Starling holds that although the experimental proof of the conclusion that the blood carries the protein from the alimentary canal by means of amino-acids is beset with many difficulties, there is a prior reason to believe that such is the case. Experiments in this direction are somewhat confusing. Starling, therefore, submits three possible processes which may take place.

At the present time it is impossible to decide with any certainty as to which of these views of the fate of the ingested protein is correct. It is possible that all three processes may take place:

(a) A proportion of the protein may be built up in the cells. lining the alimentary canal to form blood protein, so that this organ would have to be regarded as an important blood-forming organ.

(b) Another portion, representing the amount required to replace the tissue waste of the body, is absorbed into the blood stream as amino-acids, in which form it is carried to the tissues and reintegrated into the protein characteristic of each tissue.

(c) A third portion, probably the major portion of the protein, does not reach the tissues at all as a nitrogenous compound, but undergoes deaminization in the intestinal wall, the nitrogen being carried rapidly to the liver and converted into urea and then excreted by the kidneys, while the non-nitrogenous moiety is carried to the tissues, to which it serves as a ready and important source of energy.

Some of the matters discussed in the foregoing paragraphs reiterate the views of the most prominent authorities on the question of food absorption, but it is emphasized in this manner in order to drive home the fact that the modern theory of the absorption of food rests mainly upon the supposition that this process is one with which the amino-acids are chiefly concerned. As to the exact manner in which the amino-acids act, definite knowledge is lacking, but the recent investigations of the leaders of research in this direction tend to prove that they pursue the course which has been already clearly stated-that proteins are split by the digestive juices into their constituent amino-acids, and that these substances pass as such into the blood stream.

THE RÔLE OF THE WHITE BLOOD CORPUSCLES IN

THE ABSORPTION OF FOOD

When food is in the course of absorption the leukocytes accumulate in the mucous membrane of the gut. After a meal has been absorbed they are found in the blood in very large quantities. Schäfer describes them as loading themselves with fat and carrying the fat to the chyle where the fat is set free by the dissolution of the leukocyte.

Leukocytosis is said to be most pronounced during the digestion of proteins. These leukocytes that ingest bacteria are termed "phagocytes." The "phagocytosis theory of Metchnikoff" assumes that the functions of the leukocytes is to ingest foreign particles, including bacteria with which they come into contact, in the manner that the ameboid leukocytes are known to do. This function was the foundation for Wright's opsonic theory, which was to the effect that the power of the leukocytes to ingest bacteria depends upon the presence in the plasma of certain substances to which the name of opsonin was given, which render the bacteria in such a state that they are particularly susceptible or sensitive to attacks by the leukocytes. Howell states that they also aid in the absorption of fats from the intestine. They aid in the absorption of peptones from the intestine. They take part in the process of blood coagulation. They are held to maintain the normal protein composition of the blood plasma.

Howell further comments "that the blood proteins are peculiar, and they are not formed directly from the digested food. It is possible that the leukocytes, which are the only phagocytic cells in the body, aid in keeping up the normal supply of proteins. From this standpoint they might be regarded, in fact, as unicellular glands, the products of their metabolism serving to maintain the normal composition of the blood plasma. When the products of digestion of proteins are in the capillary tubes the leukocytes are chemotactic towards them, and as they exhibit the same tendency towards decomposing tissue, Hofmeister has suggested that most of the proteins are absorbed by uniting with the leukocyte body. Decomposition of the whole or part of the cell is supposed to set them free. As the agreement among physiologists appears to be general that proteins are absorbed as amino-acids, and enter the blood stream as such, and, moreover, when the rapidity and volume of absorption and the relatively small number of leukocytes found in the blood are considered, this theory will hardly bear close analysis. However, there are still some indications that blood proteins may be formed by leukocytes and that they busy

themselves in securing sufficient protein to supply the needs of the blood tissue alone.

Perhaps most writers on physiology have reached the conclusion that the work of the leukocytes is limited to the removal of dead or injured tissue. At any rate, much further investigation is called for before any positive statement is made concerning the rôle of the leukocytes during absorption.

OSMOSIS, DIALYSIS AND DIFFUSION

At one time certain known physical and chemical energies were considered to be sufficient to explain the facts of absorption, namely, osmosis, dialysis, diffusion, and imbibition, which consists in the passage of fluid under pressure through a membrane. These were thought to be the principal means of absorbing food material. These factors undoubtedly play an important rôle in the passage of solutions through the alimentary mucous membrane and the walls of the blood vessels. The part which the physical factors play is probably most pronounced in the absorption of water and crystalloids. The nature of the fluid within the digestive tract, and the movements of the walls of the stomach and intestines by means of which the material to be absorbed is brought into intimate contact with the absorbing membrane, are additional factors which influence absorption.

But before dealing with this phase of the question it may be as well to define the terms diffusion, dialysis and osmosis.

