*Albuminoids* form a small class of foods, of minor importance, which are similar to proteids in composition, but differ from them in being unable to rebuild the tissues. Gelatin, a constituent of soup and obtained from bones and connective tissue by boiling, is the best known of the albuminoid foods. On account of the nitrogen which they contain, proteids and albuminoids are often classed together as _nitrogenous foods_. *Carbohydrates.*—While the carbohydrates are not so essential to life as are the proteids, they are of very great value in the body. They are composed of carbon, hydrogen, and oxygen, and are obtained mainly from plants. There are several varieties of carbohydrates, but they are similar in composition. All of those used as food to any great extent are starch and certain kinds of sugar. *Starch* is the carbohydrate of greatest importance as a food, and it is also the one found in the greatest abundance. All green plants form more or less starch, and many of them store it in their leaves, seeds, or roots (Fig. 60). From these sources it is obtained as food. _Glycogen_, a substance closely resembling starch, is found in the body of the oyster. It is also formed in the liver and muscles of the higher animals, being prepared from the sugar of the blood, and is stored by them as reserve food (Chapter XI). Glycogen is, on this account, called _animal starch_. Starch on being eaten is first changed to sugar, after which it may be converted into glycogen in the liver and in the muscles. [Fig. 60] Fig. 60—*Starch grains* in cells of potato as they appear under the microscope. (See practical work.) *Sugars.*—There are several varieties of sugar, but the important ones used as foods fall into one or the other of two classes, known as _double sugars_ (disaccharides) and _single sugars_ (monosaccharides). To the first class belong _cane sugar_, found in sugar cane and beets, _milk sugar_, found in sweet milk, and _maltose_, a kind of sugar which is made from starch by the action of malt. The important members of the second class are _grape sugar_, or dextrose, and _fruit sugar_, or levulose, both of which are found in fruits and in honey. The most important of all sugars, so far as its use in the body is concerned, is _dextrose_. To this form all the other sugars, and starch also, are converted before they are finally used in the body. The close chemical relation between the different carbohydrates makes such a conversion easily possible.