THE LYMPHATIC SYSTEM. 



The lymphatic system is a system of vessels which occur abundantly in almost 

 all portions of the body and converge and anastomose to form two or more main 

 trunks, which open into the subclavian veins just before they are joined by the 

 internal jugular. The vessels contain a fluid termed lymph, usually colorless, and 

 containing numerous corpuscles known as lymphocytes. Since the latter usually 

 come under observation as they circulate within the blood-vessels, the detailed 

 account of the lymphocytes is given in connection with blood-corpuscles (page- 684). 

 In those vessels which have their origin in the wall of the small intestine, 

 however, the contained fluid has, especially during digestion, a more or less milky 

 appearance, owing to the lymphocytes being loaded with particles of fat which they 

 have taken up from the intestinal contents. On this account, these vessels are 

 usually spoken of as ladeals, although it must be recognized that they are merely 

 portions of the general lymphatic system. 



In certain respects the vessels of the system strongly resemble the veins, closely 

 associated with which they take their origin embryologically and into which they 

 finally pour their contents in the adult. They arise from a capillary net-work, their 

 walls have a structure closely resembling that of the veins, they are abundantly 

 supplied with valves, and it may be said that the fluid which they contain flows from 

 the tissues towards the heart. With these similarities there are combined, however, 

 marked differences. One of the most important of these consists in the fact that 

 the capillaries are closed and do not communicate with any centrifugal set of vessels, 

 as the venous capillaries do with the arterial ; and another important difference is 

 to be found in the frequent occurrence upon the lymphatic vessels of character- 

 istic enlargements, the lymphatic nodes or so-called glands (lymphoglandulae), quite 

 different from anything occurring in connection with the veins. 



Lymph-Spaces. — Throughout practically all regions of the body spaces of 

 varying size, occupied by a clear, more or less watery fluid, exist, and to these 

 the term lymph-spaces has been applied (Fig. 781). It was long believed that 

 they were directly continuous with the lymphatic capillaries, that the latter, indeed, 

 opened out from them, the spaces forming the origins of the capillaries. There is, 

 however, a growing tendency to dispute this view and to regard the lymphatic 

 capillaries as being quite independent of the spaces, — the entire lymphatic system, 

 in fact, being a closed system, except for its communications with the subclavian 

 veins. Since, however, the lymphatic capillaries form net-works in the tissues 

 which bound these spaces, interchange of their contents with those of the capillaries 

 is by no means difficult, the lymphocytes, even, passing on occasion through the 

 walls of the capillaries into the spaces and returning again to the interior of the 

 capillaries. 



If a colored fluid be injected into the portal vein it will pass through the walls of 

 the venous capillaries and invade the spaces of the interlobular hepatic connective 

 tissue, and later it will flow away bv the hepatic lymph-capillaries. By varying the 

 extent of the injection it will be found that the lymphatic vessels will be injected when 

 the lymph-spaces are completely filled, but will not be when the spaces are only 

 partially injected (Mall), so that it may be concluded that the extravasation from the 

 portal capillaries is primarily into the hepatic lymph-spaces and thence makes its way 

 into the lymph-capillaries. 



The spaces vary greatly in size, existing in certain tissues even between the 

 individual cells. They are more evident, however, in the connective tissues, reaching 

 a considerable size in areolar tissue, where they form a continuous net-work, and, 

 since the blood-vessels are usually surrounded by a greater or less amount of con- 

 nective tissue, lymph-spaces are quite distinct along their courses, forming what are 

 known as the perivascular lymph- spaces (Fig. 782). In other regions of the body 

 somewhat extensive spaces occur which have been regarded as belonging to the 



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