Salivary Glands

Saliva is a fluid that is continuosly secreted by glands associated with the mouth. Ordinarily, just enough saliva is secreted to keep the mucous membranes of the mouth and pharynx moist, but when food enters the mouth secretion increases so the saliva can lubricate, dissolve, and begin the chemical breakdown of the food. The mucous membrane lining the cheeks contains many small glands, the buccal glands, that secrete small amounts of saliva. There are also minor salivary glands in the lips, tongue, and hard and soft palates that secrete small amounts of saliva. However, most saliva is secreted by the major salivary glands, accessory structures that lie outside the mouth and pour their contents into ducts that empty into the oral cavity. There are three pairs of salivary glands: parotid, submandibula, and sublingual glands.

The parotid glands are located inferior and anterior to the ears between the skin and the masseter muscle. They are compound tubuloacinar glands. Each secretes into the oral cavity vestibule via a duct, calles parotid (Stensen’s) duct, that pierces the buccinators muscle to open into the vestibule opposite the upper second molar tooth. The submandibular glands, which are compound acinar glands, are found beneath the base of the tongue in the posterior part of the floor of the mouth. Their ducts, the submadibular (Wharton’s) glands, run superficially under the mucosa on their side of the midline of the floor of the mouth and enter the oral cavity proper on either side of the lingual frenulum. The sublingual glands, also compound acinar glands, are superior to the submandibular glands, and their ducts, the lesser sublingual (Rivinus’s) ducts, open into the floor of the mouth in the oral cavity proper.

The parotid gland receives its blood supply from branches of the external carotid artery and is drained by vessels that are tributaries of the external jugular vein. The submandibular gland is supplied by branches of the facial artery and drained by tributaries of the facial vein. The sublingual gland is supplied by the sublingual branch of the lingual artery and the submental branch of the facial artery and is drained by tributaries of the sublingual and submental veins.

The salivary glands receive both sympathetic and parasympathetic innervations. The sympathetic fibers form plexuses on the blood vessels that supply the glands and serve as vasoconstrictors. The parotid gland receives sympathetic fibers from the plexus on the external carotid artery, whereas the submandibular and sublingual glands receive sympathetic fibers that contribute to the sympathetic plexus and accompany the facial artery to the glands. The parasympathetic fibers of the glands consist of secretomotor fibers to the glands.

The fluids secreted by the buccal glands, minor salivary glands, and the three pairs of major salivary glands constitute saliva. Amounts of saliva secreted daily vary considerably but range from 1000 to 1500 ml. Chemically, saliva is 99.5 percent water and 0.5 percent solutes. Among the solutes are salts, dissolved gases, and various organic substances, including urea and uric acid, serum albumin and globulin, mucin, the bacteriolytic enzyme aysozyme, and the digestive enzyme salivary amylase, are also present. Salivary amylase initiates the breakdown of starch. Saliva continues to be secreted heavily some time after food is swallowed. This flow of saliva washes out the mouth and dilutes and buffers the chemical remnants of irritating substances.