438 ABSORPTION. 



In all the lymphatic vessels, beginning a short distance 

 from their plexus of origin, are found numerous semilunar 

 valves, generally arranged in pairs, with their concavities 

 looking toward the larger trunks. These folds are formed 

 of -the two inner coats; but the fold formed of the lining 

 membrane is by far the wider, so that the free edges of the 

 valves are considerably thinner than that portion which is 

 attached directly to the vessel. In some of the vessels, at 

 the point where one lymphatic communicates with another, 

 there is a valve formed of two folds, one of which is much 

 wider than the other but in the valves situated in the course 

 of the vessels, the curtains are of equal size. The valves are 

 very numerous in all of the lymphatics ; but they are most 

 abundant in the superficial vessels. Sappey counted from 

 sixty to eighty in the vessels of the arm, from the fingers 

 to the axillary glands, and from eighty to one hundred in 

 the long vessels of the lower extremities. 1 The distance 

 between the valves is from one-twelfth to one-eighth of an 

 inch, near the origin of the vessels, and from one-quarter to 

 one-third of an inch, in their course. In the lymphatics situ- 

 ated between the muscles, the valves are less numerous. 

 They are always relatively few in the vessels of the head and 

 neck and in all that have a direction from above downward. 

 Though there are a number of valves in the thoracic duct, 

 they are not so numerous here as in the smaller vessels. 



In their anatomy and general properties, the lymphatics 

 bear a close resemblance to the veins. Though much thinner 

 and more transparent, their coats have nearly the same ar- 

 rangement. The arrangement of valves is entirely the same ; 

 and in both systems, the folds prevent the reflux of fluids when 

 the vessels are subjected to pressure. A number of forces 

 (which will be considered hereafter) combine to produce the 

 flow of lymph and chyle in the absorbent system. Among 

 these is intermittent pressure from surrounding parts, which 



1 Op. cit., tome i., p. 618. 



