The Submandibular Gland

EMBRYOLOGY

The submandibular gland begins to form at the 13 mm stage as an epithelial outgrowth into the mesenchyme forming the floor of the mouth in the linguogingival groove. This proliferates rapidly, giving off numerous branching processes that eventually develop lumina. Initially the developing gland opens into the floor of the mouth posteriorly, lateral to the tongue. The walls of the groove into which it drains come together to form the submandibular duct. This process commences posteriorly and moves forward so that ultimately the orifice of the duct comes to lie anteriorly below the tip of the tongue close to the midline.

ANATOMY

The submandibular gland consists of a larger superficial lobe lying within the digastric triangle in the neck and a smaller deep lobe lying within the floor of the mouth posteriorly (Figure 1.10). The two lobes are continuous with each other around the posterior border of the mylohyoid muscle. As in the parotid gland, the two "lobes" are not true lobes embryologically, as the gland arises as a single epithelial outgrowth. However, surgically it consists of the two lobes as described above. It is a mixed seromucinous gland.

The Superficial Lobe

The superficial lobe lies within the digastric triangle. Its anterior pole reaches the anterior belly of a

Figure 1.10. The relationship of the superficial and deep lobes of the submandibular gland. Cross-sectional anatomy (a). The superficial lobe from outside (b). The relationship of the deep and superficial lobes to the mylo-hyoid muscle (c).

the digastric muscle and the posterior pole reaches the stylomandibular ligament. This structure is all that separates the superficial lobe of the submandibular gland from the parotid gland. It is important to realize just how close the lower pole of the parotid is to the posterior pole of the sub-mandibular gland, as confusion can arise if a mass in the region is incorrectly ascribed to the wrong anatomical structure (Figure 1.2). Superiorly, the superficial lobe lies medial to the body of the mandible. Inferiorly it often overlaps the intermediate tendon of the digastric muscles and the insertion of the stylohyoid. The lobe is partially enclosed between the two layers of the deep cervical fascia that arise from the greater cornu of the hyoid bone and is in intimate proximity of the facial vein and artery (Figure 1.11). The superficial layer of the fascia is attached to the lower border of the mandible and covers the inferior surface of the superficial lobe. The deep layer of fascia is attached to the mylohyoid line on the inner aspect of the mandible and therefore covers the medial surface of the lobe.

The inferior surface, which is covered by skin, subcutaneous fat, platysma, and the deep fascia, is crossed by the facial vein and the cervical

Figure 1.10. The relationship of the superficial and deep lobes of the submandibular gland. Cross-sectional anatomy (a). The superficial lobe from outside (b). The relationship of the deep and superficial lobes to the mylo-hyoid muscle (c).

Figure 1.11. Superficial dissection of the left submandibular gland. The investing layer of the deep cervical fascia is elevated off of the submandibular gland and the facial vein is identified.

Figure 1.12. Deep dissection of the left submandibular gland. With the submandibular gland retracted, the facial artery is identified in proximity to the facial vein.

i branch of the facial nerve, which loops down from the angle of the mandible and subsequently innervates the lower lip. The submandibular lymph nodes lie between the salivary gland and the mandible. Sometimes one or more lymph nodes may be embedded within the salivary gland.

The lateral surface of the superficial lobe is related to the submandibular fossa, a concavity on the medial surface of the mandible, and the attachment of the medial pterygoid muscle. The facial artery grooves its posterior part lying at first deep to the lobe and then emerging between its lateral surface and the mandibular attachment of the medial pterygoid muscle from which it reaches the lower border of the mandible.

The medial surface is related anteriorly to the mylohyoid from which it is separated by the mylo-hyoid nerve and submental vessels. Posteriorly, it is related to the styloglossus, the stylohyoid ligament, and the glossopharyngeal nerve separating it from the pharynx. Between these, the medial aspect of the lobe is related to hyoglossus muscle from which it is separated by styloglossus muscle, the lingual nerve, submandibular ganglion, hypoglos-sal nerve, and deep lingual vein. More inferiorly, the medial surface is related to the stylohyoid muscle and the posterior belly of the digastric.

The Deep Lobe

The deep lobe of the gland arises from the superficial lobe at the posterior free edge of the mylo-hyoid muscle and extends forward to the back of the sublingual gland (Figure 1.12). It lies between the mylohyoid muscle inferolaterally, the hyoglos-sus and styloglossus muscles medially, the lingual nerve superiorly and the hypoglossal nerve and deep lingual vein inferiorly.

The Submandibular Duct

The submandibular duct is about 5 cm long in the adult. The wall of the submandibular duct is thinner than that of the parotid duct. It arises from numerous tributaries in the superficial lobe and emerges from the medial surface of this lobe just behind the posterior border of the mylohyoid. It crosses the deep lobe, passing upward and slightly backward for 5 mm before running forward between the mylohyoid and hyoglossus muscles. As it passes forward, it runs between the sublingual gland and genioglossus to open into the floor of the mouth on

Figure 1.12. Deep dissection of the left submandibular gland. With the submandibular gland retracted, the facial artery is identified in proximity to the facial vein.

the summit of the sublingual papilla at the side of the lingual frenum just below the tip of the tongue. It lies between the lingual and hypoglossal nerves on the hyoglossus. At the anterior border of the hyoglossus muscle it is crossed by the lingual nerve. As the duct traverses the deep lobe of the gland it receives tributaries draining that lobe.

Blood Supply and Lymphatic Drainage

The arterial blood supply arises from multiple branches of the facial and lingual arteries. Venous blood drains predominantly into the deep lingual vein. The lymphatics drain into the deep cervical group of nodes, mostly into the jugulo-omohyoid node, via the submandibular nodes.

Nerve Supply to the Submandibular Gland

Parasympathetic Innervation

The secretomotor supply to the submandibular gland arises from the submandibular (sublingual) ganglion. This is a small ganglion lying on the upper part of the hyoglossus muscle. There are additional ganglion cells at the hilum of the gland. The submandibular ganglion is suspended from the lingual nerve by anterior and posterior filaments (Figure 1.13).

Figure 1.13. Clinical photograph showing the relationship of the lingual nerve to the submandibular gland.

The parasympathetic secretomotor fibers originate in the superior salivatory nucleus and the preganglionic fibers, then travel via the facial nerve, chorda tympani, and lingual nerve to the ganglion via the posterior filaments connecting the ganglion to the lingual nerve. They synapse within the ganglion, and the postganglionic fibers innervate the submandibular and sublingual glands (Figure 1.9). Some fibers are thought to reach the lower pole of the parotid gland.

Sympathetic Innervation

The sympathetic root is derived from the plexus on the facial artery. The postganglionic fibers arise from the superior cervical ganglion and pass through the submandibular ganglion without syn-apsing. They are vasomotor to the vessels supplying the submandibular and sublingual glands. Five or six branches from the ganglion supply the sub-mandibular gland and its duct. Others pass back into the lingual nerve via the anterior filament to innervate the sublingual and other minor salivary glands in the region.

Sensory Innervation

Sensory fibers arising from the submandibular and sublingual glands pass through the ganglion without synapsing and join the lingual nerve, itself a branch of the trigeminal nerve.

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