ANATOMY OF LYMPHATIC AND LACTEAL VESSELS 



243 



are not the result of infiltration in the tissue. It does not appear that 

 the vessels composing this plexus vary much in size. They are quite 

 elastic, and after distention by injection, they return to a very small 

 diameter when the fluid is allowed to escape. 



The lacteal system presents essentially the same anatomical charac- 

 ters as the general lymphatics, and the vessels are filled with colorless 

 lymph during the intervals of digestion. In many situations the lym- 

 phatics present in their course little solid structures, called lymphatic 

 glands, although, as regards structure and office, they are not true 

 glandular organs. The smallest capillary lymphatics have a diameter 

 of about Z^-Q of an inch (83 ft). This may be taken as their average 

 diameter in primitive plexuses. The plexus, when the vessels are 

 abundant, as they are in certain parts of the cutaneous surface, resem- 

 bles an ordinary plexus 

 of capillary bloodvessels, 

 except that the walls of 

 the lymphatics are thin- 

 ner and their diameter is 

 greater. The vessels are 

 lined by endothelial cells, 

 the borders of which are 

 brought into view by the 

 action of silver nitrate. 



The smallest lym- 

 phatic vessels are by far 

 the most abundant. They 

 are arranged in the form 

 of a fine plexus, superficially situated in the skin. A second plexus 

 exists just beneath the skin, composed of vessels of much greater 

 diameter. The skin is thus enclosed between two plexuses of capillary 

 lymphatics. A plexus analogous to the superficial plexus of the skin 

 is found just beneath the surface of the mucous membranes. These 

 may, indeed, be classed with the superficial lymphatics. The deep 

 lymphatics are much larger and less abundant, and their origin is less 

 easily made out. These accompany the deeper veins in their course. 

 They receive the lymph from the superficial vessels. 



No valvular arrangement is found in the smallest lymphatics ; but 

 the vessels coming from the primitive plexuses, as well as the large 

 vessels, contain valves in great numbers. These valves, being closely 

 set in the vessels, give to them, when filled with injection, a peculiar 

 and characteristic beaded appearance. 



The course of the lymphatics usually is direct. As they pass toward 



Fig. 51. Lymphatic plexus, showing the endothelium 

 (Belaieff). 



