PHYSIOLOGICAL 345 



liver, without being chemically changed in any way. The fatty 

 acids and glycerol absorbed, however, have a very different fate; 

 they immediately combine with one another to form fats in the 

 lining cells of the intestinal wall, from which they are discharged, 

 not into the blood stream, but into special channels (lacteals) in the 

 form of a milky emulsion (chyle). These lacteals lead to the thoracic 

 duct and finally are connected with the blood-stream at a point near 

 the left shoulder in man. 



4. Intermediate Metabolism. — Of the amino-acids absorbed 

 from the intestine, a large part is used by the body as a source of 

 energy, while a smaller part is built up into proteins. Very little is 

 known of this latter process, but a short digression on the nature of 

 proteins and their occurrence in the body may be of service here. 

 Emil Fischer showed conclusively that the essential feature of the 

 structure of the enormous protein molecule was a long chain of 

 amino-acids, such as — 



Glycine CH^ . NH,— COOH 

 Alanine CH3— CH . NH,— COOH 

 Tryptophane C CH^ . CH . NH^ 



/\ I 



CeH^ CH COOH 



NH 

 Tyrosine CgH^ . CH— CH,— CH . NH — COOH 



Cysteine CH, . SH— CH . NH,— COOH 



and so on. When these are united to form chains in a protein mole- 

 cule, each amino-acid loses the elements of water, one hydrogen 

 atom from its — NH, (amino) group, and one hydrogen and one 

 oxygen from its — COOH (carboxyl) group, so that a glycine 

 molecule within a protein has the structure 



CH,.NH-- 



— CO 



being united to other amino-acids on either hand. Hence when 

 proteins are digested, for each molecule of amino-acid set free one 

 molecule of water must be incorporated into its structure; such a 

 process of splitting is called hydrolysis. 



Many of the proteins of the body appear to consist solely of 

 amino-acids, such as the albumin and globulin of the blood serum 

 and the elastin of elastic fibres. But the proteins very often unite 

 with other types of substance, and examples of such "conjugated" 

 proteins are the red haemoglobin of the blood, the casein of milk, 

 and the vitellin of egg-yolk, which contain phosphorus, mucin (used 

 throughout the body as a lubricating agent), which contains sugar, 



