CHAPTER XXIII. 

 NUTRITION. 



WE can DOW 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, carbo-hydrates, salts, and water, introduced by the 

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

 tory apparatus ; whilst 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 outgoings 

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

 manner at the minimal expenditure necessary for the sustenation 

 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 proportion of 



