as not limited to substances prepared by the action of the digestive fluids, but holds that it can absorb undigested food, such as white of egg, although it is probable that even here there is a splitting up of the protein molecule into smaller molecules before absorption takes place. Therefore he asserts that nutrient enemata based upon the knowledge have saved many lives. The opinions of eminent physiologists diverge considerably with respect to the absorptive powers of the large intestine. The whole matter is in a somewhat confused state. It is, however, likely that the powers of absorption of the large intestine have been unduly minimized by some physiologists. The fact must not be lost sight of that in the small intestine the villi are especially adapted for absorptive purposes, and that in the large intestine there are no villi. In the large intestine there are some solitary glands which perform as much absorptive work as can be expected from their limited number, but in the opinion of Pohlmann the mucous membrane of the large intestine is an important factor in absorption.

As an excretory organ the large intestine of carnivora and civilized man plays a prominent rôle and perhaps its main value lies rather in its excretory functions than in its absorptive powers.

ABSORPTION OF FOOD

Now that the absorptive powers of the alimentary tract and the means or most probable means by which this process is accomplished have been considered, the next obvious subject for discussion is the absorption of the various kinds of foodstuffs. The substances to be absorbed are peptones, glucose and emulsified fats-the products of digestion; in addition to water and various salts. It may be stated in a general way that foods such as water and soluble salts are absorbed unchanged. On the other hand, the organic foods are changed considerably. As pointed out previously, absorption occurs mainly through the blood vessels by way of the portal tributaries, and through the lymphatic vessels or lacteals. Speaking broadly, the proteins and carbohydrates are absorbed by the blood capillaries of the villi and the fats by the lacteals.

Halliburton, in common with all modern physiologists, does not attribute to known physical laws a complete explanation of the absorption of food material in the body. It has been definitely proven that diffusion and osmosis occur in the intestine, but this does not mean that absorption proceeds solely from mechanical action. Such action plays a

part, but in all likelihood a small part, in the absorption that goes on in a healthy human being.

Much has yet to be learned regarding absorption, and as mentioned before the energy that controls absorption is doubtless some form of imbibition. Halliburton lays stress on the fact that a marked feature during absorption is the increased activity of the lymphocytes which lie beneath the epithelium; the number of these cells increases conspicuously. The surmise has therefore been advanced that these cells share in the work of transporting absorbed materials.

ABSORPTION OF FATS

Theories of Fat Absorption. The manner in which the fats are absorbed has long been a stumbling block to physiologists and numerous theories have been brought forward to explain the mode of fat absorption. According to most recent views set forth by Howell, the emulsified neutral fat is either ingested directly by the epithelial cells, the fat droplets enter between the epithelial cells in the so-called cement substance, the fat droplets are ingested by leukocytes that lie between the epithelial cells or, lastly, that the fat is first split into fatty acid and glycerol and is absorbed in the epithelial cells in these forms. The last theory is that most greatly favored. While allowing that during digestion the epithelial cells contain fat droplets, Howell is of the opinion that these droplets are formed in situ by a synthesis of the absorbed glycerol and fatty acids. Even granting that the fat is absorbed in solution as fatty acids and glycerol, Howell still thinks that the mechanism of fat absorption remains unexplained. The most favored theory of the mechanism of fat absorption, restated clearly, is that the emulsified fats are split into fatty acids and glycerol which are taken up by the epithelial cells and passed into the stroma of the villi, there to be reconverted into minute fat globules.

Halliburton points out that in explaining the mechanism of fat absorption, the difficulty was to show how the fat first entered the columnar epithelium. Munk and, later, Moore and Lockwood, have demonstrated conclusively that fat is completely broken down in the intestine into glycerol and fatty acids. Preliminary emulsification is advantageous for the formation of these substances, but not essential. According to Halliburton, then, not only is fat entirely absorbed as glycerol and fatty acids, but the latter, in great measure, are first converted into soaps; that is, compounds of the fatty acids and alkalis. As Pohlmann remarks, it is a moot point as to whether fatty acids and glycerol pass directly

into the lymph channels which traverse the villus and finally unite to form the lacteal, or whether the lymphocytes, so abundantly found in the stroma, carry the small fat globules from the epithelial cells directly into the lacteal.

Munk found out that the soap compounds of the fatty acids and alkalis pass readily through the striated borders of the intestinal epithelial cells; and these cells perform the synthetic act of building them into fat once more, the fat so formed appearing in the form of small globules, surrounding or becoming mixed with the protoplasmic granules that are ordinarily present. Munk also discovered as a result of his investigations that on feeding an animal on fatty acids the chyle contains fat, showing that the necessary glycerol must have been formed by protoplasmic activity during absorption.

