122 READINGS IN BIOLOGICAL SCIENCE 



laries, through whose walls it passes by diffusion and filtration. But there 

 are two possible return routes. The fluid (lymph) either may re-enter the 

 capillaries and be carried onward in the venous stream or may enter the 

 lymph capillaries. The exact mechanism is not known. At any rate, once 

 in a lymph capillary, the fluid slowly moves on in a devious course through 

 larger and larger vessels, empties into the large vein mentioned, and so 

 eventually returns to the heart. 



From this it is apparent that in composition the lymph must resemble 

 blood. It consists of those constituents of blood which are able to penetrate 

 the capillary wall, plus elements that may be added to it by the tissues. It 

 contains no red corpuscles and has much less protein than blood plasma. 

 Otherwise it closely resembles blood plasma in composition. 



At rather frequent intervals along the course of a lymph vessel are struc- 

 tures called lymph nodes. These are made up essentially of a network of 

 connective tissue, in the meshwork of which are located two special kinds 

 of cells. These are (a) cells which mature into one kind of white blood 

 cell — the lymphocytes — and (b) phagocytic cells which possess the same 

 capacity as ameba for engulfing particles. As the lymph flows through the 

 vessels, its course is interrupted by the nodes, through which it must pass, 

 trickling through the packed lymphoid cells and phagocytes. The lymph 

 nodes, as we may judge, play a large part in determining the peculiar func- 

 tions of the lymphatic system. 



What are these functions? In the first place, the lymph system helps 

 return tissue fluids to the blood circulation. In these fluids are some of the 

 waste products of metabolism on the way to excretion. But why cannot 

 this return be carried out adequately by the capillaries and veins of the 

 blood-circulatory system? Why have the two routes of return? With our 

 present information we cannot answer these questions entirely satisfac- 

 torily. It seems to be true, however, that solid particles seem to be able to 

 get into the lymphatic capillaries much easier than into the blood capil- 

 laries. Then, as the fluid trickles through the lymph nodes, some of these 

 solid particles are filtered out and thereby prevented from entering the 

 blood stream. In the lymph nodes near the lungs of city dwellers, for 

 example, so many particles of dust and soot are filtered out that in the 

 course of a normal lifetime the nodes become very dark or even black in 

 appearance. 



Bacteria in the lymphatics may be filtered out and phagocytized in the 

 lymph nodes. 



Sugars and amino acids are absorbed chiefly into the blood capillaries 

 directly. But the fats that we ingest, digest, and absorb pass mainly into 

 the lymphatics. Of course, even the fat soon gets into the blood stream 

 via the lymphatics. 



Another function of the lymphatics is the manufacture of lymphocytes. 

 They course along with the lymph and, with it, enter the blood stream. 



