LYMPHATIC VESSELS 141 



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 consid- 

 ered as forming the beginning of the lymphatic system. Recent 

 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 nitration, so the tissue juices, as the predecessors of 

 lymph, enter the lymphatic vessels by similar processes of secre- 

 tion, osmosis, and filtration. Lymph is also formed by absorption, 

 which occurs chiefly in the alimentary tract. 



Under similar conditions the lymph as well as the blood will 

 coagulate, the fibrin forming a firm, colorless clot in which the 

 leucocytes are entangled. Because of their tendency to adhere to 

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

 current the lymphatic corpuscles 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 capillaries, 

 form anastomosing meshes in all tissues where blood capillaries 

 are found. They are most abundant in the perivascular connect- 

 ive tissues, where they form a dense plexus about the wall of the 

 blood vessels. 



The wall of the lymphatic capillary, like that of the blood 

 capillary, consists of a single layer of endothelium. This endo- 

 thelium probably forms a complete lining for the lymphatic 

 capillary and is continuous through larger and larger vessels with 

 that of the veins, from which, according to Sabin,* the lymphatics 

 are originally developed. 



The relation of the lymphatic capillaries to the tissue spaces 

 is not as yet definitely settled. It was formerly thought that these 

 spaces were continuous with the lymphatic capillaries, but the 

 more recent observations, represented by those of MacCallum,f 

 seem to show that the capillaries of the lymphatic system, like 



* Amer. J. Anat, 1902. 



f Johns Hop. Hosp. Bull., 1903 ; and Arch. f. Anat., 1902. 



