The blood-vascular system has, as a part of its function, the collection of substances from the various tissues of the body which are to be conducted to the other tissues. In carrying on this function it is assisted by a second system of collecting vessels, the lymphatics. This second system resembles the blood-vascular system in many ways, but differs markedly in others. Like the blood-vascular system, it is made up of minute endothelial-lined capillaries, where the absorption of substances occurs, and of larger conducting vessels. It differs from the blood-vascular system' in two important particulars. While the blood-vascular system is provided with a pumping mechanism by which its fluid content is driven through a complete circuit from the heart, through artery, capillary, vein and back to the heart, the lymphatics merely conduct fluid from, the capillaries to the larger vessels, which eventually empty their contents into the large veins of the neck. The second important difference between the two systems is found in the presence, along the course of the lymphatic vessels, of glands or nodes (fig. 553) [lymphoglandulee] in which the vessels branch out into lymph capillaries. These are lined, as are the absorbing capillaries, with a single layer of endothelial cells, thus permitting an interchange of substances between the contents of the lymph capillaries and the lymphoid tissue around them. Our present knowledge does not permit an exact statement of the complete extent of the lymphatic system. While, in a general way, the lymphatics may be said to be present wherever blood-capillaries occur, there are certain tissues where lymphatics have not been definitely demonstrated. The general constitution of the lymphatic system will be considered under three heads - (1) the capillaries, (2) the collecting vessels and (3) the lymphoid organs.

The lymphatic capillary, like the blood-capillary, is the portion of the lymphatic system which is chiefly concerned in the specific function of this system. In the blood-capillaries, where the blood is separated from the outside tissues by a single layer of flat endothelial cells, there occurs the interchange of fluid substances and of cells, while the heart, arteries and veins serve to transport the blood, modified in the capillaries, to other parts of the body. Similarly in the lymphatic system, it is in the capillaries, both those most peripheral and those in the lymph nodes, where the absorption and interchange of fluid substances and of cells takes place. Consequently it becomes of prime importance to obtain a clear understanding of the structure of the lymphatic capillaries, their relation to the other tissues, and their mode of functioning. At the outset, however, it must be admitted that our knowledge on this subject is far from complete.

Closely associated with the lymphatic capillaries and vessels is a group of glandular structures known as lymphoid organs. They consist, essentially, of groups of round lymphoid cells, lying in a meshwork of reticulum fibers, and having often a definite relationship to the blood or lymph vessels.

Our knowledge of the lymphatic system has been very greatly increased during the past ten years by studies on its mode of development. Previous to 1902 nothing definite was known about the primary development or the mode of growth of the lymphatic system. It was concluded by some (Budge, Gullard and Saxer) that the lymphatics arise from undifferentiated mesenchyme cells; Ranvier believed that they arise from veins by budding of the endothelium; while Sala described them as arising partly from the mesenchyme and partly from venous endothelium.

The lymph which enters the lymphatic capillaries passes over into collecting vessels (ducts), which carry it through the lymph-glands (nodes) to the large veins at the base of the neck. The lymph-vessels course in the loose subcutaneous tissues, in the connective tissues between muscles and organs, often accompanying the arteries and veins, sometimes forming networks around them. An idea of their arrangement can be best obtained by glancing at the illustrations of the lymphatics of special regions.

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