The parotid is the largest of three salivary glands. It lies on the side of the face, in front of the ear, and extends deeply into the space behind the ramus of the lower jaw. Its weight varies from 5 to 8 drachms. It has a duct called the parotid or Stenson's duct. It is about 2 1/2 inches long, and about a line and a half in thickness. Its orifice is opposite the crown of the second molar tooth of the upper jaw. The submaxillary gland weighs about 2 to 2 1/2 drachms, and is situated on the inner surface of the inferior maxillary. The duct is named Wharton's, and is about 2 inches in length. Its orifice is found under the tongue. The sublingual gland weighs about a drachm. It is situated on the floor of the mouth. The ducts are called the ducti Rivintiani. They are from 8 to 20 in number. They may be seen when the tip of the tongue is lifted up. Saliva. Mixed saliva, as it appears in the mouth, is a thick, glairy, generally frothy, turbid fluid. The quantity of saliva secreted in 24 hours varies. The average amount is probably from two to three pints in 24 hours. The composition of saliva is: Water, 994.10 Solids, 5.90 The solids are: Pyaline, 1.41 Fat, 0.07 Epithelium and Mucus, 2.13 { Sulphocyanide of Potassium, } { Phosphate of Soda, } Salts, {    ,,     ,, Lime, } 2.29 {    ,,     ,, Magnesia, } { Chloride of Sodium, } {    ,,    ,, Potassium, } ---- 5.90 The specific gravity varies from 1.004 to 1.008. The rate at which saliva is secreted is subject to considerable variation. When the tongue and muscles concerned in mastication are at rest, and the nerves of the mouth are subject to no unusual stimulus, the quantity secreted is not more than sufficient, with the mucus, to keep the mouth moist. The purposes served by saliva are of several kinds: 1. Acting mechanically in conjunction with mucus, it keeps the mouth in a due condition of moisture, and facilitates the movements of the tongue in speaking, and the mastication of the food. 2. It serves also in dissolving sapid substances and rendering them capable of exciting the nerves of taste. 3. By mixing with the food during mastication, it makes it a soft pulpy mass, such as may easily be swallowed. 4. Saliva performs a chemical part in the digestion of food. It transforms starchy substances into dextrine and grape sugar. Starch is a carbohydrate--carbon 18, hydrogen 30, oxygen 15. C18H30O15 + 3H2O = C6H12O6 + 2(C6H10O5) + 2H2O (Grape sugar.) (Dextrine.)