ABSOEPTfON. 303 



bundles dilate, and lymph is sucked from the peritoneal cavity, through 

 the widely open stomata, into the interfascicular lymphatics. During 

 expiration, the spaces between the concentric tendon bundles dilate, and 

 the lymph is squeezed into the lymphatics towards the pleura! surface 

 (Klein). It thus appears probable that during health there is a con- 

 tinued sucking in of lymph from the peritoneum into the lymphatics 

 by the " pumping " action of the diaphragm; and there is doubtless an 

 equally continuous exudation of fluid from the general serous surface of 

 the peritoneum. When this balance of transudation and absorption is 

 disturbed either by increased transudation or some impediment to absorp- 

 tion, an accumulation of fluid necessarily take place (ascites). 



Stomata have been found in the pleura; and as they maybe presumed 

 to exist in other serous membranes, it would seem as if the serous cavi- 

 ties, hitherto supposed closed, form but a large lymph-sinus or widening 

 out, so to speak, of the lymph-capillary system with which they directly 

 communicate. 



FIG. 219. Peritoneal surface of septum cisternae lymphatic magnae of frog. The storaata, 

 some of which are open, some collapsed, are surrounded by germinating endothelium. x 160. 

 (Klein.) 



Structure of Lymphatic Vessels. The larger vessels are very like 

 veins, having an external coat of fibre-cellular tissue, with elastic fila- 

 ments; within this, a thin layer of fibro-cellular tissue, with plain 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 epithe- 

 lium; 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 at- 

 tached, give them a peculiar beaded or knotted appearance. 



Current of the Lymph. With the help of the valvular mechanism 

 (1) all occasional pressure on the exterior of the lymphatic and lacteal 

 vessels propels the lymph towards the heart: thus muscular and other 



