92 ELEMENTARY PHYSIOLOGY less, ii 



The cause of this flow is not so immediately apparent as it 

 is in the case of the blood, for the lymphatic system pos- 

 sesses no central pump, such as the heart, to keep the 

 lymph in motion.' In the absence then of any obviously 

 propulsive mechanism, to what may we attribute this 

 continual passage of lymph along the lymphatics ? 



The flow is in reality brought about by several causes. 

 We may point out in the first place that some force, whose 

 nature will be considered in the next Lesson, is at work 

 to determine the initial exit of fluid from the blood-vessels 

 into the lymph-spaces. This force must obviously tend 

 to drive out the lymph already in those spaces, into and 

 along the channels leading from them. Further, as we 

 have seen, the blood-jiressure in the large veins near the 

 heart is very small and is certainly nnich less than it is in 

 the capillaries ; and since the lymphatics originate at the 

 capillaries and discharge their contents into the great 

 veins, this difference of pressure at the two ends of the 

 system must tend to cause an onward flow of lymph. 

 Here also the movements of respiration play a part, for, 

 as will be seen when dealing with respiration, the pressure 

 in the great veins is suddenly diminished at. each inspira- 

 tion and lymph is thus sucked out of the thoracic duct, no 

 reversal of this action being possible at expiration because 

 of the valves guarding the end of the duct. But the one 

 great and potent cause of lymph-flow is the presence, all 

 along the course of the lymphatics, of valves whose action 

 is, however, only brought into play by the movements of 

 the body. As in the veins (p. 36) so in the lymphatic 

 vessels ; when any pressure is applied to their outside the 

 lymph is driven out of the squeezed part, and since the 

 valves only open towards the junction of the thoracic duct 

 with the venous system, the lymph is thereby driven 

 along in the desired direction. 



1 The frog possesses four lymph hearts, placed in two pairs at the 

 upper and lower end of the backbone Their strnctural arrangement is 

 very similar to that of the blood-honrt, and, V)eing rhythmically con- 

 tractile, they pump the lymph into the venous system. 



