THE DIGESTIVE APPARATUS AND NUTRITION 



342. The second agent of absorption is the lymph cells 

 in the tissues of the walls of the alimentary canal. It is 

 chiefly by the lymphatics that the fat of the food gets into 

 the circulation. Two changes are effected in the fats by 

 the digestive juices in the small intestine to prepare them 

 for absorption by the lymphatic vessels, which are in the 

 intestines called lacteals, because the presence of fat ren- 

 ders the fluid they contain milky. First, by the mixture 

 of the bile and pancreatic juice with the food an emulsion 

 is formed ; that is, the fat is broken up into minute 

 particles, which float in the liquid, giving it a milky 

 appearance. Secondly, a chemical union takes place 

 between some of the acid fats and the alkaline secretions 

 of the digestive organs, a process called saponification, 

 which is the making of soap. The digested food in its 

 milky form is called chyle, and this is collected in larger and 

 larger tubes, and finally poured by the great thoracic duct 

 into the blood of the jugular vein. Being then carried to 

 the right auricle of the heart, it passes to the right ven- 

 tricle and thence to the lungs, in whose capillaries it 

 meets with the oxygen from the air, and having returned 

 to the left side of the heart is distributed throughout the 

 system. 



343. The Lymphatic Vessels are delicate tubes which 

 drain the spaces between the cells of the tissues, gradu- 

 ally uniting to form the main lymphatic vessels, of which 

 the thoracic duct is the largest. These vessels are sup- 

 plied with numerous valves, like those of the veins, to 

 prevent the reflow of their contents, and the opening of 

 the thoracic duct is guarded by a valve. The flow of the 

 fluid is kept up by the pressure in the capillaries, which 

 is greater than is the pressure in the veins into which it 

 is emptied, and by the movements of the body, which 



MACY'S PHYS. 15 