A chapter dealing with absorption of foodstuffs would be obviously incomplete without reference to the views of Starling, and attention may be drawn here to certain of that distinguished physiologist's statements. concerning the absorption of fat. Arguing from the postulate that the fats eaten by an animal in excess of its daily requirements are stored up in the body in the form in which they are ingested, and that each cell of the body probably possesses in itself the mechanism for the utilization of these neutral fats and for effecting in them the various changes involved in the successive stages of their disintegration and oxidation through which they are finally converted to CO2 and water, Starling contends that the problem of fat absorption is ultimately one of the simplest with which we have to deal. It merely involves the transference of the neutral fat of the food to the circulating fluids in such a form that it can be carried by them to the place where it is required for the metabolism of the body or where it may be stored up as a reserve substance. Wherever it is present in health fat should be regarded as food reserve, whether in the subcutaneous or intramuscular tissue, within the bones, or the cells of the cartilage, or of glandular organs. In its wide distribution fat in the animal tissues may be compared to that of starch in the vegetable kingdom, though the glycogen of liver and muscle bears the closer resemblance. Yet fat and starch are physiologically interchangeable.

It may be further stated that after the removal of storage fat, its utilization, according to the hypothesis of Leathes, is brought about through the agency of the liver, in which such fat is transformed by a process of "desaturation" into fat-like bodies or lipoids, which are thence. distributed in a utilizable form to the important organs of the body,

including, of course, the liver itself. There is no space in this chapter to discuss at any length the elaborate and carefully-thought-out presentation of Starling's views. Among other salient points to which reference is made is the description by Brücke of the muscular mechanism of absorption. This mechanism was commented on in an earlier part of the chapter when considering the manner in which absorption by the small intestine was carried out; namely, that repeated contractions of the muscle fibers of the villus would tend to empty the spaces into the central lacteal, and this in turn into the submucous plexus of lymphatics, so that the lymph in the spaces is constantly renewed and passes laden with absorbed fat particles into the valved lymphatics of the mesentery. Starling thinks it probable that the muscular mechanism of absorption plays an important part in the absorption of fat, although it is difficult to furnish any experimental proof of the manner in which this mechanism works.

An extremely important part is played by the bile as well as by the pancreatic juice in the process of absorption of fats. Bile aids the digestion of fat, the bile salts acting in the same way as the co-enzyme of pancreatic lipase. Bile is also a solvent of fatty acids and Halliburton suggests that it probably assists fat absorption by reducing the surface tension of the intestinal contents. Membranes moistened with bile allow fatty materials to pass through them more readily than would be otherwise the case.

According to Moore and Lockwood, quoted by Howell, the reason that the bile salts aid the absorption of the split fats is probably because the bile salts dissolve the fatty acids readily and then bring them into contact in soluble form with the epithelial cells. If the intestine be deprived of its bile contents by drainage by means of a fistula from the gall bladder or duct, a great deal of the fatty food is not absorbed and can be found in the feces. It is easy by direct observation to prove that the lymphatic vessels take up the greater part of the fat ingested. To demonstrate this it is only necessary to examine the lymphatics of the intestine of an animal after the administration of a meal rich in fats. These vessels are distended with milky chyle, while, according to Starling, the same fluid is found filling the cisterna lymphatica magna (receptaculum chyli) and the thoracic duct. It has been conclusively shown that a part, probably the greater part, of the fat absorbed from the intestine is conveyed thence by the lymphatic vessels to the venous circulation by which it is distributed to the various tissues of the body. It has not been determined as yet how it leaves the blood stream.

But many experiments have shown that some of the fat which is absorbed does not travel by the way of the lymphatics and the thoracic duet. In fact, after deducting the amount of fat that escapes absorption and is lost in the feces, the amount that may be recovered from the thoracic duct is less than that taken in the food.

Frank, referred to by Howell, has advanced the theory that some of the fat is absorbed directly by the blood vessels of the villi. The portion thus absorbed enters the portal vein and passes through the liver before reaching the general circulation. The liver holds back more or less of the fat taking this route, as it is found that during absorption the liver cells show an accumulation of fat droplets in their interior. Starling says that it is not known what happens to the fat that is not taken up by the lymphatics and holds that it is difficult to imagine that "any large proportion of this lost fraction is absorbed into the blood stream in the form of soaps," pointing out that Munk has shown soaps injected into the blood act as potent poisons. Emphasis may be laid upon the point that if absorption is active, the meshes of the villus contain a number of free fat granules and the leukocytes in these meshes are also generally found full of these granules.

Function of the Leukocytes.-Zawarykin and Schafer, referred to by Starling, impute an importance to the leukocytes in the transference of the granules from epithelial cells to the central lacteal. It was supposed that the leukocytes took up the fat granules extruded by the epithelial cells at the base of the villi and thence found their way into the central lacteal, where they broke down, in this manner furnishing the molecular basis of the chyle as well as its protein constituents. Heidenhain was strongly opposed to the view and pointed out that many of the granules staining darkly with osmic acid were not necessarily fat and that the number of leukocytes within the villi were hardly sufficient to account for the amount of material observed. Reuter is of the opinion that the epithelial cells take up fat in a dissolved condition through the striated border, and deposit it as granules of neutral fat in the inner portion of the protoplasm. From here the fat is passed on by the protoplasm by the side of the nucleus and extruded in the form of very fine granules in the deeper parts of the interepithelial clefts, which thus function as true. excretory organs for the epithelial cells.

The view is still held that the fats are taken up by the epithelial cells from the intestine as fine particles of neutral fat, the chief use of the pancreatic juice being to aid the formation of an emulsion of fat in the intestines. Starling does not agree with this conception of the manner