SALIVA (la Saline, Fr. ; der Speichel, Germ. ; la Sciliva, Ital.). — The saliva is a fluid secreted by a series of glands placed about the max llary region. These glands, viz. the parotids, submaxillaries, and sub linguals, pour their secretions into the cavity of the mouth on either side. In consequence of this arrangement, it has always been a matter of difficulty to obtain saliva in a per fectly pure state, the secretion of the mouth interfering, by admixture, with the exhibition of the natural qualities of the saliva, and more especially with its microscopic characters. It occasionally happens that the fluid can be ob tained more directly from the gland in cases of salivary fistula affecting the parotid duct, but it is to be doubted whether we ought to look for the secretion in its normal state in such instances. No attempts have, as yet, been made to determine whether or not saliva, as obtained from the different glands, is iden tical in character ; but so far as general ob servation guides us, there appears no variation in its constitution as secreted from these dif ferent sources.
Qieantity.—The quantity of saliva secreted during the day has never been very accurately ascertained. It has been said that about twelve ounces are produced during the twenty-four hours, but it is highly probable that much more than this is excreted by the adult in health. The data for the statement above mentioned are most imperfect. Mitscherlich made expe riments on a patient suffering from fistula of the stenonian duct, and succeeded in obtaining about 2k ounces troy of saliva from the one parotid in twenty-four hours. The saliva col lected during the same time from the mouth amounted in this experiment to six times more than that collected from the one gland : we may, therefore, conclude that the subject of this experiment was secreting from 16 to 20 ounces troy of saliva during the twenty four hours. Mitscherlich observed that when the nerves were not excited by the motion of the muscles of mastication, or of those of the tongue, no saliva flowed, but that motion of these parts induced secretion.
The presence of food in the mouth caused a rapid flow of saliva, which was more espe cially noticed when the first portions were in troduced. Long mastication appeared to cause excessive secretion, and the more sti mulating the nature of the food, the larger uas the quantity of saliva produced.
The uses of -the saliva will be best con sidered when we have described its general qualities.
Physical qualities. — The constitution of saliva has been investigated by several che mists. It possesses the fifflowing general physical characters : — When freshly ob tained from the mouth it is opalescent, vis cid, and colourless. It sepamtes by rest into an upper stratum of clear fluid, and a lower portion made up of the same fluid in admixture with epithelium scales and mucus. Under the microscope, saiiva shows the pre sence of epithelium scales swollen mucus globules, and substances of various forms, ap parently shreds of scales and ruptured cells. There are also fatty particles, varying in size, and bright granules. Some of the mucous globules are remarkably transparent, and smaller than the more opake.
Specific gravity. —The specific gravity of healthy saliva is about 1007.9, according to the experiments of Dr. Wright. It is denser after food has been taken. Mitscherlich gives the specific gravity of saliva at 1006'1 to 1008.8, which agrees with Dr. Wright's ob servations.
Some discrepancy of opinion exists as to the reaction of saliva in respect to alka linity or acidity. Tiedemann and Gmelin, and also Schultze, state fresh saliva to be alkaline. The latter chemist, has, indeed, atteinpted to define its saturating power. He also considers that it may become acid if retained long in the mouth, and that its alka linity when fresh is dependent on ammonia. This is denied by Mitscherlich, who says that no ammonia is given off when fresh saliva is heated, and that the alkalinity depends on the presence of a fixed alkali.