STRUCTURE OF LYMPHATIC VESSELS. 357 



case, yet unproved, the latter mode of origin does not 

 occur. 



In structure, the lymphatic and lacteal vessels are veiy 

 like veins ; having, according to Kolliker, an external 

 coat of fibro-cellular tissue, with elastic filaments ; within 

 this, a thin layer of fibre-cellular tissue, with organic 

 muscular fibres, which have, principally, a circular direc- 

 tion, and are much more abundant in the small than in 

 the larger vessels ; and again, within this, an inner elastic 

 layer of longitudinal fibres, and a lining of epithelium ; 

 and numerous valves. The valves, constructed like those 

 of veins, and with the free edges turned towards the 

 heart, are usually arranged in pairs, and, in the small 

 vessels, are so closely placed, that when the vessels are 

 full, the valves constricting them where their edges are 

 attached, give them a peculiar braided or knotted ap- 

 pearance (fig. 95). 



With the help of the valvular mechanism, all occasional 

 pressure on the exterior of the lymphatic and lacteal ves- 

 sels propels the lymph towards the heart : thus muscular 

 and other external pressure accelerates the flow of the 

 lymph as it does that of the blood in the veins (see p. 181). 

 The actions of the muscular fibres of the small intestine, 

 and probably the layer of organic muscle present in each 

 intestinal villus (p. 315), seem to assist in propelling the 

 chyle : for, in the small intestine of a mouse, Poiseuille 

 saw the chyle moving with intermittent propulsions that 

 appeared to correspond with the peristaltic movements of 

 the intestine. But for the general propulsion of the lymph 

 and chyle, it is probable that, together with the vis a tergo 



vein. d. Radial vein. e. Median vein. /. Ulnar vein. The lymph- 

 atics are represented as lying on the deep fascia. 



f Fig. 93. Superficial lymphatics of right groin and upper part of 

 thigh, i (after Mascagni). I. Upper inguinal glands. 2'. Lower inguinal 

 or femoral glands. 3, 3. Plexus of lymphatics in the course of the 

 long saphenous vein. 



