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

36 страница из 88

Water exuded from blood into lymph may be reabsorbed into the blood near the place where it was poured out, or it may reach the blood via the thoracic duct. It would seem that the former is the natural, the latter the emergency route; the former the course taken when an organ is tranquil, the latter a necessity when the organ is active. If the large lymphatic vessels of a limb are cut, no lymph escapes from them so long as the limb is at rest. When the muscles contract lymph begins to flow. If the limb is flexed and extended by hand, lymph flows. If the muscles are squeezed or massaged, lymph flows. As the flow is set up both by active contraction of the muscles and by passive movements in which the muscles do not take part, it clearly must be due to external pressure on the lymphatic vessels. As they are provided with valves, squeezing them converts them into pumps. The fluid which they contain is bound to go forwards. Additional fluid is squeezed into them from the tissue-spaces. To a large extent, therefore, the outflow of lymph from contracting muscles is to be explained as the result of the pressure which the swelling muscles exert upon the lymphatic vessels within their sheaths. But there is another factor which must not be overlooked, although it cannot readily be estimated. When a muscle is actively contracting its bloodvessels dilate. There is a greater exudation of lymph; and reabsorption by blood is not equal to the exudation. The surplus leaves the limb by the lymphatic vessels. A gland is never at rest. In the intervals between the ejection of its secretion its cells are preparing materials for the next outflow. Lymph is always flowing from a gland; its amount increases as the activity of the gland increases. More lymph leaves the blood when the gland is exceptionally active than when it is relatively quiet. Some of it is not reabsorbed into the blood. A certain proportion of the waste products of the active gland are hurried away by the overflow system in the direction of the thoracic duct.