783. Absorption is the function by which external substances are received into the body, and the component particles of the body are taken up from one part of the system, and deposited in some other part. So universal and constant is the operation, that there is not a fluid nor a solid, not a surface nor a tissue, not an external nor an internal organ, which is not, in its turn, the seat and the subject of the process. By its action the component particles of the living body are kept in a state of perpetual mutation. 784. The plant in a humid atmosphere increases in weight. The nutritive matter of the plant diffused in the soil is taken up by its capillary rootlets, or by the spongolæ which are attached to them, and conveyed into the system. The fall of dew or rain upon leaves promotes the growth of the plant. Leaves placed on water are capable of preserving not only their own vitality, but that of the branches and twigs to which they are attached. These phenomena show that the process of absorption is carried on by the plant. 785. The evidence of the absorbing power possessed by the animal is still more striking. 786. 1. If an animal be immersed in water the amount of which is ascertained by measure, its head being kept out of the water, so that none can enter the mouth, the body increases in weight and the water diminishes in quantity. If certain animals, as snails, are plunged in water impregnated with colouring matter, the fluids in the interior of their body soon acquire the colour of the water by which they are surrounded. Frogs, previously kept for some time in dry air, when placed in water, absorb a quantity equal in weight to their whole body. 787. 2. In a humid atmosphere the animal increases in weight still more than the plant. 788. 3. If a quantity of water be injected into any of the great cavities of the body, as into that of the peritoneum, the whole of the fluid after a certain time disappears; it is spontaneously removed. 789. 4. If in the progress of disease a fluid be poured into any cavity of the body, as often happens in dropsy, the whole of the fluid is removed, sometimes spontaneously and quite suddenly; but more often slowly, under the influence of medicinal agents. 790. 5. Certain substances, whether applied to an external or an internal surface, produce specific effects on the system, just as when they are received into the stomach or injected into the blood-vessels. Mercury in mere contact with the skin, but more rapidly when the application is aided by friction, produces the same specific action upon the salivary glands, and the same general action upon the system as when the preparation of the metal is received into the stomach. By the like external and local application arsenic, opium, tobacco, and other narcotics produce their distinct and peculiar effects on the nervous system, and their remote and general effects on the other systems.