706 THE LYMPHATIC SYSTEM 



The blood-vessels, which enter and leave at the hilus, send branches into the follicles and 

 into the medullary cords. 



The enormous widening of the lymph-stream in the lymph-node from the vasa afferentia 

 to the capillaries — like a brook widening out into a pond — causes a very great diminution in 

 the rate of flow of the Ij'^mph. Thus there is present in the gland a very slowly moving stream 

 of lymph, which is separated from the lymphoid tissue outside by a single layer of flattened 

 endotheUal cells. There is thus possible an easy interchange of substances, and an opportunity 

 for the passage, through the endothelium, of wandering cells. While the entire mode of func- 

 tioning of the lymph-gland is not clear, it is known that lymphocytes, formed here, enter the 

 lymph-stream, and that substances such as, for instance, carbon granules, or leucocytes laden 

 with bacteria, are checked in their course by the lymph-gland. 



Arrangement. — The lymph-glands are so arranged throughout the body that 

 all the h'mph which enters the lymphatic capillaries must pass through one or 

 more lymph-glands on its way to the veins. 



It is possible that this rule may have exceptions, although none have yet been definitely 

 proved. Thus, some of the small lymphatics which join the thoracic duct may enter it without 

 having passed through a gland. Moreover, there is often found (fig. 551) a direct anastomosis 

 between an afferent and an efferent lymphatic vessel. 



Most of the glands are collected in certain regions, where they form centers 

 toward which the lymphatic vessels radiate. Such groups are termed regional 

 glands. The glands forming such a group are connected with one another by 

 numerous anastomoses, which are termed lymphatic plexuses [plexus lymphatici]. 

 In addition to the regional glands there are many isolated glands which lie along the 

 course of the lymph-vessels, and through which pass the vessels draining a much 

 more limited capillary area. Such glands are termed intercalated glands. 



4. THE DEVELOPMENT OF THE LYMPHATIC SYSTEM 



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. 



Regarding the mode of growth and spreading of the lymphatics, various theories were like- 

 wise held. KoUiker, His, Goethe and, later, Sala held that growth takes place by the suc- 

 cessive addition of mesenchyme cells; Langer, Rouget, and Ranvier maintained that growth 

 takes place by sprouting of the endothehum (fig. 553). S. Mayer thought that new lymphatics 

 are derived from transformed blood-capillaries. 



Miss Sabin in 1902 gave the first clear picture of the mode of origin and growth of the 

 lymphatic system, and our present knowledge is largely based upon her discoveries. She 

 showed by injections of embryo pigs that the lymphatics of the skin appear first in four regions 

 of the body — two on each side at the base of the neck, and two in the inguinal region — in the 

 form of sacs which are connected with the veins. From these four regions the lymphatics 

 spread out step by step over the skin of the entire body, in the form of a richly anastomosing 

 capillary plexus. Since the publication of Miss Sabin's paper, numerous studies have been 

 made on the mode of development of lymphatics in many different animals, including man. 

 The results of these studies indicate that the lymphatic endothelium first appears in the form 

 of buddings-out from the veins in certain well-defined regions of the embryo. As to the exact 

 manner of this primary origin views differ. Miss Sabin, in her first paper, held that it arises 

 by budding from the veins. F. T. Lewis held that it is formed by the transformation of plexuses 

 01 blood-capillaries. This view was accepted by Miss Sabin, and verified by Huntington and 

 McClure. Stromsten recui-red to Sala's view that the first lymphatic endothelium arises in 

 part from venous endothelium, and in part from the mesenchyme cells. Hoyer and his pupila 

 find that the fir.st lymphatics arise as buds from the veins. This has also been found (1912) by 

 E. R. and E. L. Clark in chick embryos. 



Thus far six regions have been found, in which lymphatics develop from the veins — in the 

 neck, on each side, at the angle formed by the internal jugular and subclavian veins; in the pelvis, 

 on each side, along the iliac veins; and two unpaired sets in the region of the renal veins, one 

 ventral to the aorta, the mesenteric, and one dorsal to the aorta, retroperitoneal. In these 

 six regions the lymphatics soon coalesce to form large sacs, the jugular, iliac, mesenteric and 

 retroperitoneal. The sacs are later broken up into the primary sdts of lymph-nodes. The 

 receptaculuni chyli develops in the region of the retroperitoneal sac. 



From these primary aiilagcs derived from the veins the lymphatics spread out into the 

 various organs and tissues of the body. The cutaneous lymphatics spread out from the two 

 jugular and two iliac regions (Sabin), the lymphatics of the intestine from the mesenteric sac 

 (Heuer). 



The method by which this exiension of the i)rimary lymphatics occurs is still in dispute, 

 but there .seems to be conclu.sive evidence that it takes place by the sproutingof the endothelium 

 (fig. 553); that the cndotheliuin of the lymphatics, derived from the veins, is a specific, inde- 

 pendent tissue, and that all new lymphatic endothelium is formed from lymphatic endothelium, 



