The proteolytic ferment pepsin is active only in an acid medium. Yet apart from its digestive function as an ally of pepsin, hydrochloric acid by itself also exerts a valuable disintegrating action on certain constituents of the food. Possibly the most important results of the presence of free hydrochloric acid in the great chamber into which food is first received are due to its disinfective property. It destroys all the putrefactive germs which accompany the food, and many germs which, if introduced into the blood, would give rise to disease. It also destroys the germs which multiply in the stomach towards the end of each interval between two meals. When withdrawn from the body, gastric juice will keep an indefinite time, if evaporation of the acid be prevented. =Pancreas.=—In structure the pancreas presents a marked resemblance to the salivary glands. Probably this resemblance is merely superficial. Minute examination reveals points, apparently of great morphological importance, in which they differ. In the gland-tubes of the salivary glands, and, indeed, in all glands with the exception of the pancreas, secreting cells project into the lumen. The secreting cells of the pancreas are invested internally by a layer of flattened scales (intra-acinar cells). They lie, therefore, between the basement membrane which invests them externally and this second layer of flattened cells which separates them from the lumen of the tube. At a very early date in embryonic life the gland-cells of the pancreas are filled with highly refracting granules. As this occurs long before any digestive action is called for, it may be taken as indicating that the pancreas has functions which other glands—the salivary, for example—do not possess. These granules do not, however, appear in all parts of the tubes. Certain portions of the tubes remain undeveloped—fail, that is to say, to acquire a secreting function—even in adult life. Such patches of cells, not disposed in gland-tubes, are known as islands of Langerhans. When the pancreas is over-stimulated by artificial means, leading to its extreme exhaustion, large portions of its glandular substance return to this primitive condition. The gland-cells not only discharge their stores of granules, but they lose the greater part of their cell-protoplasm. It would seem that, in their effort to meet the demand for ferments, they use up their own cell-substance in their manufacture. Having exhausted their coal, they stoke the furnace with the looms and furniture of the mill. It may be that other glands would do the same if it were possible to stimulate them as strongly as the pancreas can be stimulated. The result is probably due to the extreme susceptibility of the pancreas to the action of secretin, a substance made in the intestine. Secretin can be isolated and injected into the blood. We shall refer again to this chemical stimulation of the pancreas when tracing the progress of food through the alimentary canal.