From these statements, it is obvious that what we term absorption is something more than a simple diffusion of soluble substances from the alimentary tract into the blood current. The process is much more complex than appears on the surface, and our lack of definite knowledge, in spite of numerous efforts to unravel the mystery, merely strengthens the view that we are dealing here with an obscure physiological problem, and not a simple physical one. Digestion induces a splitting up of the food proteid into fragments, large or small, while incidental to absorption there is apparently a reconstruction, or synthesis, of proteid from the fragments so formed. The process seems somewhat costly, physiologically speaking, yet when one considers the variety of proteids consumed as food, it is easy to comprehend how essential it is that in some manner, as in absorption, there be opportunity for construe-tion of the specific proteids of the blood and lymph.

We find an analogous process in the absorption of fats. As we have seen, the fats of the food are broken apart in the small intestine into glycerin and free fatty acid, a portion of the latter, and perhaps all, combining with the alkali of the intestinal juices to form soluble soaps, or sodium salts of the respective fatty acids. The neutral fats present in animal and vegetable foods are all alike in containing the glyceryl radicle, but they differ in the character of the fatty acids present. Further, one form of animal fat, like that from beef, may contain quite a different proportion of stearin, palmitin, and olein than is present in the fat of another animal, like mutton. By digestion, however, they are all broken apart into fatty acid and glycerin. These acids and their salts can be readily detected in the intestine, but they are not found in the blood or lymph, yet shortly after fatty food is taken the lymph is seen to be milky from fat. Obviously, the fatty acids liberated in the intestine are absorbed, either as soluble soaps or as free fatty acids dissolved in bile, but as they pass through the epithelial cells of the intestine into the lacteal radicles, there is a synthesis or reconstruction of fat; and as a result, neutral fats and not soaps are found in the lymph. Here, then, we have a process quite analogous to what apparently occurs in the absorption of proteid, though less complex; and it is possible that this is one of the factors which aids in the formation of a specific fat mixture corresponding, in a measure, to the type of fat present in the particular species. It is well understood that the fat of an animal's tissues may be modified somewhat by the character of the fat fed, yet in spite of this there is a certain degree of constancy in composition which calls for explanation. Sheep and oxen feeding in the same pasture have fat widely different in the proportion of stearin, palmitin, etc. The fat of man's tissues is fairly definite in composition, yet he eats a great variety of fatty foods. One man may consume large amounts of hard mutton fat with its relatively large content of stearin, while another individual may take his fat mainly in the form of the soft butter fats, with their relatively large content of olein and palmitin. In both cases, the fat of the man's tissues will be essentially the same. To be sure, the changes that take place in the tissue cells, reinforced by the construction of fat from other sources, may be partly responsible for this constancy of composition, but the transformations incidental to absorption are quite possibly, in some measure, helpful thereto.

The great bulk of the digested food material is absorbed from the small intestine, and there are two pathways open through which the absorbed material can gain access to the blood. The one path leads directly to the liver, and substances taking this course are exposed to the action of this organ, before they enter into the general circulation. The other path is through the lacteal or lymphatic system, and constitutes a roundabout way for substances to enter the blood stream, since they must first pass through the thoracic duct before entering the main circulation. As a general truth, it may be stated that fats are absorbed through the latter channel, while carbohydrates and proteids follow the first path. The innumerable blood capillaries in the villi of the intestine take up the products resulting from the digestion of proteids and carbohydrates, through which they are passed into the portal vein, and thereby distributed throughout the liver. This means that both carbohydrates and proteids - or their decomposition products - are exposed to a variety of possible changes in this large glandular organ, before they can enter into the tissues of the body. As we have seen, practically all carbohydrate food is converted into a monosaccharide, principally dextrose, in the alimentary tract; and it is in this form of a simple sugar that the carbohydrate passes into the blood. This might easily mean a pound of sugar absorbed during the twenty-four hours, and would obviously give to the blood a high degree of concentration, unless the excess was quickly disposed of. Sugar is very diffusible, and if it accumulates to any extent in the blood it is quickly gotten rid of by excretion through the kidneys. This, however, is wasteful, physiologically and otherwise, and does not ordinarily occur except in diseased conditions. Further, physiologists have learned that a certain small, but definite, amount of sugar in the blood is a necessary requirement in nutrition, and it is the function of the liver to maintain the proper carbohydrate level.

We must again emphasize the great importance of carbohydrate food; there is a far larger amount of starchy food consumed than of any other foodstuff, and it is more readily available as a source of energy. Its presence in the blood, in the form of sugar, is constantly demanded, but it must be kept within the proper limits for the uses of the different tissues and organs of the body. The liver serves as an effective regulator, maintaining, in spite of all fluctuations in the supply and demand, a definite percentage of sugar such as is best adapted to keep the tissues of the body in a normal and healthy condition. This regulation by the liver is rendered possible through the ability of the hepatic cells to transform the sugar brought to the gland into glycogen, so-called animal starch, which iS stored up in the liver until such time as it is needed by the body. The process is one of dehydration, the reverse of what takes place in the intestine when ordinary Btarch is converted into maltose and dextrose. The efficiency of this regulating mechanism depends also upon the ability of the liver to transform glycogen into sugar, presumably through the agency of an enzyme in the hepatic cells. Hence, glycogen may be looked upon as a temporary reserve supply of carbohydrate, manufactured and stored in the liver during digestion, when naturally large amounts of sugar are passing into the portal blood, and to be drawn upon whenever from any cause the content of sugar in the blood threatens to fall below normal. Obviously, there must he some delicate machinery for the adjustment of these opposite changes in the liver, and we may well believe that it is associated with the composition of the blood itself, which in some fashion stimulates and inhibits, as may be required, the functional activity of the liver, or its component cells. In any event, we have in this so-called glycogenic function of the liver a most effective means for accomplishing the complete and judicious utilization of all the sugar formed from the carbohydrates of the food, after it has once passed beyond the confines of the alimentary tract into the blood; preventing all loss, and at the same time guarding against all danger, from undue accumulation of sugar in the circulation. We see, too, how wise the provision that all sugar should pass from the alimentary canal into the portal circulation and not by way of the lymphatics, since by the latter channel the regulate ing action of the liver would be mainly lost. Further, recalling how soluble and diffusible sugar is, we may well marvel that it practically all passes from the intestine by way of the blood, and escapes entry into the lymphatics. Surely, this marked shunning of the other equally accessible pathway affords a striking illustration of selective action such as might be expected in a physiological process, but not in harmony with the ordinary physical laws of osmosis or diffusion. In conformity with this statement, it may be mentioned that appropriate experiments have clearly demonstrated that the different sugars available as food are not absorbed from the intestine in harmony with their diffusion velocity, but show deviations therefrom which can be explained only on the ground that the intestinal wall exercises some selective action, due to the living cells composing it. Likewise interesting in their bearing on nutrition are the observations of Hofmeister,1 who finds by experiments on dogs that the assimilation limit of the different sugars shows marked variation. Thus, dextrose, levulose, and cane-sugar have the highest assimilation, while milk-sugar is far less easily and completely assimilated. If this is equally true of man, it indicates that starchy foods, with their ultimate conversion into dextrose, are to be ranked as having a high assimilation limit, thus affording additional evidence of their high nutritive value.