THE CIRCULATION 127 



The Movement of the Lymph. Just as we find veins 

 returning from every part of the body, we can make out, 

 though with much greater difficulty, small vessels bringing 

 lymph in the same general direction, that is, toward the 

 thorax. Like the veins, they unite as they draw near the 

 heart, and the great majority eventually contribute to a 

 trunk known as the thoracic duct. This springs from the 

 union of many branches in the abdominal cavity, pierces 

 the diaphragm, and can be traced upward in front of 

 the spinal column until it empties into a great vein which 

 is bringing the blood from the left shoulder toward the 

 right auricle. A comparatively insignificant group of 

 lymphatics centers at an outlet in the corresponding posi- 

 tion on the right. 



The onward movement of the lymph in its channels is 

 parallel with that of the venous blood, but is incomparably 

 slower. It is sometimes almost entirely arrested. In the 

 last chapter it was stated that we cannot confidently say 

 whether the lymphatics drain all the microscopic spaces 

 in the tissues or whether they rise in small definite enclos- 

 ures. In either case they are bearing away a certain over- 

 flow of fluid and may be regarded as supplementing the 

 service of the veins. As to the cause of the halting move- 

 ment of their contents, the simplest statement that can 

 be made is somewhat as follows: The formation of new 

 lymph in all the organs crowds away the lymph previously 

 in the beginnings of the lymphatics, and this is the central 

 fact to be considered. The energy required is derived 

 partly from the heart, since liquid may be forced out of 

 the capillaries by its transmitted pressure, and partly from 

 other sources too obscure to be discussed. The lymph 

 is generally referred to as a carrier of waste, but a partial 

 exception must be made in favor of the lymph coming 

 from the intestinal area during digestion. This lymph 

 may contain absorbed food, principally fat. It was in the 

 mesentery that lymphatics distended with milky liquid 

 were first seen. They were called lacteals because of their 

 appearance, and this term is still used in a local sense. 



