We have left to the last the two constituents which at the present time possess perhaps the greatest interest and importance, the proteins and the ash. The proteins of bread consist of several substances, the differences between which, for the present purpose, may be neglected, and we may assume that for all practical purposes the proteins of bread consist of one substance only, namely gluten. The importance of gluten in conferring on wheat flour the property of making light spongy loaves has already been insisted upon. No doubt this property of gluten has a certain indirect bearing on the nutritive value of bread by increasing its palatability. But gluten being a protein has a direct and special part to play in nutrition, which is perhaps best illustrated by following one step further the comparison between the animal body and a steam engine. It has been pointed out that starch, sugar, and fat play the same part in the body as does the fuel in a steam engine. But an engine cannot continue running very long on fuel alone. Its working parts require renewing as they wear away, and coal is no use for this purpose. Metal parts must be renewed with metal. In much the same way the working parts of the animal body wear away, and must be renewed with the stuff of which they are made. Now the muscles, nerves, glands and other working parts of the body are made of protein, and they can only be renewed with protein. Consequently protein must be supplied in the diet in amount sufficient to make good from day to day the wear and tear of the working parts of the body. It is for this reason that the protein of bread possesses special interest and importance.