416 HANDBOOK OF PHYSIOLOGY. 



jection of the lymphatic vessels of the diaphragm, through the stomata on its 

 peritoneal surface, may readily be obtained if artificial respiration be carried 

 on for about half an hour. In this way it has been found that in the rabbit 

 the lymphatics are arranged between the tendon bundles of the centrum ten- 

 dineum ; and they are hence termed interfascicular. The centrum tendiiieum 

 is coated by endothelium on its pleural and peritoneal surfaces, and its substance 

 consists of tendon bundles arranged in concentric rings toward the pleural 

 side and in radiating bundles toward the peritoneal side. 



The lymphatics of the anterior half of the diaphragm open into those of the 

 anterior mediastinum, while those of the posterior half pass into a lymphatic 

 vessel in the posterior mediastinum, which soon enters the thoracic duct. 

 Both these sets of vessels, and the glands into which they pass, are readily 

 injected by the method above described ; and there can be little doubt that 

 during life the flow of lymph along these channels is chiefly caused by the 

 action of the diaphragm during respiration. As it descends in inspiration, 

 the spaces between the radiating tendon bundles dilate, and lymph is sucked 

 from the peritoneal cavity, through the widely open stomata, into the inter- 

 fascicular lymphatics. During expiration, the spaces between the concentric 

 tendon bundles dilate, and the lymph is squeezed into the lymphatics toward 

 the pleural surface (Klein) . It thus appears probable that during health there 

 is a continued 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 perito- 

 neum. When this balance of transudation and absorption is disturbed either 

 by increased transudation or some impediment to absorption, an accumulation 

 of fluid necessarily takes place (ascites) . 



Structure of Lymphatic Vessels. The larger vessels as before men- 

 tioned are very like veins, having an external coat of areolar tissue, with 

 elastic filaments; within this, a thin layer of areolar tissue, with un- 

 striped muscular 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 toward 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 beaded or knotted 

 appearance. 



The Lymph Flow. 



The flow of the lymph toward the point of its discharge into the veins 

 is brought about by several agencies. With the help of the valvular 

 mechanism (1) all occasional pressure on the exterior of the lymphatic 

 and lacteal vessels propels the lymph onward: thus muscular and other 

 external pressure accelerates the flow of the lymph as it does that of 

 the blood in the veins. The actions of (2) the muscular fibres of tho 



