338 ABSORPTION. 



with the great veins. But we have seen that the lymph is not derived entirely from 

 the blood, a considerable portion resulting from interstitial absorption in the general lym- 

 phatic system and from the absorption of certain nutritive matters by the chyliferous 

 vessels. These are, physiologically, the most important constituents of the lymph and 

 chyle ; and they are taken up simply to be carried to the blood and do not pass again 

 from the general vascular system into the lymphatics. 



As far as the mode of origin of the lymph and chyle has any bearing upon the move- 

 ments of these fluids in the lymphatic vessels, there is no difference between the imbibi- 

 tion of new materials from the tissues or from the intestinal canal, and the transudation 

 of the liquid portions of the blood ; for the mechanism of the passage of liquids from the 

 blood-vessels is such that the motive power of the blood cannot be felt. An illustration 

 of this is in the mechanism of the transudation of the liquid portions of the secretions. 

 The force with which fluids are discharged into the ducts of the glands is enormous and 

 is independent of the action of the heart, being due entirely to the force of transudation 

 and secretion. This is combined with the force of imbibition, and with it forms one of 

 the important agents in the movements of the lymph and chyle. These movements are 

 studied with great difficulty. One of the first peculiarities to be observed is, that, under 

 normal conditions, the vessels are seldom distended, and the quantity of fluid which they 

 contain is subject to considerable variation. As far as the flow in the vessels of medium 

 size is concerned, the movement is probably continuous, subject only to certain moment- 

 ary obstructions or accelerations from various causes. But, in the large vessels situated 

 near the thorax and in those within the chest, the movements are in a marked degree 

 remittent, or they may even be intermittent. All experimenters who have observed the 

 flow of lymph or chyle from a fistula into the thoracic duct have noted a constant 

 acceleration with each act of expiration, and an impulse synchronous with the pulsa- 

 tions of the heart has been frequently observed. 



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

 numerous valves by which different portions may become isolated, render it impossible 

 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 amount of distention of the 

 thoracic duct is exceedingly variable, its capacity not infrequently being many times 

 increased during active absorption. 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 pene- 

 trate these vessels is very great. This is illustrated by the experiment of ligating the 

 thoracic duct ; for, after this operation, unless communicating vessels exist by which the 

 fluids can be discharged into the venous system, their accumulation is frequently suffi- 

 cient to rupture the vessel. 



The general rapidity of the current in the lymphatic vessels has never been accurate- 

 ly estimated. As a natural consequence of the variations in the distention of these ves- 

 sels, the rapidity of the circulation must be subject to constant modifications. Beclard, 

 making his calculation from the experiments of Colin, who noted the quantity of fluid 

 discharged in a given time from fistulous openings into the thoracic duct, estimates that 

 the rapidity of the flow in this vessel is about one inch per second. This estimate, how- 

 ever, can be only approximative ; 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. 



Causes of the Movements of the Lymph and Chyle. 



Various influences combine to produce the movements of fluids in the lymphatic sys- 

 tem, some being constant in their operation, and others, intermittent or occasional. 

 These will be considered, as nearly as possible, in the order of their relative importance. 



