SECTION XII. 



Nutrition. 



It has been found that the blood is constantly losing portions 

 of its constituents in the formation of the different secretions and 

 excretions of the body and in supplying nutritive substances to the 

 different tissues. The excretions and the substances removed from the 

 blood in supplying the tissues with nutriment are to be regarded as 

 permanent losses to the blood, since in the former case the substances 

 so separated are conducted directly without the body, while in the latter 

 case they undergo modifications in the tissues which deprive them of all 

 nutritive value. In the case of secretions, on the other hand, it has 

 been found that the losses which the blood undergoes in their formation 

 are temporary, the matters removed from the blood in these processes 

 being again returned to it after the secretions have fulfilled their function. 

 The milk alcme is an exception to this statement. 



On the other hand, we have seen that in absorption from the 

 alimentary canal the blood is constantly receiving additions. The 

 balance between this income and outgo of the blood constitutes nutrition. 

 If the income exceed the outgo the body increases in weight ; if the 

 latter predominates, the body loses weight. 



Two methods are open to us for studying the processes which are 

 concerned in the maintenance of nutritive equilibrium. We have found 

 that the income of the body or the substances which enter the blood and 

 lymph in the form of peptone, sugar, albumen, salts, and fats are con- 

 stituted of the elements of carbon, hydrogen, oxygen, and nitrogen in 

 various proportions. It has been seen that the waste products of the 

 animal body are urea, carbon dioxide, water, and salts. 



We know that nearly all the carbon taken in the food is removed by 

 the lungs and skin in the form of carbon dioxide; we know that nearly 

 all the nitrogen taken in with the food is excreted in the form of urea. 

 All the hydrogen is excreted in the form of water, while the oxygen 

 leaves the animal economy in combination with carbon as carbon dioxide, 

 or with hydrogen as H 2 0. 



The attempt to trace the intermediary stages through which the 

 constituents of the food pass before reaching these final excretory- 

 products is accompanied by the greatest difficulty. But little is known 

 as to the way in which these substances are transformed one into the 



(659) 



