STRUCTURE OF LYMPHATIC AND LACTEAL VESSELS 249 



In all but the capillary lymphatics, although the walls are very thin, 

 three distinct coats can be distinguished. The internal coat consists 

 of an elastic membrane lined with oblong endothelial cells. This coat 

 readily gives way when the vessels are forcibly distended. The middle 

 coat is composed of longitudinal fibres of connective tissue, with delicate 

 elastic fibres, and non-striated muscular fibres arranged transversely. 

 The external coat is composed of the same structures as the middle coat, 

 but most of the fibres are arranged longitudinally. In this coat the 

 muscular fibres do not form a continuous sheet, but are collected into 

 separate fasciculi, which have a direction either longi- 

 tudinal or oblique. The fibres of connective tissue are 

 abundant and unite the vessels to the surrounding parts. 

 The internal and the middle coats are closely adherent 

 to each other; but the external coat may readily be 

 separated from the others. Bloodvessels have been 

 found in the walls of the lymphatics, and the existence 

 of vasomotor nerves is probable. 



The walls of the lymphatic vessels are closely ad- 

 herent to the surrounding tissues ; so closely, indeed, that 

 even a small portion of a vessel is detached with diffi- 

 culty, and the vessels, even those of large size, can not be 

 followed out and isolated for any considerable distance. 



In all the lymphatic vessels, beginning a short dis- 

 tance from their plexuses of origin, are semilunar valves, 

 usually arranged in pairs with their concavities looking 

 toward the larger trunks. These folds are formed of 

 the middle and inner coats; but the fold formed from 

 the lining membrane is by far the wider, so that the free p . 

 edges of the valves are thinner than the portion attached of the lymphatics 

 to the vessel. The valves are most abundant in the ( 

 superficial vessels. The distance between the valves is ^ to | of an 

 inch (2 to 3 millimeters) near the origin of the vessels, and J to -J- of an 

 inch (6 to 8 millimeters) in their course. In the lymphatics between 

 the muscles the valves are less abundant. They are relatively few in the 

 vessels of the head and neck and in all that have a direction from above 

 downward. Although there are valves in the thoracic duct, they are 

 not so abundant as in the smaller vessels. 



In their anatomy and general properties, the lymphatics bear a close 

 resemblance to the veins. Although much thinner and more trans- 

 parent, their coats have nearly the same arrangement. The arrange- 

 ment of valves is the same ; and in both systems, the folds prevent 

 reflux of liquids when the vessels are subjected to pressure. 



