216 MANUAL OF PHYSIOLOGY. 



The movements of the chyle depend on the same forces, with 

 the addition of the power used in the contraction of the villi, 

 which pump the chyle from the lacteal radicles into the network 

 of valved vessels in the submucous tissue. 



The commencement of the thoracic duct is, moreover, placed in 

 the abdominal cavity, and therefore is constantly under the influ- 

 ence of the positive pressure exerted by the abdominal wall on 

 the contained viscera. The rest of the duct is in the thorax, where 

 the pressure is habitually negative, but where certain variations 

 coincident with inspiration and expiration take place, which must 

 aid the onward flow of fluid in a vessel containing valves so 

 closely set. 



