Now, the blood comes into direct contact with every one of these tissues. The ultimate cells which form the tissues are constantly being bathed by the myriads of minute blood-vessels which bring to the cells the raw material needed for their continued renewal. These cells are able to select from the nutritive fluid whatever they require to repair their waste, and to provide for their renewed activity. At the same time, the blood, as it bathes the tissues, sweeps into its current and bears away the products of waste. Thus the waste occurs in the tissues and the means of repair are obtained from the blood. The blood is thus continually being impoverished by having its nourishment drained away. How, then, is the efficiency of the blood maintained? The answer is that while the ultimate purpose of the food is for the repair of the waste, its immediate destination is the blood.[19] 129. Absorption of Food by the Blood. How does the food pass from the cavity of the stomach and intestinal canal into the blood-vessels? There are no visible openings which permit communication. It is done by what in physics is known as _endosmotic_ and _exosmotic_ action. That is, whenever there are two solutions of different densities, separated only by an animal membrane, an interchange will take place between them through the membrane. To illustrate: in the walls of the stomach and intestines there is a network of minute vessels filled with blood,—a liquid containing many substances in solution. The stomach and intestinal canal also contain liquid food, holding many substances in solution. A membrane, made up of the extremely thin walls of the blood-vessels and intestines, separates the liquids. An exchange takes place between the blood and the contents of the stomach and bowels, by which the dissolved substances of food pass through the separating membranes into the blood. Illustration: Fig. 46.—Cavities of the Mouth, Pharynx, etc. (Section in the middle line designed to show the mouth in its relations to the nasal fossæ, the pharynx, and the larynx.) A, sphenoidal sinus; B, internal orifice of Eustachian tube; C, velum palati; D, anterior pillar of soft palate; E, posterior pillar of soft palate; F, tonsil; H, lingual portion of the pharynx; K, lower portion of the pharynx; L, larynx; M, section of hyoid bone; N, epiglottis; O, palatine arch This change, by which food is made ready to pass into the blood, constitutes food-digestion, and the organs concerned in bringing about this change in the food are the digestive organs.