374 NUTRITION. 



acts of nutrition, is attended with a corresponding loss of 

 matter that has become changed into the nitrogenized ele- 

 ments of excretion. It is the intermediate series of phe- 

 nomena that is so obscure. 



The nutrition of the nitrogenized elements of the tissues 

 may be greatly modified by the supply of new matter. For 

 example, a diet composed of nitrogenized matter in a readily 

 assimilable form will undoubtedly affect favorably the devel- 

 opment of the corresponding tissues of the body ; and, on the 

 other hand, a deficiency in the supply will produce a corre- 

 sponding diminution in power and development. The modi- 

 fications in nutrition due to supply have, however, certain 

 well-defined limits. An excess taken as food is not discharged 

 in the faeces, nor does it pass out in the form in which it 

 entered in the urine ; but it apparently undergoes digestion, 

 becomes absorbed by the blood, and increases the quantity 

 of nitrogenized excrementitious matter discharged, particu- 

 larly the urea. This fact is shown by the great increase in 

 the elimination of urea produced by an excess of nitrogen- 

 ized food. 1 Whether the nitrogenized matter that is not 

 actually needed in nutrition be changed into urea in the 

 blood, or whether it be appropriated by the tissues, increas- 

 ing the activity of their disassitnilation, is a question difficult 

 to determine experimentally. Certain it is, however, that 

 an excess of nitrogenized food is thrown off in nearly the 

 same way as an excess of inorganic matter ; the difference 

 being that the latter passes out in the form in which it has 

 entered, and the former is discharged in the form of nitro- 

 genized excrementitious matter. 



Development of Power and Endurance ly Exercise and 

 Diet (Training). The nutrition of the nitrogenized ele- 

 ments of the body is greatly influenced by functional 

 exercise. This is partly local and partly general in its 

 effects. For example, by the persistent exercise of particu- 



1 See page 225. 



