S E C J 1 O N T I 



THE LYMPHATIC SYSTEM 



Revised for the Fifth Edition 

 By ELIOT R. CLARK, A.B., M.D. 



ASSOCIATE IN ANATOMY, JOHNS HOPKINS MEDICAL SCHOOL 



I. GENERAL ANATOMY OF THE LYMPHATIC SYSTEM 



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-hned 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 pro- 

 vided 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) 

 [lymphoglandulse] in which the vessels branch out into lymph capillaries. These are lined, as are 

 the absorbing capillaries, with a single laj^er 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 where- 

 ever 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. 



1. THE LYMPHATIC CAPILLARIES 



The lymphatic capillary, like the blood-capillary, is the portion of the lymph- 

 atic 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 laj'^er of flat endothehal cells, there occurs the interchange of fluid substances 

 and of cells, while the heart, arteries and veins serve to transport the blood, modi- 

 fied 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 l3^mph 

 nodes, where the absorption and interchange of fluid substances and of cells takes 

 place. Consequently it becomes of prime importance to obtain a clear under- 

 standing 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. 



Historical. — Previous to the development of microscopic anatomy, in the middle third of the 

 19th century, there was no accurate knowledge of such small structures as the lymphatic capil- 

 lary. In order to explain the absorption of substances by the lymphatics, as well as the passage 

 of substances from the blood-vessels through the tissues, various theories were invented. Promi- 

 nent among such theories was that of the "vasa serosa," of H. Boerhaave and other 18th century 

 anatomists and physiologists, which was perhaps most elaborately developed by Bichat, 1801-03. 

 According to this theory there are two sets of minute vessels, too smaU for the passage of cellu- 

 lar elements. The one set leads from the blood-capillaries onto the various surfaces of the body 

 and into the loose spaces in the tissues — the "exhalants. " The other set leads from the body 

 surfaces (including the serous cavities) and the loose spaces in the tissues to the lymphatics — 

 the "inhalants" or " absorbants, " which take in fluids by a sucking action. 



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