_IV.--Food the Fuel of Life_ We have still to account for the use in food of substances which are destitute of nitrogen but are known to be necessary to animal life. Such substances are starch, sugar, gum, and pectine. In all of these we find a great excess of carbon, with oxygen and hydrogen in the same proportion as water. They therefore add an excess of carbon to the nitrogenised constituents of food, and they cannot possibly be employed in the production of blood, because the nitrogenised compounds contained in the food already contain exactly the amount of carbon which is required for the production of fibrine and albumen. Now, it can be shown that very little of the excess of this carbon is ever expelled in the form either of solid or liquid compounds; it must be expelled, therefore, in the gaseous state. In short, these compounds are solely expended in the production of animal heat, being converted by the oxygen of the air into carbonic acid and water. The food of carnivorous animals does not contain non-nitrogenised matters, so that the carbon and hydrogen necessary for the production of animal heat are furnished in them from the waste of their tissues. The transformed matters of the organs are obviously unfit for the further nourishment of the body--that is, for the increase or reproduction of the mass. They pass through the absorbent and lymphatic vessels into the veins, and their accumulation in these would soon put a stop to the nutritive process were it not that the blood has to pass through a filtering apparatus, as it were, before reaching the heart. The venous blood, before returning to the heart, is made to pass through the liver and the kidneys, which separate from it all substances incapable of contributing to nutrition. The new compounds containing the nitrogen of the transformed organs, being utterly incapable of further application in the system, are expelled from the body. Those which contain the carbon of the transformed tissues are collected in the gall-bladder as bile, a compound of soda which, being mixed with water, passes through the duodenum and mixes with chyme. All the soda of the bile, and ninety-nine-hundredths of the carbonaceous matter which it contains, retain the capacity of re-absorption by the absorbents of the small and large intestines--a capacity which has been proved by direct experiment. The globules of the blood, which in themselves can be shown to take no share in the nutritive process, serve to transport the oxygen which they give up in their passage through the capillary vessels. Here the current of oxygen meets with the carbonaceous substances of the transformed tissues, and converts their carbon into carbonic acid, their hydrogen into water. Every portion of these substances which escapes this process of oxidation is sent back into the circulation in the form of bile, which by degrees completely disappears.