Page:Structure and functions of the body; a hand-book of anatomy and physiology for nurses and others desiring a practical knowledge of the subject (IA structurefunctio00fiskrich).pdf/39

 *ally during digestion, when fat is present. Here it is called chyle. Otherwise it is generally a clear, transparent and slightly opalescent fluid, which, owing to the presence of fibrin, clots when drawn from the body and allowed to stand. In fact, it resembles blood plasma very closely in composition and, as it also contains a certain number of corpuscles or leucocytes that just correspond to the white corpuscles of the blood, it is practically blood without the red corpuscles. These leucocytes have considerable power of amœboid movement and are thought by some to play an important part in the absorption of food.

Owing to intracapillary pressure, the lymph transudes into the lymph spaces and bathes the tissues, being carried away again by the lymphatics. The amount of transudation is determined by the blood pressure—the greater the pressure, the greater the amount of transudation—and is increased by some organic action of the cells in the walls of the vessels. In the process of transudation a certain amount of solid matter goes through the wall of the vessel and it is probable that certain protein elements can be carried thus from the blood-vessels to the lymphatics, though they do not pass through the capillary wall as readily as other substances. Some lymph is also probably formed by the action of the tissues themselves, though the process is not understood.

All muscular movements, active or passive, including the respiratory movements, tend to drive the lymph on its way by pressure, the valves of the vessels keeping it from flowing back. Moreover, its flow is from the capillaries to the veins or from a region of high pressure to one of less pressure. There is probably also some contraction in the walls of the vessels themselves, and the continual formation of lymph helps to drive it along. If an obstruction to the circulation occurs, however, back-*pressure results and causes too great transudation. In that event a limb becomes swollen, pale, and generally cool. It pits on pressure, the pressure driving the lymph