Nutriment exclusive of air is received into the body by means of the alimentary canal. The great receiver of air is the lungs, but it also penetrates the body through the pores of the skin, and at these points carbonic acid is given off as in the lungs. The body is often compared to a steam-engine, which takes in raw material in the form of fuel and converts it into force or power. Food, drink, and air are the fuel of the body,--the things consumed; heat, muscular and intellectual energy, and other forms of power are the products. Food, during the various digestive processes, becomes reduced to a liquid, and is then absorbed and conveyed, by different channels constructed for the purpose, into the blood, which contains, after being acted upon by the oxygen of the air in the lungs, all those substances which are required to maintain the various tissues, secretions, and, in fact, the life of the system. Some of the ways in which the different kinds of food nourish the body have been found out by chemists and physiologists from actual experiments on living animals, such as rabbits, dogs, pigs, sheep, goats, and horses, and also on man. Often a scientist becomes so enthusiastic in his search for knowledge about a certain food that he gives his own body for trial. Much valuable work has been done in this direction during the last decade by Voit, Pettenkofer, Moleschott, Ranke, Payen, and in this country by Atwater. No one can explain all the different intricate changes which a particle of food undergoes from the moment it enters the mouth until its final transformation into tissue or some form of energy; but by comparing the income with the outgo, ideas may be gained of what goes on in the economy of the body, and of the proportion of nutrients used, and some of the intricate and complex chemical changes which the different food principles undergo in the various processes of digestion, assimilation, and use.[22] Probably hundreds of changes take place in the body, in its various nutritive functions, of which nothing is known, or they are entirely unsuspected, so that if we do our utmost with the present lights which we possess for guidance to health, we shall still fall far short of completeness. The subject of food and nutrition, viewed in the light of bacteriology and chemistry, is one of the most inviting subjects of study of the day, and is worthy of the wisest thought of the nation.