Iodide of potassium is very quickly excreted by the kidneys, so that the great bulk of it passes out of the body in a short time after it has been taken. But a little of it is retained very persistently for a length of time. There may be more than one reason for this. It is possible that it becomes combined with albuminous matters of the blood and tissues, and this combination being only slowly broken up, the elimination of the drug continues for a length of time. Another reason appears to be that it is excreted even more readily by the salivary glands than by the urine. The saliva in which it is contained is swallowed, the iodide is again absorbed from the stomach and carried by the circulation to the salivary glands. It thus goes on in a continual round from mouth to stomach and from stomach to mouth (Fig. 126). Iodide of iron, and probably other iodides, are eliminated by the saliva in the same way. Iodide of iron occurs in the saliva either when injected into the artery of the gland or when absorbed from the stomach. When lactate of iron and iodide of potassium are introduced simultaneously, or at a short interval after each other, into the stomach, so that iodide of iron is formed there by their combination, iodide of iron is found in the saliva.1 But if they are injected separately into the blood, iodide of potassium alone without any iron appears in the saliva. Iodine probably causes other substances besides potassium and iron to appear in the saliva when they are combined with it. It probably does so to quinine, for when iodide of potassium and quinine are given together in a mixture, patients frequently complain of a very persistent bitter taste in the mouth much more marked than when the quinine is given in simple solution with acid.

1 Bernard, Physiologic Experimentale, tom. ii. p. 99.

Fig. 126.   Diagram of the gastro salivary circulation.

Fig. 126. - Diagram of the gastro-salivary circulation.

Uses. - Saliva is useful in keeping the mouth moist, and thus facilitating mastication, solution, deglutition, and the movement of the tongue in speaking. By moistening the fauces, it also prevents or lessens thirst. A pebble placed under the tongue, or masticated, will keep up a slight flow of saliva, and may be useful for these purposes. Where this is insufficient, dilute acids are employed. As the flow of blood to the glands is greatly increased through secretion, sialagogues have been used as derivatives to lessen inflammation, congestion, and pain, in other parts of the head, as in toothache, earache, and inflammation of the ear, nose, or scalp.

Saliva has also a digestive action on starch, and increase of the flow may be advantageous in imperfect digestion of this substance. When swallowed, the saliva stimulates the secretion of gastric juice, and increased salivary secretion therefore tends to aid the gastric digestion of proteids. To obtain this object it is best to chew a piece of ginger, pellitory, or rhubarb.