220 TEXT-BOOK OF PHYSIOLOGY. 



Routes for the Absorbed Food. Physiologic experiments have dem- 

 onstrated that the agents concerned in the removal of the products of 

 digestion after their absorption from the interior of the villus are : 



1. The veins of the gastro-intestinal tract, which converge to form the 



portal vein. 



2. The lymph- vessels of the small intestine, which converge to empty into 



the thoracic duct. 



The products of digestion find their way into the general circulation by 

 these two routes, as follows: (See Fig. 94). 



The water, inorganic salts, proteins, and sugar after entering the blood- 

 vessels of the villus are carried by the blood of the intestinal veins directly 

 into the liver by the portal vein; after circulating through the capillaries of 

 the liver and being influenced by the liver cells, they are discharged by the 

 hepatic veins into the inferior or ascending vena cava. 



The fats after entering the lymph-radicle of the villus are carried by the 

 lymph-stream of the intestinal lymph-vessels and emptied into the recep- 

 taculum chyli from which they ascend into the thoracic duct, by which 

 they are discharged into the blood at the junction of the left subclavian and 

 internal jugular veins. 



Forces Aiding the Movement of Lymph and Chyle. The force 

 which primarily determines the movement of the lymph has its origin in the 

 beginnings of the lymph-vessels, the lymph-spaces, and depends on a dif- 

 ference in pressure here and at the termination of the thoracic duct. The 

 rise of pressure in the lymph-spaces is due to the continual production of 

 lymph, either by filtration or secretor activity of the capillary walls. As 

 soon as the pressure rises above that in the thoracic duct a forward move- 

 ment of lymph takes place. Other things being equal, the rate of move- 

 ment will be proportional to the difference of pressure. The first movement 

 of the chyle, its passage from the lymph-capillary in the villus into the sub- 

 jacent lymph- vessel, has been attributed to a shortening of the villus and a 

 compression of the capillary by the contraction of the non-striated muscle- 

 fibers by which it is surrounded. With the entrance of the chyle into the 

 subjacent lymph- vessel there is a distention of the vessel and a rise in pres- 

 sure. When the muscle-fibers relax, regurgitation is prevented by the 

 closure of the valves at the base of the villus. The elastic tissue of the 

 lymph-vessel now recoils and forces the chyle toward the thoracic duct. 

 After the emptying of the lymph-capillary the conditions as far as pressure 

 is concerned are favorable for the absorption of new material. The rhythmic 

 contractions of the intestinal wall undoubtedly aid in the movement of lymph 

 and chyle. It is quite possible that the walls of the general lymphatic system 

 aid the forward movement of lymph by more or less rhythmic contractions 

 of their contained muscle-fibers. 



Inasmuch as the lymph-vessels lie in situations in which they are sub- 

 ject to compression by muscles during contraction, it is probable that the 

 fluid in the vessels will be forced onward toward the thoracic duct at each 

 compression, a backward movement being prevented by the closure of the 

 valves which are everywhere present in the vessels. Experimental observa- 

 tions have demonstrated the truth of this supposition. Alternate contraction 

 and relaxation of the muscles of the leg will, in an animal at least, increase 



