LYMPHATIC VESSELS. 



139 



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FIG. 162. 



Connective tissue from the submucous layer of 

 the small intestine of a cat, showing one 

 blood vessel, b. v.; three lymphatic 

 vessels, 1. v. ; and numerous intercellular 

 spaces, i. s. 



two or more. From its development the lymphatic system is a part of 

 the venous system, consisting of endothelial tubes ramifying in connective 

 tissue, anastomosing with each other 

 or ending blindly. Its striking char- 

 acteristic is that it is wholly afferent; it 

 is like a venous system which has no 

 corresponding arteries. The fluid 

 within it is derived from the intercellu- 

 lar tissue fluids. 



The smaller lymphatic vessels 

 may be studied advantageously in sec- 

 tions of the small intestines from animals in which intestinal digestion 

 is in progress. The lymphatics are then dilated. They appear as spaces 

 in the connective tissue (Fig. 162) which are sharply denned, thus contrast- 

 ing with the intercellular 

 spaces. Their distinct lining 

 is due to endothelium, the 

 nuclei of which are often 

 seen. They have the struc- 

 ture of capillaries but are of 

 larger size; blood vessels of 

 similar caliber have thicker 

 walls. The lymphatic ves- 

 sels often appear empty or 



contain a granular coagulum, whereas red blood corpuscles are to be 

 expected in the blood vessels. A structure containing many red corpus- 

 cles may be safely regarded as a blood vessel, but obviously an empty 

 vessel is by no means a lym- 

 phatic. Occasional red cor- 

 puscles find their way into 

 lymphatic vessels. In silver 

 nitrate preparations (Fig. 

 163) the lymphatic endo- 

 thelium is seen to be simi- 

 lar to that of the blood 

 vessels. Valves are numer- 

 ous even in small lymphatic 

 vessels. They are folds of 

 endothelium such as would 



result if the distal part of the vessel were pushed forward into the 

 proximal part. The vessels are often distended on the proximal side of 



FIG. 163. SILVER NITRATE PREPARATION OF A LYMPHATIC 

 VESSEL FROM A RABBIT'S MESENTERY, SHOWING THE 

 BOUNDARIES OP THE ENDOTHELIAL CELLS, AND A 

 BULGING JUST BEYOND A VALVE. 



Lymphatic vessel from a section of a human bronchus, show- 

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

 smooth muscle fibers are seen at m. f. 



