22 8 TEXT-BOOK OF PHYSIOLOGY. 



rhythmic contractions of the intestinal wall undoubtedly aid in the 

 movement of lymph and chyle. 



It is quite possible that the walls of the general lymphatic system 

 aid the forward movement of lymph by more or less rhythmic con- 

 tractions of their contained muscle -fibers. 



Inasmuch as the lymph-vessels lie in situations in which they 

 are subject to compression by muscles during contraction, it is prob- 

 able that the fluid in the vessels will be forced onward toward the 

 thoracic duct at each compression, a backward movement being 

 prevented by the closure of the valves which are everywhere present 

 in the vessels. Experimental observations have demonstrated the 

 truth of this supposition. Alternate contraction and relaxation of 

 the muscles of the leg will, in an animal at least, increase considerably 

 the flow as well as the production of lymph from the thoracic duct. 

 ^Massage has a similar influence. The respiratory movements also 

 aid the How of both lymph and chyleTFom "tBe thoracic :"duct and 

 larger lymph-vessels into the venous blood. During inspiration the 

 negative pressure of the thorax is increased, the increase being pro- 

 portional to the extent of the inspiration. The positive pressure of 

 the air within the lungs on the thoracic structures, venae cavae, thoracic 

 duct, etc., being at the same time diminished, there is an expansion of 

 and a fall of pressure in the thoracic duct and venae cavae. As the 

 lymph in the abdominal portion of the thoracic duct is subjected to 

 the higher intra-abdominal pressure, its contents are forced ener- 

 getically toward the end of the thoracic duct. During expiration 

 the reverse conditions obtain. As the negative pressure diminishes 

 and the positive intrapulmonary pressure increases the upper part of 

 the thoracic duct is compressed and the lymph is forced 'into the 

 subclavian vein at its junction with the internal jugular. Regurgita- 

 tion here is prevented by a closure of the valves. 



