lymphatic circulation

The direction of flow of the lymph is from the tissues toward the large lymphatic trunks, the thoracic and the right lymphatic duct. The flow is maintained in this direction mainly by a difference in pressure at the two ends. At the opening of the large trunk's into the jugular veins the pressure is very low, in the vein, in fact, it may be zero or even negative as compared witli the atmospheric pressure. The opening between the lymph vessel and the vein is protected by a valve which opens toward the vein, and the lymph, therefore, will flow into the vein as long as the pressure in the latter is lower than that in the lymphatic duct. At the other extremity of the system, in the tissue spaces to which the lymphatic capillaries are distributed, the pressure, on the contrary, is high. Its exact amount is not known, but, since the pressure in the blood capillaries is equal to 40-60 nims. Hg. the pressure in the liquid of the surrounding tissues must also be considerable. The tissues are, in fact, in a condition of turgidity owing to the pressure of the lymph in the tissue-spaces. This difference in pressure at the two ends of the lymphatic system is the main constant factor in moving the lymph. It is obvious that in the long run it is dependent upon the pressure within the blood-vessels and therefore upon the force of the heart beat. The contractions of the heart supply the energy, not only for the movement of the blood, but also for the much slower movement of the lymph. The circulation of the lymph is aided, however, by many accessory factors. In some animals there are genuine lymph hearts upon the course of the vessels,— that is, pulsatile expansions of the lymph vessels whose force of beat, controlled by valves, is directly applied to moving the lymph. No such structures are found in the mammalia, but according to some observers the large receptacle at the teginning of the thoracic duct, receptaeulum chyli may undergo contractions, and is, besides, under the influence of motor anif inhibitory nerves. Such movements, if they occur, must lie equivalent to the action of a lymph heart in their influence upon the flow of lymph. The flow of lymph or chyle in the intestinal area is also, without doubt, greatly assisted by the peristaltic and especially by the rhythmic contractions of the musculature of the intestines. The volume of the lymph in this region U especially large and the lymph capillaries and veins are provided with valves. Rhythmical contractions of the musculature of the intestine must squeeze the lymph toward the thoracic duct, acting like a local pump to accelerate the How of lymph. A similar influence is exerted by the contractions of the skeletal muscles. The compression exerted by the shortened fibers squeeze? the lymph vessels and, on account of the valves present, forces the lymph onward toward the larger ducts. The flow of lymph from the resting muscles —the arms and legs, for instance—^is normally small in quantity, but during muscular exercise and massage it is obviously increased. This increase may be observed in experimental work by placing a cannula in the thoracic duct. Active or passive movements of the limbs under these conditions will cause a noticeable increase in the outflow from the duct. Still another factor which exercises an influence upon the flow of lymph is foimd in the respiratory movements of the thorax. At each inspiration the pressure within the thorax is diminished (increase of negative pressure), and this factor influences the lymph How in several ways: By increasing the flow of blood through the large veins at the edge of the thorax, jugulars and subclavians, it doubtless aspirate* lymph from the thoracic and right lymphatic ducts into these veins. More-When we place a cannula in the thoracic duct and measure the outflow directly it is found to be exceedingly slow and variable. Older measurements (Weiss) indicate that it has a velocity in the duct in the neck of about 4 mms. per second, but this velocity changes naturally with the conditions influencing the production of lymph in the tissues. Heidenhain estimates that for a dog weighing 10 kgms. "the total outflow from the thoracic duct in 24 hours is equal to 640 c.c. Munk and Rosenstein, from observations upon a case with a lymph fistula, estimated that in man the flow may be equal to 60 to 100 or 120 c.c. per hour. over, by lowering the pressure upon the intrathoracio portion of the thoracic duct it also aspirates the lymph from the abdominal portion of this vessel.