The digestion of dextrin has already been mentioned in connection with that of starch, both saliva and pancreatic juice forming dextrin during the digestion of starch and acting upon it with the production of maltose. Complete hydrolysis of dextrin, as by boiling with acid, yields glucose as the sole product.

Glycogen, (C6H10O5)x, plays much the same ro1e in animals which starch plays in plants, and is sometimes called "animal starch" -Glycogen also takes the place of starch as reserve carbohydrate in fungi and other forms of plant life not provided with the chlorophyll apparatus. It is a white, amorphous powder, odorless and tasteless, which swells up and apparently dissolves in cold water to an opalescent colloidal dispersion which is not cleared by repeated filtration, but loses its opalescence on addition of a very small amount of potassium hydroxide or acetic acid. Water solutions (dispersions) of glycogen are readily precipitated by alcohol. When treated with iodine they react yellow-brown, red-brown, or deep red. Hydrolysis of glycogen yields glucose only, as end-product.

Glycogen occurs in the lower as well as the higher animals, and in all parts of the body, but is especially abundant in the liver. The amount of glycogen in the liver depends to a great extent upon the condition of nutrition of the animal. In the average of seven experiments by Schondorff in which dogs were fed for the production of as much glycogen as possible, 38 per cent of that found was in the liver, 44 per cent in the muscles, 9 per cent in the bones, and the remaining 9 per cent in the other tissues of the body. But the distribution of glycogen in the body as shown by these experiments was quite variable, even among animals of the same species which had been fed in the same way. It is well known, too, that some species store glycogen in their muscles to a greater extent than others, attempts even having been made to distinguish analytically between horseflesh and beef by the difference in their glycogen content. The storage of glycogen in the body is promoted by rest as well as by liberal feeding, and stored glycogen is used up rapidly during active muscular work.

Cellulose, (C6H10O5)x, the chief constituent of wood and of the walls of plant cells generally, is an anhydride of glucose and can be made to yield the latter when hydrolyzed by suitable treatment with strong acid. Typical cellulose of mature fiber (such as cotton, linen, or wood fiber) is, however, quite resistant to the action of dilute acids or of ordinary enzymes and passes through the digestive tract for the most part unchanged. The toughness of the cellulose differs with the stage of growth or maturity, and some of the less resistant forms of cellulose, such as that of tender white cabbage, may disappear from the digestive tract in appreciable amounts. Experiments to determine whether the cellulose thus disappearing is digested to sugar and absorbed or merely decomposed by bacteria in the digestive tract have not given conclusive results. According to Swartz: "In any event, the quantities of cellulose which the alimentary tract of man is capable of absorbing are, apparently, too small for it to play a r61e of any importance in the diet of a normal individual." The cellulose in the food may, however, serve a very useful purpose in giving bulk to the food residues and thus facilitating their passage along the digestive tract.

Hemicelluloses is a term somewhat loosely applied to polysaccharides, usually occurring as constituents of cell walls in plants, which are not digested by the starch-splitting enzymes but are usually much more readily hydrolyzed by acid than is cellulose. In many plant tissues the hemicellulose consists chiefly of pentosans; in other cases it is largely mannan or ga-lactan.

Mannans, (C6H10O5)x, anhydrides of mannose, are widely distributed in the vegetable kingdom and, as Swartz points out, show great differences in solubility, ranging from the readily soluble mucilaginous forms found in certain legumes to the horny matter of such seeds as the date, a form of mannan which was long confused with true cellulose. The experiments of Swartz upon the mannan of salep showed it to disappear completely in its passage through the human digestive tract, although tests with individual digestive enzymes gave negative results. In what way and to what extent the mannan thus disappearing from the digestive tract becomes available in nutrition is still a subject of investigation.

Galactans, (C6H10O5)x, anhydrides of galactose, are widely distributed in plants. They occur in the seeds of legumes and to a slight extent in the cereals also, in by-products of beet sugar manufacture and abundantly in several of the algae and lichens, including Chinese moss, agar-agar, and Irish moss. The pectins are said to consist largely of galactans, apparently either in combination or admixture with pentosans and perhaps other complexes as well. The galactans differ in their solubilities and apparent digestibility when eaten by man or other animals, but on the whole do not appear to be of much nutritive value. Those of agar-agar and Irish moss, which are most used as food, are not digested.

Levulans is the term under which a number of polysaccharides of the composition (C6H10O5)x, and yielding fructose (levulose) on hydrolysis have been described. The most important of these, at least so far as is at present known, is inulin,a white, powdery substance occurring in the tubers of the Jerusalem artichoke and to a less extent in the bulbs of onions and garlic as well as in various parts of plants not commonly used for food. By the action of acids inulin is very readily hydrolyzed to levulose, but the digestive juices do not seem to contain enzymes capable of hydrolyzing inulin and it appears to be of practically no importance as human food.