

MOVEMENT OF THE LYMPH. 209 



to the veins. Along their entire extent these vessels are subject 

 to certain forces which must materially aid the flow of the lymph 

 stream. The first of these is the pressure exerted on the small 

 vessels by the movement of the muscles in the neighborhood. 

 The second is the unequal distribution of atmospheric pressure, 

 which has full force on the peripheral channels, but is kept off 

 the thoracic duct and its termination, as already mentioned, by 

 the rigidity of the thoracic wall, which, together with the tendency 

 of the elastic lungs to shrink, causes a permanent negative pressure 

 in the thoracic cavity through which the duct passes. And lastly, 

 the thin-walled lymphatics are everywhere surrounded with very 

 elastic textures, inclosed in an elastic skin, which exert an amount 

 of pressure, sufficient to empty and press together the walls of the 

 vessels after death, and therefore during life must have consider- 

 able influence upon the fluid they contain. 



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. 



18 



