Numerous glands secrete the watery, somewhat viscous fluid known as saliva into the oral cavity. Small salivary glands are widely scattered under the lining of the oral cavity and are named, according to their location, labial, buccal, palatine, and lingual glands. The three chief, large, paired salivary glands are the parotid, submandibular, and sublingual.
The parotid gland, the largest of the salivary glands, is roughly shaped as a three-sided wedge, which is fitted in anterior and inferior to the external ear. The triangular superficial surface of the wedge is practically subcutaneous, with one side of the triangle almost as high as the zygomatic arch and the opposing angle at the level of the angle of the mandible. The anteromedial side of the wedge abuts against and overlaps the ramus of the mandible and the related masseter and medial pterygoid muscles. The posteromedial side of the wedge turns toward the external auditory canal, mastoid process, sternocleidomastoid, and digastric (posterior belly) muscles. The parotid (Stensen) duct leaves the anterior border of the gland and passes superficial to the masseter muscle, at the anterior border of which it turns medially to pierce the buccinator muscle and then the mucous membrane of the cheek near the second maxillary molar.
The submandibular gland lies in the submandibular triangle but overlaps all three sides of the triangle, extending superficial to the anterior and posterior bellies of the digastric muscle and deep to the mandible, in the submandibular fossa. Most of the gland is inferior to the mylohyoid muscle, but a deep process extends superior to the muscle. The submandibular (Wharton) duct at first runs anteriorly with the deep process and then in close relation to the sublingual gland (first inferior and then medial to it) to reach the sublingual caruncle at the summit of which it opens, next to the lingual frenulum.
The sublingual gland, the smallest of the three paired salivary glands, is located deep to the mucous membrane of the floor of the mouth, where it produces the sublingual fold. It lies superior to the mylohyoid muscle in relation with the sublingual fossa on the mandible. In contrast to the parotid and submandibular glands, which have quite definite fibrous capsules, the lobules of the sublingual gland are loosely held together by connective tissue. About 12 sublingual ducts leave the superior aspect of the gland and open individually through the mucous membrane of the sublingual fold. Some of the ducts from the anterior part of the gland may combine and empty into the submandibular duct. This is apparently prone to considerable variation.
The nerve supply of the large salivary glands is dis- cussed in a later segment on the innervation of the mouth and pharynx and the autonomic nervous system. Microscopically, the large salivary glands appear as compound tubular-alveolar glands. The secretions of these glands are serous and mucous and mucous with serous demilunes, with different proportions of these in different glands. The parotid gland is almost entirely serous, the submandibular gland is predominantly serous but with some mucous alveoli containing serous demilunes, and the sublingual gland varies to quite an extent in composition in different parts of the gland but, for the most part, is predominantly mucous with serous demilunes. In the parotid and submandibular glands, the alveoli are joined by intercalated ducts with low epithelium to portions of the duct system, which are thought to contribute water and salts to the secretion and, hence, are called secretory ducts. The epithelium of the ducts is at first cuboidal, then columnar, and may finally be stratified cuboidal near the opening of the duct. It should be noted that the appearance of serous demilunes is an artifact of specimen preparation and that during life, the serous-secreting cells of each acinus sit side by side with the mucous-secreting cells.