LYMPHATIC VESSELS 



I8 7 



FIG. 178. SILVER NITRATE PREPARATION OF A LYMPHATIC 

 VESSEL FROM A RABBIT'S MESENTERY, SHOWING THE 

 BOUNDARIES OF THE ENDOTHELIAL CELLS, AND A BULG- 

 ING JUST BEYOND A VALVE. 



often the connecting strands cannot be demonstrated." (Subsequently, Bremer 

 demonstrated such strands in great abundance.) 



"4. The endothelium of the embryonic lymphatics is sometimes seen to be con- 

 tinuous with that of the veins" i.e., in certain places, as in connection with the jugular 

 sac, the origin of the lymphatic vessels from the venous endothelium can be clearly 

 seen; this fact is conclusively demonstrated by Huntington and McClure, who use 

 the term " veno-lymphatic " for transitional vessels (Amer. Journ. Anat., 1910, vol. 

 10, pp. 177-311). 



LYMPHATIC VESSELS IN THE ADULT. In sections of the intestine from 

 an animal in which intestinal digestion was in progress, lymphatic vessels 

 may readily be found between 

 the muscle layers (Fig. 177). 

 Their walls are decidedly 

 thinner than those of blood 

 vessels of the same caliber, 

 and their contents are typi- 

 cally a granular or fibrinous 

 coagulum free from red cor- 

 puscles, but containing an 

 occasional lymphocyte. It 



must be remembered, however, that blood vessels seen in sections are 

 not infrequently empty, and that blood corpuscles may be taken into 

 the lymphatic vessels. Having learned to recognize the lymphatics in 

 such favorable situations as the intermuscular tissue, one may readily 

 identify them in the connective tissue layer internal to the circular 

 muscle of the intestine, and in the connective tissue around the bron- 

 chioles in the lung; in the 

 embryonic lung they are 

 very conspicuous. They 

 may then be sought for in 

 various organs, but a sharp 

 distinction must be drawn 

 between the endothelium- 

 lined lymphatic vessels and 

 the interfibrillar tissue spaces. 

 When prepared with silver 

 nitrate, the outlines of the en- 

 do thelial cells are seen to resemble those of blood vessels (Fig. 178), and in 

 the larger lymphatic vessels the endothelium with the underlying connec- 

 tive tissue forms a tunica intima. These lymphatics (0.2-0.8 mm. in 

 diameter), are often composed of three coats, though loose in texture. The 

 media contains circular smooth muscle fibers and a small amount of elastic 

 tissue; and the externa is composed of longitudinal connective tissue and 

 scattered bundles of longitudinal muscle. Thus they resemble the veins 



FIG. 179. 



Lymphatic vessel from a section of a human bronchus, 

 showing a valve, v. ; distal to the branch, br. Bundles 

 of smooth muscle fibers are seen at m. f. 



