OP THE LYMPHATIC VESSELS. 309 



467. The surfaces of the lymphatic vessels, like those of 

 all the vessels, are the one cellular and adherent, the other 

 smooth and free: the latter presents a multitude of valves. 



These valves, which are of a semilunar or parabolic form, 

 are mostly arranged in pairs, and are large enough to close 

 tho vessel completely. They are generally placed at unequal 

 intervals, excepting in the vessels of the testicles, where they 

 occur nearly every line, which gives them more than any 

 other the appearance of a chaplet. They are more or less 

 close, according to the parts, without their being more parti- 

 cularly so in the branches than in the twigs; in certain vessels 

 there occurs spaces of several inches without valves: the 

 thoracic duct is especially remarkable in this respect. In 

 some points the insertion of a small vessel in a larger one is 

 only furnished with a single valve. In some places of the 

 trunks annular valves, that do not entirely close the canal, are 

 found. The insertion of the trunks into the subclavian veins 

 is furnished with a double valve, which effectually prevents 

 the reflux of the blood from entering into their cavity. All 

 these valves, like those of the veins and arteries, are formed 

 of a duplicature of the inner membrane. 



468. Lymphatic vessels are formed of two membranes, 

 very distinct in their principal trunks. 



The external, cellular and unequal or exterior is united to 

 the surrounding cellular tissue, which invests it with a sheath; 

 more deeply, it is distinctly fibrous or filamentous: it is even 

 supposed that muscular fibres have been observed in it. The 

 inner membrane is very thin. 



Small sanguineous vessels, arteries and veins, have been fol- 

 lowed up into the thickness of the outer membrane; some say 

 that they have also seen in it lymphatic vessels. No one has 

 ever been able to perceives nerves in them. 



469. The parietes of the lymphatic vessels, although very 

 thin and transparent, are dense and very resistant, much more 

 so than those of the veins, taking into consideration the differ- 

 ence of their thickness. Nevertheless, these vessels are exten- 

 sible, and also very retractile. Elasticity is manifest in them: 



