ABSORPTION 223 



at once enter the lymph-radicle and become constituents of the lymph- 

 stream, to which they impart a white, milky appearance. If the abdomen 

 of an animal in full digestion be opened, the lymph-vessels of the mesentery 

 present themselves as distinct white threads. An examination of the fluid 

 they contain, known as chyle, shows the presence of fat-granules of micro- 

 scopic size. With the passage of the chyle into the thoracic duct it also 

 assumes the same milky appearance. For this reason the lymphatics of 

 the mesentery were erroneously termed lacteals. The chyle as obtained 

 from these lymph-vessels possesses the same qualitative though not quanti- 

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

 of fat in the former. Indeed, chyle may be regarded as lymph with the 

 addition of fat. 



With the absorption of the last of the fat-granules the contents of the 

 lymph-vessels of the mesentery the lacteals begin to lose their white 

 appearance near the intestine and as the fat-granules pass toward the 

 thoracic duct, the lymph-vessels gradually become transparent and disappear 

 from view. A similar change in the appearance of the thoracic duct ensues, 

 as the last portions of the fat ascend the duct to be discharged into the blood- 

 stream. 



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. 88.) 



The water, inorganic salts, proteins or amino-acids, and sugar after 

 entering the blood-vessels of the villus are carried by the blood of the in- 

 testinal 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 fat-granules, synthesized in the epithelial cells, after entering the 

 lymph-radicle of the villus are carried by the lymph-stream of the intestinal 

 lymph-vessels and emptied into the receptaculum 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 



