Lymphatic Glands.—The lymphatic glands, sometimes called lymph nodes, are small and somewhat rounded bodies situated along the course of the lymphatic tubes. They vary in size, some of them being an inch or more in length. The lymph vessels generally open into them on one side and leave them on the other (Figs. 28 and 30). They are not glands in function, but are so called because of their having the general form of glands. They provide favorable conditions for the development of white corpuscles (page 29). They also separate harmful germs and poisonous wastes from the lymph, thereby preventing their entrance into the blood.
Relations of the Lymph, the Blood, and the Cells.—While the blood is necessary as a carrying, or transporting, agent in the body, the lymph is necessary for transferring materials from the blood to the cells and vice versa. Serving as a physiological "go between," or medium of exchange, the lymph enables the blood to minister to the[pg 069] needs of the cells. But the lymph and the blood, everything considered, can hardly be looked upon as two separate and distinct liquids. Not only do they supplement each other in their work and possess striking similarities, but each is made in its movements to pass into the vessels occupied by the other, so that they are constantly mixing and mingling. For these and other reasons, they are more properly regarded as two divisions of a single liquid—one which, by adapting itself to different purposes,[27] supplies all the conditions of a nutrient fluid for the cells.
Movements of the Lymph.—As compared with the blood, the lymph must be classed as a quiet liquid. But, as already suggested, it has certain movements which are necessary to the purposes which it serves. A careful study shows it to have three well-defined movements as follows:
1. A movement from the capillaries toward the cells.
2. A movement from the cells toward the capillaries.
3. A movement of the entire body of lymph from the lymph spaces into the lymphatics and along these channels to the ducts through which it enters the blood.
By the first movement the cells receive their nourishment. By the second and third movements the lymph, more or less laden with impurities, is returned to the blood stream. (See Figs. 28 and 30.)
Causes of the Lymph Movements.—Let us consider first the movement through the lymph tubes. No pump, like the heart, is known to be connected with these tubes and[pg 070] to supply the pressure necessary for moving the lymph. There are, however, several forces that indirectly aid in its flow. The most important of these are as follows:
1. Blood Pressure at the Capillaries.—The plasma which is forced through the capillary walls by pressure from the heart makes room for itself by pushing a portion of the lymph out of the lymph spaces. This in turn presses upon the lymph in the tubes which it enters. In this way pressure from the heart is transmitted to the lymph, forcing it to move.
2. Variable Pressure on the Walls of the Lymph Vessels.—Pressure exerted on the sides of the lymph tubes by contracting muscles tends to close them up and to push the lymph past the valves, which, by closing, prevent its return (Fig. 29). Pressure at the surface of the body, provided that it is variable, also forces the lymph along. The valves in the lymph vessels serve the same purpose as those in the veins.