Читать книгу The Body at Work: A Treatise on the Principles of Physiology онлайн

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Other illustrations might be given showing how the plasma of blood is altered in composition while it is passing out of, or after it has passed out of, capillary bloodvessels. Perhaps it would be more logical to start on the outer side of the walls of the capillaries; since blood may, very properly, be regarded as a tissue, dependent, like all other tissues, upon diffusion from lymph for the nutrient materials that it needs. In the wall of the alimentary canal it receives supplies via the lymph. It drops them in the liver, its garde-manger, to pick them up again as they are wanted. The torrent of lymph which the thoracic duct discharges into the veins of the neck conveys the fat which could not traverse the walls of the capillary bloodvessels, and much of the reserve of food which the blood had deposited in the liver. Only about one-quarter of the fluid of the body (one-thirteenth of the body-weight) is included within the blood-system; but this enclosed fluid, owing to the fact that it is kept in circulation by the heart, replenishes and purifies the much larger quantity which does not circulate. The unenclosed lymph has in particular situations a chemical composition which varies widely from that of the blood. Imagine a marsh through which a river flows—the vast plains of water-plants on the Nile above Fashoda, for example. There is a constant interchange between the flowing water of the river and the stagnant water of the marsh. In any given part of the marsh the quality of the water will depend upon what it has been able to take from, and what it has given back to, the river; upon what the water-plants have taken from it, and what they have added to it. Boats which cannot penetrate the walls of reed keep to the open channel of the Nile. Fish swim, now in the river, now in the narrow passages and open pools of the marsh. So it is, in a way, with the fluid in the spaces and cavities of the lymphatic system and in the bloodvessels which traverse them, and with its migratory inhabitants. In our extravagant analogy read leucocytes for fish. Fish have two reasons for wandering from river to marsh. Amongst the water-weeds they hunt for food; they seek quiet places in which to breed. In this matter the analogy holds good. A leucocyte may be overtaken with cell division anywhere—in the blood-stream or in a lymph-vessel. But cell division very rarely occurs except in certain favoured spots. The breeding-places chosen by leucocytes are sheltered situations in connective tissue where the blood-supply is abundant, and the eligibility of such a spot is much increased by its being near to a field where their services are likely to be called for. The nests of connective tissue made by the leucocytes are of three kinds, termed respectively diffuse adenoid tissue, lymph-follicles, and lymphatic glands. The connective tissue beneath the mucous membrane of the whole of the respiratory tract—trachea, bronchi, and bronchioles—is diffuse adenoid tissue. It presents no special structure, but its spaces are packed with leucocytes in various stages of cell division, and young leucocytes, or lymphocytes, as they are usually named. Some of the lymphocytes make their way into the blood or into the lymph. Others, acquiring their full dimensions, scour the epithelium which lines the respiratory tract for germs and other foreign bodies which are drawn into the tract with inspired air. They may be seen pushing aside the cells of the lower strata of the epithelium, on their way to the surface, or returning to the subepithelial connective tissue with germs, or particles of soot, or débris of epithelial cells which they have taken into their substance (ssss1).