DIGESTION 145 



present impossible to define, the system can reconstruct its 

 proteins from amides and other simple products of their 

 degradation. 



The animal economy receives, and after due digestive pre- 

 paration absorbs, three classes of food nitrogenous, fatty, and 

 carbohydrate. If either of the two latter kinds be deficient 

 in the diet, the body can to a certain extent produce it from 

 the other two. What is the special value of each kind of food ? 

 What use is made of it ? Before attempting to answer these 

 questions, we must endeavour to trace the further history of 

 the foods after they have traversed the wall of the alimentary 

 canal. 



After leaving the stomach and intestines, the foods follow 

 two different routes. Proteins and carbohydrates are carried 

 by the portal vein to the liver. Fats are carried by the 

 thoracic duct to the general circulation. An excess of fat 

 is found in the blood in all parts of the body after a meal rich 

 in fat. The eventual destination and fate of fatty foods is 

 unknown. Under certain circumstances they are added to the 

 fatty deposits in connective tissue ; but if no additional fat is 

 being laid down, they go to other tissues, in which they are oxi- 

 dized into carbonic acid and water. When the amount absorbed 

 is excessive, a certain quantity of fat may be stored in the liver. 

 In the cells of this organ it is housed for a time, in order that 

 it may be distributed to the tissues after they have used 

 up the supplies which first reach them through the general 

 blood-stream. 



Proteins are completely lost to sight after they are ab- 

 sorbed into the blood. They take part, of course, in the 

 formation of growing tissue, blood-corpuscles, skin, hair, nails. 

 It is also common to speak of them as making good the wear 

 and tear of active tissues, although it is very doubtful whether 

 we can legitimately speak of the wear and tear of tissues. 

 The protoplasm which does the work of the body is not worn 

 out in the same way as the materials of which a machine is 

 made. There is no friction to rub it down. Proteins, like 

 other foods, are used up as sources of muscular energy and 

 heat. Eventually they are reduced to urea, carbonic acid, 

 and water. Chemists naturally seek for substances inter- 

 mediate in constitution between proteins and urea. They 



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