STRUCTURE OF LYMPHATIC VESSELS. 353 



^examining the process of absorption under the micro- 

 scope, Recklinghausen noticed that the milk-globules ran 

 towards small natural openings or stomata between the 

 epithelial cells, and disappeared by passing vortex-like 

 through them. The stomata, which had a roundish outline, 

 were only wide enough to admit two .or three milk-globules 

 abreast, and never exceeded the size of an epithelial cell. 

 Openings of a similar kind have been found by Dybskowsky 

 in the pleura ; and as they may be presumed to exist in 

 other serous membranes, it would seem as if the serous 

 cavities, hitherto supposed closed, form but a large widen- 

 ing out, so to speak, of the lymph-capillary system with 

 which they directly communicate. 



In structure, the medium-sized and larger lymphatic 

 vessels are very like veins ; having, according to Kolliker, 

 an external coat of fibro-cellular tissue, with elastic 

 filaments; within this, a thin layer of nbro-cellular 

 tissue, with organic muscular fibres, which have, princi- 

 pally, a circular direction, 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 appearance (fig. 99). 



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. 1 70). 

 The actions of the muscular fibres of the small intestine, 

 and probably the layer of organic muscle present in each 

 intestinal villus (p. 307), seem to assist in propelling the 



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