282 ABSORPTION. 



the walls of the blood-capillaries and small veins (p. 138), and 

 could pass with still less trouble, probably, through the com- 

 paratively ill-defined w r alls of the capillaries which contain 

 lymph. 



Observations of Recklinghausen have led to the discovery 

 that in certain parts of the body openings exist by which lym- 

 phatic capillaries directly communicate with parts hitherto 

 supposed to be closed cavities. If the peritoneal cavity be in- 

 jected with milk, an injection is obtained of the plexus of lym- 

 phatic vessels of the central tendon of the diaphragm ; and on 

 removing a small portion of the central tendon, with its peri- 

 toneal surface uninjured, and examining the process of absorp- 

 tion under the microscope, 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 out- 

 line, were only wide enough to admit two or three milk-glob- 

 ules 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 widening 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 exter- 

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

 this, a thin layer of fibro-cellular tissue, with organic muscu- 

 lar fibres, which have, principally, 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 vessels 

 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. 143). The 

 actions of the muscular fibres of the small intestine, and 

 probably the layer of organic muscle present in each intesti- 

 nal villus (p. 246), seem to assist in propelling the chyle : for, 



