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digestion, as soon as they have been disintegrated and emulsioned by the action of the intestinal fluids. As digestion proceeds, they accumulate in larger quantity, and gradually fill the whole lacteal

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system and the thoracic duct. As they are discharged into the subclavian vein, and mingled with the blood, they can still be distinguished in the circulating fluid, as a mixture of oily molecules and granules, between the orifice of the thoracic duct and the right side of the heart. While passing through the pulmonary circula

tion, however, they disappear. Precisely what becomes of them, or what particular chemical changes they undergo, is not certainly known. They are, at all events, so altered in the blood, while passing through the lungs, that they lose the form of a fatty emulsion, and are no longer to be recognized by the usual tests for oleaginous substances.

The absorption of fat from the intestine is not, however, exclusively performed by the lacteals. Some of it is also taken up, under the same form, by the blood vessels. It has been ascertained by the experiments of Bernard' that the blood of the mesenteric veins, in the carnivorous animals, contains, during intestinal digestion, a considerable amount of fatty matter in a state of minute subdivision. Other observers, also (Lehmann, Schultz, Simon), have found the blood of the portal vein to be considerably richer in fat than that of other veins, particularly while intestinal digestion is going on with activity. In birds, reptiles, and fish, furthermore, according to Bernard, the intestinal lymphatics are never filled with opaque chyle, but only with a transparent lymph; so that these animals may be said to be destitute of lacteals, and in them the fatty substances, like other alimentary materials, are taken up altogether by the blood vessels. In quadrupeds, on the other hand, and in the human subject, the absorption of fat is accomplished both by the blood vessels and the lacteals. A certain portion is taken up by the former, while the superabundance of the fatty emulsion is absorbed by the latter.

A difficulty has long been experienced in accounting for the absorp tion of fat from the intestine, owing to its being considered as a nonendosmotic substance; that is, as incapable, in its free or undissolved condition, of penetrating and passing through an animal membrane by endosmosis. It is stated, indeed, that if a fine oily emulsion be placed on one side of an animal membrane in an endosmometer, and pure water on the other, the water will readily penetrate the substance of the membrane, while the oily particles cannot be made to pass, even under a high pressure. Though this be true, however, for pure water, it is not true for slightly alkaline fluids, like the serum of the blood and the lymph. This has been demonstrated by the experiments af Matteucci, in which he made an einulsion with an alkaline fluid containing 43 parts per thousand of caustic potass. Such a solution has no perceptible alkaline

'Leçons de Physiologie Expérimentale. Paris, 1856, p. 325.

taste, and its action on reddened litmus paper is about equal to that of the lymph and chyle. If this emulsion were placed in an endosmometer, together with a watery alkaline solution of similar strength, it was found that the oily particles penetrated through the animal membrane without much difficulty, and mingled with the fluid on the opposite side. Although, therefore, we cannot explain the exact mechanism of absorption in the case of fat, still we know

fasting.

Fig. 44.

that it is not in opposition to the ordinary phenomena of endosmosis; for endosmosis will take place with a fatty emulsion, provided the fluids. used in the experiment be slightly alkaline in reaction.

It is, accordingly, by a process of endosmosis, or imbibition, that the villi take up the digested fatty substances. There are no open orifices or canals, into which the oil penetrates; but it passes di

INTESTINAL EPITHELIUM; from the Dog, while rectly into and through the substance of the villi. The epithelial cells covering the external surface of the villus are the first active agents in this absorption. In the intervals of digestion (Fig.

Fig. 45.

INTESTINAL EPITHELIUM; from the Dog, during the digestion of fat.

44) these cells are but slightly granular and nearly transparent in appearance. But if examined during the digestion and absorption of fat (Fig. 45), their substance is seen to be crowded with oily particles, which they have taken up from the intestinal cavity by absorption. The oily matter then passes onward, penetrating deeper and deeper into the substance of the villus, until it is at last received by the capillary vessels and lacteals in its centre.

The fatty substances taken up by the portal vein, like those absorbed by the lacteals, do not at once enter the general circulation, but pass first through the capillary system of the liver. Thence they are carried, with the blood of the hepatic vein, to the right side of the heart, and subsequently through the capillary system of the lungs. During this passage they become altered in character, as above described, and lose for the most part the distinguishing characteristics of oily matter, before they have passed beyond the pulmonary circulation.

But as digestion proceeds, an increasing quantity of fatty matter finds its way, by these two passages, into the blood; and a time at last arrives when the whole of the fat so introduced is not destroyed during its passage through the lungs. Its absorption taking place at this time more rapidly than its decomposition, it begins to appear, in moderate quantity, in the blood of the general circulation; and, lastly, when the intestinal absorption is at its point of greatest activity, it is found in considerable abundance throughout the entire vascular system. At this period, some hours after the ingestion of food rich in oleaginous matters, the blood of the general circulation everywhere contains a superabundance of fat, derived from the digestive process. If blood be then drawn from the veins or arteries in any part of the body, it will present the peculiar appearance known as that of "chylous" or "milky" blood. After the separation of the clot, the serum presents a turbid appearance; and the fatty substances, which it contains, rise to the top after a few hours, and cover its surface with a partially opaque and creamy-looking pellicle. This appearance has been occasionally observed in the human subject, particularly in bleeding for apoplectic attacks occurring after a full meal, and has been mistaken, in some instances, for a morbid phenomenon. It is, however, a perfectly natural one, and depends simply on the rapid absorption, at certain periods of digestion, of oleaginous substances from the intestine. It can be produced at will, at any time, in the dog, by feeding him with fat meat, and drawing blood, seven or eight hours afterward, from the carotid artery or the jugular vein.

This state of things continues for a varying length of time, according to the amount of oleaginous matters contained in the food. When digestion is terminated, and the fat ceases to be introduced in unusual quantity into the circulation, its transformation and decomposition continuing to take place in the blood, it disappears gradually from the veins, arteries, and capillaries of the

general system; and, finally, when the whole of the fat has been disposed of by the nutritive processes, the serum again becomes transparent, and the blood returns to its ordinary condition.

In this manner the nutritive elements of the food, prepared for absorption by the digestive process, are taken up into the circulation under the different forms of albuminose, sugar, and chyle, and accumulate as such, at certain times, in the blood. But these conditions are only temporary, or transitional. The nutritive materials soon pass, by catalytic transformation, into other forms, and become assimilated to the pre-existing elements of the circulating fluid. They thus accomplish finally the whole object of digestion; which is to replenish the blood by a supply of new materials from without. There are, however, two other intermediate processes, taking place partly in the liver and partly in the intestine, at about the same time, and having for their object the final preparation and perfection of the circulating fluid. These two processes require to be studied, before we can pass on to the particular description of the blood itself. They are: 1st, the secretion and reabsorption of the bile; and 2d, the production of sugar in the liver, and its subsequent decomposition in the blood.

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