The parotid gland, the largest of the three, varies in weight from 14 to 28 gm. It lies upon the side of the face, immediately below and in front of the external ear. The main portion of the gland is superficial, somewhat flattened and quadrilateral in form, and is placed between the ramus of the mandible in front and the mastoid process and Sternocleidomastoideus behind, overlapping, however, both boundaries. Above, it is broad and reaches nearly to the zygomatic arch; below, it tapers somewhat to about the level of a line joining the tip of the mastoid process to the angle of the mandible. The remainder of the gland is irregularly wedge-shaped, and extends deeply inward toward the pharyngeal wall.
The gland is enclosed within a capsule continuous with the deep cervical fascia; the layer covering the superficial surface is dense and closely adherent to the gland; a portion of the fascia, attached to the styloid process and the angle of the mandible, is thickened to form the stylomandibular ligament which intervenes between the parotid and submaxillary glands.
The anterior surface of the gland is moulded on the posterior border of the ramus of the mandible, clothed by the Pterygoideus internus and Masseter. The inner lip of the groove dips, for a short distance, between the two Pterygoid muscles, while the outer lip extends for some distance over the superficial surface of the Masseter; a small portion of this lip immediately below the zygomatic arch is usually detached, and is named the accessory part (socia parotidis) of the gland.
The posterior surface is grooved longitudinally and abuts against the external acoustic meatus, the mastoid process, and the anterior border of the Sternocleidomastoideus.
The superficial surface, slightly lobulated, is covered by the integument, the superficial fascia containing the facial branches of the great auricular nerve and some small lymph glands, and the fascia which forms the capsule of the gland.
The deep surface extends inward by means of two processes, one of which lies on the Digastricus, styloid process, and the styloid group of muscles, and projects under the mastoid process and Sternocleidomastoideus; the other is situated in front of the styloid process, and sometimes passes into the posterior part of the mandibular fossa behind the temporomandibular joint. The deep surface is in contact with the internal and external carotid arteries, the internal jugular vein, and the vagus and glossopharyngeal nerves.
The external carotid artery lies at first on the deep surface, and then in the substance of the gland. The artery gives off its posterior auricularbranch which emerges from the gland behind; it then divides into its terminal branches, the internal maxillary and superficial temporal; the former runs forward deep to the neck of the mandible; the latter runs upward across the zygomatic arch and gives off its transverse facial branch which emerges from the front of the gland. Superficial to the arteries are the superficial temporal and internal maxillary veins, uniting to form the posterior facial vein; in the lower part of the gland this vein splits into anterior and posterior divisions. The anterior division emerges from the gland and unites with the anterior facial to form the common facial vein; the posterior unites in the gland with the posterior auricular to form the external jugular vein. On a still more superficial plane is the facial nerve, the branches of which emerge from the borders of the gland. Branches of the great auricular nerve pierce the gland to join the facial, while the auriculotemporal nerve issues from the upper part of the gland.
The parotid duct (ductus parotideus; Stensen's duct) is about 7 cm. long. It begins by numerous branches from the anterior part of the gland, crosses the Masseter, and at the anterior border of this muscle turns inward nearly at a right angle, passes through the corpus adiposum of the cheek and pierces the Buccinator; it then runs for a short distance obliquely forward between the Buccinator and mucous membrane of the mouth, and opens upon the oral surface of the cheek by a small orifice, opposite the second upper molar tooth. While crossing the Masseter, it receives the duct of the accessory portion; in this position it lies between the branches of the facial nerve; the accessory part of the gland and the transverse facial artery are above it. The parotid duct is dense, its wall being of considerable thickness; its canal is about the size of a crow-quill, but at its orifice on the oral surface of the cheek its lumen is greatly reduced in size. It consists of a thick external fibrous coat which contains contractile fibers, and of an internal or mucous coat lined with short columnar epithelium.
Vessels and Nerves.—The arteries supplying the parotid gland are derived from the external carotid, and from the branches given off by that vessel in or near its substance. The veins empty themselves into the external jugular, through some of its tributaries. Thelymphatics end in the superficial and deep cervical lymph glands, passing in their course through two or three glands, placed on the surface and in the substance of the parotid. Thenerves are derived from the plexus of the sympathetic on the external carotid artery, the facial, the auriculotemporal, and the great auricular nerves. It is probable that the branch from the auriculotemporal nerve is derived from the glossopharyngeal through the otic ganglion. At all events, in some of the lower animals this has been proved experimentally to be the case.