In some of the lower animals small muscular sacs occur in the course of the main lymph channels, which pump the lymph into the great veins by contracting rhythmically, much in the same way as the heart.
In man and the higher animals no such lymph hearts have been found; the onward movement of the fluid depends chiefly on the pressure under which the irrigation stream leaves the blood vessels. The fluid in the blood vessels, as we shall presently see, is under considerable pressure, which causes the plasma to leave the capillaries. Hence, if a lymphatic trunk be tied, its tributaries are filled with lymph until a considerable pressure (8-10 mm., soda solution) is developed in their radicles.
While the pressure exerted on the small tributaries of the lymph channel may become considerable, that in the thoracic duct is invariably very low, for the following reasons: The blood in the large veins into which the duct opens is under less pressure than in any other part of the vascular system, owing to the thoracic suction, or negative pressure in the thorax, caused by the elastic traction of the lungs. In fact, the pressure in the large veins, e. g., brachial, etc., varies from 0 to - 4 mm. Hg., and that in the venae cavae is always negative, except in sudden or forced expiration, and varies, according to the period of the respiratory rhythm, from - 5 mm., in inspiration, to - 2 mm. in expiration.
Fig. 94. Diagram showing the Course of the Main Trunks of the Absorbent System. The lymphatics of lower extremities (D) meet the lacteals of intestines (LAC) at the receptaculum chyli (R. C), where the thoracic duct begins. The superficial vessels are shown in the diagram on the right arm and leg (S), and the deeper ones on the arm to the left (D). The glands are here and there shown in groups. The small right duct opens into the veins on the right side. The thoracic duct opens into the union of the great veins of the left side of the neck (T).
The fact that the lymph at the origin of the small channels is at a pressure of 8 to 10 mm. of water, while at the entrance to the vein it is nil, would be sufficient to explain the movement, even if there were no other force aiding it.
It must be remembered that every lymph vessel is furnished with closely set valves, which prevent the fluid it contains from being forced backward, so that any accidental local pressure exercised on the exterior of a lymph channel helps the fluid onward to the veins. Along their entire extent these vessels are subject to certain forces which must materially aid the flow of the lymph stream. The first of these is the pressure exerted on the small vessels by the movement of the muscles in the neighborhood. The second is the unequal distribution of atmospheric pressure, which has full force on the peripheral channels, but is kept off the thoracic duct and its termination, as already mentioned, by the rigidity of the thoracic wall, which, together with the tendency of the elastic lungs to shrink, causes a permanent negative pressure in the thoracic cavity through which the duct passes. And, lastly, the thin-walled lymphatics are everywhere surrounded with very elastic textures enclosed in an elastic skin which exert an amount of pressure sufficient to empty and press together the walls of the vessels after death, and therefore during life must have considerable influence upon the fluid they contain.
The movements of the chyle depend on the same forces, with the addition of the power used in the contraction of the villi, which pump the chyle from the lacteal radicles into the network of valved vessels in the submucous tissue.
The commencements of the thoracic duct and the lacteals are placed in the abdominal cavity, and therefore are constantly under the influence of the positive pressure exerted by the abdominal wall on the contained viscera. The rest, of the duct is in the thorax, where the pressure is habitually negative. Certain variations coincident with inspiration and expiration take place in both these cavities, and must aid the onward flow of fluid in a vessel containing valves so closely set.