70 THE VITAL PROCESSES 



to supply the pressure necessary for moving the lymph. 

 There are, however, several forces that indirectly aid in its 

 flow. The most important of these are as follows: 



1. Blood Pressure at the Capillaries. The plasma 

 which is forced through the capillary walls by pressure 

 from the heart makes room for itself by pushing a portion 

 of the lymph out of the lymph spaces. This in turn 

 presses upon the lymph in the tubes which it enters. In 

 this way pressure from the heart is transmitted to the 

 lymph, forcing it to move. 



2. Variable Pressure on the Walls of the Lymph Ves- 

 sels. Pressure exerted on the sides of the lymph tubes by 



contracting muscles tends to close them 

 up and to push the lymph past the 

 valves, which, by closing, prevent its 

 return (Fig. 29). Pressure at the sur- 

 face of the body, provided that it is 

 variable, also forces the lymph along. 

 The valves in the lymph vessels serve 

 1 A |L1 f B the same purpose as those in the veins. 

 FIG. 29.- Diagram ^ Thg I ns pi ratory /w^,_ Wheri 

 to show how the muscles , , . . . . , . , 



pump lymph. A. Re- the thoracic cavlt y ls enlarged in breath- 



laxed muscle beside ing, the unbalanced atmospheric pres- 



which is a lymphatic sure is exerted from all directions 



tube A Same muscle towards the thoracic space. This not 



in state of contraction. 



only causes the air to flow into the lungs 

 (Chapter VII), but also causes a movement of the blood 

 and lymph in such of their tubes as enter this cavity. It 

 will be noted that both of the large lymph ducts terminate 

 where their contents may be influenced by the respiratory 

 movements. (See Practical Work.) 



Where the Lymph enters the Blood. The fact that the lymph is 

 poured into the blood at but two places, and these very close to each 



