Salivary Glands
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.
