44 THE BODY AT WORK 



left side of the neck and of the left arm just where they join 

 together. 



The thoracic duct provides for the overflow of lymph from 

 the spaces of the body. There is no circulation of lymph. 

 Lymph from the liver and from the intestines is constantly 

 draining into the thoracic duct, and thus returning to the 

 blood-stream by a short direct route, entering it without the 

 necessity for reabsorption through the walls of capillary 

 vessels. By no means all of this fluid has exuded from the 

 blood-stream. Much of it is water which was poured into the 

 stomach as gastric juice, and into the intestines as the secre- 

 tions of the pancreas and other glands, or imbibed through 

 the mouth and absorbed by the lymphatics of the alimentary 

 canal. The remainder of the water taken up from the ali- 

 mentary canal enters its bloodvessels. The diluted blood 

 flows to the liver, loaded with digested products which the 

 liver will store. As the blood parts with them the additional 

 water which has served for their transport exudes from the 

 capillaries of the liver into lymphatics, which empty it into 

 the thoracic duct. Large quantities of water are used in 

 washing out digested products. Secreted into the alimentary 

 canal by the digestive glands, it passes out through its wall as 

 the vehicle of digested products. Collected by lymphatic 

 vessels, it is either carried directly into the thoracic duct, or 

 passed from lymph into blood, carried by blood to the liver, 

 again transferred from blood to lymph, and borne by the 

 lymphatic vessels of the liver to the thoracic duct. 



Water exuded from blood into lymph may be reabsorbed into 

 the blood near the place where it was poured out, or it may 

 reach the blood via the thoracic duct. It would seem that 

 the former is the natural, the latter the emergency route ; 

 the former the course taken when an organ is tranquil, the 

 latter a necessity when the organ is active. If the large 

 lymphatic vessels of a limb are cut, no lymph escapes from 

 them so long as the limb is at rest. When the muscles contract 

 lymph begins to flow. If the limb is flexed and extended by 

 hand, lymph flows. If the muscles are squeezed or massaged, 

 lymph flows. As the flow is set up both by active contraction 

 of the muscles and by passive movements in which the muscles 

 do not take part, it clearly must be due to external pressure 



