required by the blood; lime and phosphates go to make bone; but important as they all are, they need not detain us further at present.
With regard to the amount of these elements which is required per day, and which is ascertained by collecting and weighing all that is given off, it is found that about ½ ounce of nitrogen and 10 ounces of carbon are necessary to an average man—i.e., weighing about 10 stone. The ½ ounce of nitrogen and about 2 ounces of the carbon are contained in 4 ounces of dry proteid, which leaves a balance of 8 ounces of carbon to be made up; and this is usually obtained by eating 4 ounces of fat and 18 ounces of carbohydrate.
Roughly speaking, these principles are contained in ¾ pound of ordinary butcher’s meat and 2 pounds of bread; but it would be well to defer considering diet for the present, until we have examined the apparatus by which the body extracts what it wants from the raw materials, and which of these offer it the least resistance.
II.
The way in which protoplasm gets its chemical requisites for growth is doubtless simply by absorbing them. Some of the lower structureless forms carry this to an absurd extreme, for when two individuals meet they fuse, and each no doubt claims to have eaten the other. As, moreover, the first thing which a cell does when it grows is to divide, the whole proceeding looks rather futile. But ready-made protoplasm of an assimilable shape is rare, and it is not often that a cell, unless it be a plant or a parasite, finds itself in a substance which can be handed straight to the nucleus without further elaboration. Usually the cell has to discharge from itself a reagent, which will develop the right chemical qualities in the matter it wants to absorb. This substance is known as an enzyme, or ferment. Ferments, however, are an expense to the cell, requiring a certain effort for their production; so, in order that they may be economized, they are, in the higher forms, poured over the food while it is in an enclosed cavity, or stomach. In
