PHYSIOLOGIC IMPORTANCE OF LYMPH 



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three collecting ducts not infrequently enter the vein 

 separately at the junction of the two veins. 



The thoracic duct is somewhat more complex 

 than other lymphatic vessels. It usually begins in 

 front of the body of the second lumbar vertebra, to 

 the right of and behind the aorta, by a dilatation of 

 the cysterna chyli. It enters the thorax through the 

 aortic hiatus and ascends through the posterior 

 mediastinum between the aorta and azygos vein. 

 Somewhere between the fourth and sixth vertebral 

 level it inclines to the left, enters the superior medi- 

 astinum, passes behind the arch of the aorta and 

 thoracic portion of the left subclavian artery into the 

 neck where, after passing in front of the left common 

 carotid artery, vagus nerve, and jugular vein, it 

 ends, as previously noted, by emptying into the 

 angle of junction of the left subclavian vein and left 

 internal jugular vein. The thoracic duct is the largest 

 lymph vessel and is composed of an endothelial layer, 

 a distinct subendothelial layer of elastic fibers, a 

 media of irregularly arranged but mainly circular 

 smooth muscle cells interspersed with elastic and 

 connective tissue fibers, which is succeeded by the 

 adventitia containing longitudinal and transverse 

 bundles of smooth muscle cells as well as blood vessels 

 and nerves. It contains valves which are quite efficient. 

 Kampmeier (106) found many more valves in the 

 thoracic duct of early embryos than in later stages. In 

 one human fetus, of 4.3 months, he found 42 valves 

 between the jugular confluence and renal arteries. In 

 older fetuses he found as few as three complete valves 

 with numerous vestiges present. Obviously, many of 

 the early valves never progress to the functional stage 

 and some vanish entirely. Kampmeier suggested that 

 the valves which did remain in postnatal life were 

 determined by areas of direct pressure on the duct 

 as, for example, in the area between aorta and 

 esophagus as they cross, an area in which a bolus of 

 food exerts pressure upon the duct. 



The above general descriptions are actually subject 

 to more exceptions than have been indicated. Studies 

 of large numbers of animals or species soon demon- 

 strate this variability 7 . McClure & Silvester (134) 

 drew attention to this variability, as far back as 1909, 

 in their report of a study of 25 species involving 50 

 mammals (primates, carnivora, rodentia, ungulata, 

 and marsupialia). In the adult cat, communication 

 between the lymphatic system and the systemic veins 

 may normally occur on each side of the body, within 

 either one of two or within two typical districts. 

 These two districts include, approximately, the angle 

 of confluence formed by the union of the external 



and internal jugular veins (common jugular angle) 

 and the angle of confluence formed by the union of 

 the external jugular and subclavian veins (jugulo- 

 subclavian angle). In the adult cat, neither one of 

 these two districts predominates as the place of com- 

 munication between the lymphatics and the veins; 

 either one of the two, or both, may serve equally in 

 this capacity. Their studies in other mammals showed 

 that these two districts were the predominant sites 

 of communication between lymphatics and veins, but 

 there was a marked variability in lymphatic arrange- 

 ment, more so than in veins, not only in different 

 species but among members of the same species. These 

 variations are presumably due to differences in the 

 establishment of these connections in the embrvo. 



One factor which has been relatively neglected in 

 recent years has been the possible existence of direct 

 lymphatic communications between veins at points 

 other than the entrance of the main ducts. Silvester 

 (200) injected 89 adult monkeys and studied their 

 lymphatic arrangements. He made the significant 

 observation that "Whenever the mesenteric or 

 inguinal lymphatic nodes of a New World species 

 were injected, the injection mass never passed from 

 the lumbar or intestinal lymphatic trunks into the 

 thoracic duct or into the anterior regions of the body, 

 but passed directly into the postcava into the region 

 of the renal veins. A more detailed examination of 

 the vessels in this region of the body revealed the fact 

 that the lymphatics of the digestive organs and of the 

 posterior extremities invariably enter the venous 

 system at the level of the renal veins." Silvester found 

 the posterior communications between the lymphatic 

 and the venous system to vary from two to nine in 

 number and to open at almost any point on the renal 

 segment of the postcava and its immediate tributaries. 

 He examined 16 different species of Old World 

 monkeys and found no evidence of these communica- 

 tions. 



It would be of great interest to know if similar 

 communications exist in the dog, rat, and man, 

 animals most frequently used in studies of the lym- 

 phatic system. Their existence might modify inter- 

 pretations based on the supposition that all lymph 

 from the viscera and posterior extremities finds its 

 way back to the blood stream only via the main 

 lymphatic channels. 



CONTRACTILITY OF LYMPHATICS 



In lower animals, as in the frog, lymph hearts 

 serve to actively propel lymph and distribute it to 



