226 THE LYMPHATIC SYSTEM 



be daily consumed in the body equal to the number added to the blood. 

 Many of course actually suffer destruction, but it seems probable that 

 a considerable number also first undergo differentiation into granulocytes, 

 and perhaps as potential hemoblasts may function as parent cells of 

 erythrocytes. 



In addition to the leukocytes lymph contains fat globules and glyco- 

 gen. These are mostly the products of absorption from the intestinal 

 tract, in which process the lymphatic vessels play an important role. In 

 the lymphatic vessels of the intestine during absorption fat globules are 

 so abundant as to impart to the lymph a milky white color; this variety 

 of lymph is termed the chyle. These fat globules are rapidly removed 

 by the lymphoid organs, since even in the presence of abundant chyle 

 only comparatively few fat globules escape into the general blood current. 

 The lymph of other portions of the body than the abdominal region, 

 therefore, contains relatively little fat. 



The lymph, unlike the blood, circulates in but one direction, viz., 

 toward the heart. .It must therefore be formed in the tissues generally. 

 The blood plasma constantly escapes through the walls of the capillary 

 vessels into the surrounding lymphatic spaces of the tissues. It is these 

 tissue spaces which have been considered as forming the beginning of 

 the lymphatic system. Eecent evidence, however, goes to show that the 

 tissue spaces are not directly connected with the lymphatic vessels, but 

 that just as the plasma exudes into the tissue spaces by processes of 

 secretion, osmosis, and filtration, so the tissue juices, as the predecessors 

 of lymph, enter the lymphatic vessels by similar processes of secretion, 

 osmosis, and filtration. Lymph is also formed by absorption, which oc- 

 curs chiefly in the alimentary tract. 



Under favorable conditions the lymph will coagulate, though more 

 slowly than blood, the fibrin forming a firm, colorless clot in which the 

 leukocytes are entangled. Because of their tendency to adhere to the 

 sides of the vessel thus circulating at the periphery of the current 

 the lymph cells are most likely to be found at the periphery in those 

 post-mortem clots which occur within the lymphatic vessels. 



LYMPHATIC VESSELS 



(Lymphatics) 



The lymphatic vessels vary in size from that of the smallest capillary 

 vessels up to that of the thoracic duct. The smaller vessels, lymphatic 



