We can 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 belonging 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 acquired by the respiratory 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 devoted to special objects.

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 outgoings of all kinds, and, except where growth is going on rapidly, an income equal to the expenditure ought not only to suffice, but ought to be the most satisfactory for the economy.

We know that animals can live for some considerable time without food, in which case a certain expenditure of material derived from the body itself 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 sustentation of the body. We shall find, however, that (i) an income equal to this minimal expenditure (that of 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 material eliminated and stored up in the body respectively varies as the income is increased; (4) and finally, that the quality of the food - i. e., the proportion of each group of food stuff present in the diet - has an important influence on the quantity required to establish the equilibrium, and that best suited to cause increase of weight or to fatten.

It will be convenient to consider the following different cases in succession.

1. No income, except oxygen, i. e., starvation.

2. An income only equal to the expenditure found during starvation.

3. Establishment of perfect nutritive equilibrium.

4. Excessive consumption.