CHAPTER XXIII. 



NUTRITION. 



We can now compare the incomings and outgoings of the 

 economy, and should now be in a position to see what light can 

 be thrown by this comparison upon the actual changes which 

 take place in the textures of the body. 



We have seen that the income is made up of substances belong- 

 ing to the same groups of materials as are found in the body, viz., 

 albumins, fats, carbohydrates, salts and water, introduced by the 

 alimentary canal, and oxygen, which is admitted by the respira- 

 tory apparatus ; while the outgoings consist of urea from the 

 kidneys, carbonic acid from the lungs, certain excrement from the 

 intestine and other mucous passages, sweat, sebaceous secretion, 

 epidermal scales, from the skin ; together with a quantity of water 

 from all these ways of exit. The milk, ova and semen may be 

 here omitted, being regarded as exceptional losses. 



In order that the body may be kept in its normal condition, it 

 is necessary that the income should at least be equal to the out- 

 goings of all kinds, and, except where growth is going on rapidly, 

 an income equal to the expenditure ought not only to suffice, but 

 be the most satisfactory amount of supply for the economy. 



We know that animals can live for some considerable time 

 without food, in which case, though there be no income, a certain 

 expenditure is necessary to sustain life, and therefore the out- 

 goings continue. We ought thus to be able to arrive in a very 

 simple manner at the minimal expenditure necessary for the sus- 

 tentation of the body. We shall find, however, that (1) an income 

 equal to this minimal expenditure (starvation) does not at all 

 suffice to keep up the body weight, and that (2) a considerable 

 margin over and above this minimum is necessary in order to 

 establish the nutritive equilibrium ; (3) further, that the propor- 

 tion of material eliminated and stored up in the body respectively 

 varies as the income is increased ; (4) and, finally, that the quality 



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