ABSORBENT SYSTEM OF MAMMALIA. 



507 



and a lining membrane : the latter consists of flat and nucleate 

 epithelial cells, adherent to a reticulate subfibrous membraniform 

 basis : it presents a smooth surface, like that of a serous membrane, 

 to the naked eye. The fibres of the middle tunic affect a circular 

 arrangement, are contractile like other fibres of the f smooth 



C " 



system,' and are also elastic. An outer tunic may be defined by 



394 



395 



Valves of Lymphatic?, a, horse; b, human ; 

 magu. CLXVIII". 



Valves of Lymphatics. 



CLXVIII". 



the longitudinal 



course 



396 



of the fibres of the condensed areolar 

 tissue mainly forming it. In the thoracic duct longitudinal 



V ^J 



fibres of the ' smooth ' kind are distinctly superadded to the outer 

 coat, and a reticulate membrane has been 

 detected between the inner and fibrous tunics. 

 In the present class, the inner tunic is folded 

 to form many and efficient valves, of the 

 ' semilunar ' form, and commonly in pairs, 

 fig. 394, rarely single : it is reflected from 

 the fibrous coat half-way across the area of 

 the vessel and then folds back upon itself to 

 return to the wall, which it continues to line 

 until it forms the next valve. The two layers 

 of the fold firmly adhere, and oifer great re- 

 sistance to any pressure upon their concavity. 

 In figure 395, a shows a side-view, b an 

 oblique, and c an end-view of the usual dis- 

 position of the valves in pairs in distended 

 lymphatics, when their free margins meet and '-- 

 close the area of the vessel to prevent the 

 lymph flowing back. Mr. Lane has figured 

 three varieties in the valves of lymphatics, fig. 

 396, near their entry into the conglobate bodies 

 called ( glands.' In A, one fold, b, was less than the other b, and 

 the margins of the outstretched folds did not meet or perfectly 



B 



Varieties of valves of Lym- 

 phatics. CLXX". 



