, and the origin of the extracranial part of the facial nerve [VII]. The parotid gland is related to several nerves and vessels as follows, (1) The external carotid artery enters the lower part of the posteromedial surface. Ascending within the substance of the gland it divides into its terminal branches (superficial temporal and maxillary) which emerge on the anteromedial surface of the gland. The posterior auricular branch of the external carotid artery arises just before the latter enters the gland. Sometimes it arises within the substance of the gland and emerges on the posteromedial surface. The transverse facial branch of the superficial temporal artery may arise within the substance of the gland. (2) The retromandibular vein lies in the substance of the parotid superficial to the external carotid artery. (3) The trunk of the facial nerve enters the upper part of the posteromedial surface. Within the gland it divides into its terminal branches which emerge from the anteromedial surface near its anterior margin. The cervical branch of the nerve emerges from the lower end of the gland. The facial nerve is often described as dividing the parotid gland into superficial and deep parts. (4) The auriculotemporal nerve passes laterally between the neck of the mandible and the superior surface of the gland. It gives branches to the gland. (5) The anterior (facial) branch of the great auricular nerve passes forwards over the superficial surface of the gland and supplies the overlying skin. Secretions of the parotid gland are collected by a system of ducts which unite to form the parotid duct. This duct emerges at the anterior margin of the gland and runs forwards across the masseter. At the anterior border of the masseter the duct turns medially and pierces the buccinator. The terminal part of the duct runs forwards deep to the mucous membrane of the cheek. It opens into the vestibule of the mouth opposite the crown of the upper second molar tooth. A detached part of the parotid gland present just above the parotid duct is called the accessory part of the parotid gland. The parotid gland is enclosed in a fibrous capsule derived from the deep cervical fascia as follows. At the lower end of the gland the investing layer of the deep cervical fascia splits into two layers. One layer ascends superficial to the parotid gland to reach the zygomatic arch to which it is attached. This fascia is thick and strong and is called the parotid fascia. The other layer ascends deep to the gland. Part of this layer forms a thickened band extending from the posterior margin of the ramus of the mandible to the styloid process. This band is the stylomandibular ligament. It separates the parotid gland from the submandibular gland. Blood Supply The parotid gland is supplied by small branches of the external carotid artery or of its terminal divisions. The veins drain into the retromandibular and external jugular veins. Lymphatic drainage

The lymph vessels from the gland drain into the deep cervical nodes after passing through the superficial parotid nodes (lying on the lateral surface of the gland) and the deep parotid nodes (lying in the substance of the gland). Innervation Secretomotor nerves reach the gland through branches from the auriculotemporal nerve. The gland also receives sympathetic nerves. The parotid gland, receives its parasympathetic innervation from fibers that initially travelled in the glossopharyngeal nerve [IX], which eventually joins a branch of the mandibular nerve [V3] in the infratemporal fossa Clinical Significance Parotid Salivary Gland and Lesions of the Facial Nerve The parotid salivary gland consists essentially of superficial and deep parts, and the important facial nerve lies in the interval between these parts. A benign parotid neoplasm rarely, if ever, causes facial palsy. A malignant tumor of the parotid is usually highly invasive and quickly involves the facial nerve, causing unilateral facial paralysis. Parotid Gland Infections The parotid gland may become acutely inflamed as a result of retrograde bacterial infection from the mouth via the parotid duct. The gland may also become infected via the bloodstream, as in mumps. In both cases the gland is swollen; it is painful because the fascial capsule derived from the investing layer of deep cervical fascia is strong and limits the swelling of the gland. The swollen glenoid process, which extends medially behind the temporomandibular joint, is responsible for the pain experienced in acute parotitis when eating. Frey's Syndrome Frey's syndrome is an interesting complication that sometimes develops after penetrating wounds of the parotid gland. When the patient eats, beads of perspiration appear on the skin covering the parotid. This condition is caused by damage to the auriculotemporal and great auricular nerves. During the process of healing, the parasympathetic secretomotor fibers in the auriculotemporal nerve grow out and join the distal end of the great auricular nerve. Eventually, these fibers reach the sweat glands in the facial skin. By this means, a stimulus intended for saliva production produces sweat secretion instead.


								
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