THE DIGESTIVE SYSTEM. 331 



The protein derivatives are absorbed into the blood-vessels. After 

 a protein meal the lymph which can be collected from the thoracic 

 duct is not increased in amount, nor does it contain an increased 

 quantity of protein. On the other hand, the absorption of a protein 

 digest is not interfered with by ligature of the thoracic duct. It 

 is, however, a matter of considerable difficulty to demonstrate the 

 presence of amino-acids in the blood of the portal vein. Experiments 

 which have been carried out show that a process of deamination takes 

 place in the intestinal wall, whereby part of the amino-acids is oxidised, 

 yielding ammonia and an oxy- or ketofatty acid, which pass into the 

 portal circulation together with unchanged amino-acids. 



The Absorption of the Monosaccharides. The products of carbo- 

 hydrate digestion all belong to the group of monosaccharides and are 

 easily diffusible substances. The chief of these is dextrose, but some 

 fructose is formed by the hydrolysis of cane sugar, and some galactose 

 by that of the sugar of rnilk. All three varieties are absorbed directly 

 into the blood stream. During absorption of a carbohydrate meal 

 more sugar is found in the portal vein than in the hepatic vein. More- 

 over, the absorption of sugar is not accompanied by an increase of 

 that substance in the lymph of the thoracic duct, and it is not inter- 

 fered with by the ligature of that structure. Disaccharides are not 

 absorbed from the intestine. 



The Absorption of Fat. The products of digestion of fat are 

 fatty acid, held in solution by the bile salts, glycerol, and a little soap. 

 The absorption of fat differs from that of amino-acids and dextrose 

 in that it takes place for the most part into the lymphatic system. 

 During the absorption of a fatty meal the lymphatics of the mesentery 

 become filled with a milky fluid called chyle, so that they are easily 

 visible to the naked eye. Chyle collected from the thoracic duct may 

 contain over 6 per cent, of fat. Ligature of the thoracic duct 

 diminishes, but does not entirely abolish fat absorption. The absorbed 

 fat reaches the blood stream by the thoracic duct, and if an animal be 

 bled during fat absorption, the plasma will be found to be milky from 

 the quantity of minute fat globules present in it. A few hours later 

 the plasma is again clear, because the small quantity of fat present 

 in it is held adsorbed by the serum proteins, the remainder having 

 either been oxidised or transferred to the fat depots of the body. 



About 98 per cent, of the fat taken as food is absorbed, but only 60 

 per cent, can be recovered in the chyle. The fate of the remaining 

 40 per cent, is unknown. It can neither be recovered from the blood 

 nor from the thoracic duct, nor does it appear in the faeces. 



The bile salts have an important influence on fat absorption, partly 



