THE VASCULAR MECHANISM. 243 



opens into the great veins is at most 2 to 3 mm. Hg, and may be negative. 

 Other and more important factors are muscular and respiratory move- 

 ments. Every muscular movement, by compressing the lymphatic 

 vessels, forces the lymph on towards the thoracic duct. With each 

 inspiration, the abdominal pressure rises and the intestinal lymphatic 

 vessels are compressed ; at the same time, the pressure on the thoracic 

 duct becomes negative, and lymph is sucked into the chest. The effect 

 of these movements is assisted by the presence of valves in the larger 

 lymphatic vessels, which prevent any backward flow of lymph. 



