=Salivary Glands.=—The secretion which accumulates in the mouth is the combined product of the sublingual, submaxillary, and parotid glands. It is a very thin, watery solution containing not more than 0·5 per cent. of solid substance. If red litmus-paper is moistened with saliva, it becomes blue, showing that the secretion is alkaline. It contains a ferment, ptyalin, which digests starch. The action of this ferment can be demonstrated by holding in the mouth for half a minute some warm starch mucilage—boiled arrowroot, for example. It quickly loses its viscidity owing to the conversion of starch into sugar. Chemically this change may be demonstrated by adding iodine-water to a specimen of the starch before and after action. Before the starch is taken into the mouth the iodine turns it blue (a characteristic reaction for starch). After it has been exposed to the digestive action of the saliva, iodine fails to colour the mixture, which now contains no starch. All the starch has been converted into dextrin and sugar. If unboiled arrowroot is placed in the mouth, some sugar is produced, but the process of conversion is very slow. It is almost impossible to digest raw starch in the mouth sufficiently to render it insusceptible to the colouring action of iodine. The sugar produced by the action of ptyalin is of the same nature as that which appears during the malting of barley. It is therefore termed “maltose.” It closely resembles grape-sugar, but is not identical with it. =The Secretion of Saliva.=—The accessibility of the salivary glands, and especially of the submaxillary, has led to their being used for a very large number of experiments. They have been studied with the aim of coming to an understanding of the mechanism of secretion in general. The glands consist of tubes of gland-cells, each tube suspended in a basket of connective tissue, in a bath of lymph (_cf._ Fig. 3). Innumerable capillary bloodvessels traverse the lymph-bath. The arteries which carry blood to the gland are supplied with nerves, which regulate their calibre, and therefore determine the amount of blood which passes through the capillaries into which they break up. The glands also are supplied with nerves which influence their functional activity. Nutrient substances and oxygen pass out of the blood into the lymph. Carbonic acid passes into the blood from the lymph. Waste products are either carried away in the lymph-stream, or make their way through the walls of the capillaries into the blood. Many problems present themselves for solution. How does the amount of work done by the gland affect its supply of blood? Does the quantity of saliva secreted vary directly with the pressure of lymph in the spaces by which the gland is surrounded? Is this pressure wholly dependent upon the pressure of the blood? Are the substances secreted by the gland supplied as such by the blood, or does the gland make the ptyalin and