Definition of the Terms.-According to Howell, "When two gases are brought into contact a hemogeneous mixture of the two is soon formed. This interpenetration of gases is spoken of as diffusion, and is due to the continual movement of the gaseous molecules to and fro within the limits of the confining space. So also when two miscible liquids or solutions are brought into contact a diffusion occurs for a similar reason, the movements of the molecules finally effecting a hemogeneous mixture. If the two liquids happen to be separated by a membrane, diffusion will still occur, provided the membrane is permeable to the liquid molecules, and in time the liquids on the two sides will be mixtures having a uniform composition. Not only water molecules, but the molecules of many substances in solution, such as sugar, may pass to and fro through membranes so that two liquids separated from each other by an intervening membrane and originally unlike in composition, may finally, by the act of diffusion, come to have the same composition." Dialysis refers more

strictly to the passage of substances dissolved in water through animal membranes osmosis, to the passage of water alone.

"In the body we deal with aqueous solutions of various substances that are separated from each other by living membranes, such as the walls of the blood capillaries or of the alimentary canal. The laws of diffusion through membranes are of importance in explaining the passage of water and dissolved substances through these living septa. In aqueous solutions, such as are found in the body, the movements of the molecules of water, as well as of the substances in solution, must be taken into account. These latter may have different degrees of diffusibility as compared with one another or with the water molecules, and it frequently happens that a membrane which is permeable to water molecules is less permeable or even impermeable to the molecules of the substances in solution. For this reason the diffusion stream of water and of the dissolved substances may be differentiated to a greater or less extent." In recent years it has become customary to limit the term osmosis to the stream of water molecules passing through a membrane, while the term dialysis or diffusion is applied to the passage of the molecules of the substances in solution. The osmotic stream of water under varying conditions is especially important.

Osmotic Pressure. In connection with this process it is necessary to define the term osmotic pressure as applied to solutions. A perfectly satisfactory explanation of the nature of osmotic pressure has not yet been furnished. According to Halliburton, "The following simple explanation is the best. Suppose we have a solution of sugar separated by a semipermeable membrane from water; that is, the membrane is permeable to water molecules, but not to sugar molecules. The streams of water from the two sides will then be unequal; on one side we have water molecules striking against the membrane in what we may call normal numbers, while on the other side both water molecules and sugar molecules are striking against it. On this side, therefore, the sugar molecules take up a certain amount of room, and do not allow the water molecules to get to the membrane; the membrane is, as it were, screened against the water by the sugar, therefore, fewer water molecules will get through from the screened to the unscreened side and vice versa-in other words, the osmotic stream of water is greater from the unscreened water side to the screened water side than it is in the reverse direction. The more sugar molecules that are present, the greater will be their screening action; thus we see that the osmotic pressure is proportional to the number of sugar molecules in the concentration of the solution." According to Van't Hoff's hypothesis the osmotic pressure is equal to that which the dissolved sub

stance would exert if it occupied the same space in the form of a gas. The nature of the substance makes no difference; it is only the number of molecules in it which causes osmotic pressure to vary.

PRESSURE EXERTED BY CRYSTALLOIDS.-From Halliburton's researches concerning osmosis, dialysis and diffusion he thinks, "The osmotic pressure exerted by crystalloids is very considerable, but their ready diffusibility limits their influence on the flow of water in the body. Thus if a strong solution of salt is injected into the blood, the first effect will be the setting up of an osmotic stream from the tissues to the blood. The salt, however, will soon diffuse out into the tissues, and will then exert osmotic pressure in the opposite direction. Moreover, both effects will be but temporary, because excess of salt is soon eliminated by the excreting organs.'

OSMOTIC PRESSURE OF PROTEINS.-It has been generally assumed that proteins, the most abundant and important constituents of the blood, exert little or no osmotic pressure. According to Starling, they have a small osmotic pressure. As Halliburton points out, if this be so, even a slight pressure is of importance, for proteins, unlike salt, do not diffuse readily, and their effect therefore remains as an almost permanent factor in the blood. Starling places the osmotic pressure of the proteins of the blood as equal to 30 mm. of mercury. Halliburton is of the opinion "that from the theoretical standpoint we would find it difficult to imagine that a pure protein can exert more than a minimal osmotic pressure. It is made up of such huge molecules that, even when the proteins are present to the extent of 7 to 8 per cent as they are in blood-plasma, there are comparatively few protein molecules present, and these are in a state of colloidal solution, not true solution. Still by means of this weak but constant pressure it is possible to explain the fact that an isotonic or even a hypertonic solution of a diffusible crystalloid may be completely absorbed from the peritoneal cavity into the blood. The pressure observed may be due to saline materials, from which it is difficult to separate proteins. The functional activity of the tissue elements is accompanied by the breaking down of their protein constituents into such simple materials as urea, and its precursors, sulphates and phosphates. These materials pass into the lymph, and increase its molecular concentration and its osmotic pressure; thus water is attracted from the blood to the lymph, and so the volume of lymph rises and its flow increases. On the other hand, as these substances accumulate in the lymph they will in time attain there a greater concentration than in the blood, and so they will diffuse towards the blood, by which they are carried to the organs of excretion. However,