THE CIRCULATION 



107 



joins the large vein from the left arm (left subclavian vein) 



(Fig. 99). We said the lymphatics from nearly all over the 



body form the thoracic duct; 



but the lymphatics from the 



right arm and right side of the 



trunk and head form what is 



called the right lymphatic duct, 



which empties into the right 



subclavian vein just where the 



right jugular vein joins the 



latter (see Plates). 



182. What makes the 

 Lymph flow ? Did we not 

 learn that something besides 

 the heart makes the blood 

 flow ? It is the contraction 

 of the muscles and their con- 

 sequent pressure upon the 

 veins. The valves in the 

 veins make this pressure 

 effective by allowing the 

 blood to be forced in only 

 one direction. It is likewise 

 found that the lymphatics 

 have valves, and that they are 

 more abundant than those of 

 the veins. Whenever the 

 muscles contract, the lymph 

 is forced along, and the valves 

 provide that no progress made 

 shall" be lost by any backward 

 movement. Every pressure 

 leaves a part of the lymphatic 

 empty and ready to fill from behind (Fig. 100). Also, if 

 the body is pressed upon or shaken, as when riding a trot- 



FlG. 99. The Thoracic Duct. 



i, part of ribs ; a, receptaculum chyli ; b, 

 trunk of thoracic duct, opening at c into 

 junction of left jugular (/) and left sub- 

 clavian ( g") veins. The connection of these 

 veins with the superior vena cava(A) has 

 been cut across to show the thoracic duct 

 behind it ; d, lymphatic glands in the lum- 

 bar regions. 



