150 THE HUMAN MECHANISM 



13. Massage. The action of massage is only another illus- 

 tration of the same principle. By rubbing the legs and arms 

 in the direction of the heart, the blood contained in their 

 veins is forced onward and the circulation aided, precisely 

 as when a muscle contracts or one member of a limb is 

 flexed upon another. 



14. The lymphatics. Important as are the suction action 

 of the breathing movements and the pumping action of con- 

 tracting muscles as aids to the circulation of the blood, they 

 are even more important as causes of the flow of lymph along 

 the greater lymphatic trunks toward the heart. Reference to 

 the general method of origin of lymphatics, as described in 

 Chapter III, will show that the lymph in the lymph spaces, 

 unlike the blood in the capillaries, has not behind it a high- 

 pressure reservoir ; there is no such force from behind to send 

 it onward, since the lymphatics arise blindly in the tissues. 

 What, then, makes the lymph flow along the lymphatics 

 toward the heart? 



The lymphatics resemble the veins in structure, having 

 thin walls and pocket valves; like the veins, most of them 

 originate in extrathoracic organs and join or combine to 

 form larger trunks as they proceed toward the thorax. All 

 of them finally unite in two large lymphatics within the 

 thoracic cavity, and these open into the great veins near the 

 heart. (Figs. 30 and 70 should be consulted in this con- 

 nection.) It is at once clear that the breathing movements 

 must exert on the lymph within these thin-walled vessels 

 exactly the same suction action as they exert on the blood 

 in the veins, and anything which increases this suction action, 

 such as the deepened breathing movements during muscular 

 activity, must necessarily increase the flow of lymph from 

 every organ of the body. On the other hand, a pumping 

 action on the lymph in the organs results from all rhythmic 

 movements of parts of the body with reference to one another, 

 since each change of position carries with it some change of 



