304 ABSORPTION-LYMPH AND CHYLE. 



act of expiration ; and an impulse synchronous with the pulsations of the 

 heart has frequently been observed. 



The fact that the lymphatic system is never distended, and the existence 

 of the valves, by which different portions may become isolated, render it im- 

 possible to estimate the general pressure of fluid in these vessels. This is 

 undoubtedly subject to great variations in the same vessels at different times, 

 as well as in different parts of the lymphatic system. It is well known, for 

 example, that the degree of distention of the thoracic duct is very variable, 

 its capacity not infrequently being many times increased during active ab- 

 sorption. At the same time it is difficult to attach a manometer to any part 

 of the lymphatic system without seriously obstructing the circulation and 

 consequently exaggerating the normal pressure ; but the force with which 

 liquids penetrate these vessels is very great. This is illustrated by the ex- 

 periment of tying the thoracic duct ; for after this operation, unless com- 

 municating vessels exist by which the fluids can be discharged into the 

 venous system, their accumulation is frequently sufficient to rupture the 

 vessel. 



The general rapidity of the current in the lymphatic vessels has never 

 been accurately estimated. As a natural consequence of the variations in the 

 distention of these vessels, the rapidity of the circulation must be subject to 

 constant modifications. Beclard, making his calculation from the experi- 

 ments of Colin, who noted the quantity of fluid discharged in a given time 

 from fistulous openings into the thoracic duct, estimated that the rapidity of 

 the flow in this vessel was about one inch (25-4 mm.) per second. This esti- 

 mate, however, can be only approximate ; and it is evident that the flow 

 must be much less rapid in the vessels near the periphery than in the large 

 trunks, as the liquid moves in -a space which becomes rapidly contracted as it 

 approaches the openings into the venous system. 



Various influences combine to produce the movements of fluids in the 

 lymphatic system, some being constant in their operation, and others, inter- 

 mittent or occasional. These will be considered, as nearly as possible, in the 

 order of their relative importance. 



The forces of endosmosis and transudation are undoubtedly the main 

 causes of the lymphatic circulation, more or less modified, however, by influ- 

 ences which may accelerate or retard the current ; but this action is capable 

 in itself of producing the regular movement of the lymph and chyle. It is 

 a force which is in constant operation, as is seen in cases of ligation of the 

 thoracic duct, a procedure which must finally abolish all other forces which 

 aid in producing the lymphatic circulation. When the receptaculum chyli 

 is ruptured as a consequence of obstruction of the thoracic duct, the vessel 

 gives way as the result of the constant endosmotic action, in the same way 

 that the exposed membranes of an egg may be ruptured by endosmosis, when 

 immersed in water. 



The situations in which the endosmotic force originates are at the periph- 

 ery, where the single wall of the vessels is very thin, and where the extent of 

 absorbing surface is large. If liquids can penetrate with such rapidity and 



