ABSORPTION. 227 



erroneously termed lacteals. The chyle as obtained from these 

 lymph- vessels possesses the same qualitative though not quantitative 

 composition as lymph, the difference being mainly in the large excess 

 of fat in the former. Indeed) ,cJH[te- pay be regarded as lymph plus 



Routes for the Absorbed Food.^Physiologic experiments have 

 demonstrated that the agents concerned in the removal of the products 

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



1. The blood-vessels of the gastro-intestinal tract, which unite 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 circu- 

 lation by these two routes, as follows: 



The water, inorganic salts, proteids, and sugar after entering the 

 blood-vessels of the villus are carried by the blood 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 ascending vena cava. 



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

 carried by the lymph-stream into the thoracic duct, by which they are 

 poured into the blood at the junction of the left subclavian and in- 

 ternal 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 difference in pressure hereand at the termination 

 of the thoracic dUCt. Tne rise ol pressure in ttifi lyillpll-spaces is" 

 due to the continual production of lymph, cither by filtration or 

 secretory activity of the capillary walls. As soon as the pressure 

 rises above that in the thoracic duct a forward movement of lymph 

 takes place. Other things being equal, the rate of movement 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 subjacent 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 pressure. 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 re- 

 coils and forces the chyle toward the thoracic duct. After the empty- 

 ing of the lymph- capillary the conditions as far as pressure is con- 

 cerned are favorable to the absorption of new material. The 



