Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/736

Rh 678 NUTRITION are so simply imitated in the laboratory. We know also that, in fact, solutions do with the greatest ease and rapid ity pass through such a membrane as the conjunctiva, by the readiness with which the pupil may be dilated on putting a drop of atropinized fluid beneath the eyelid. But it is by no means clear that the process is so simple in the living intestine. A fresh specimen of epithelium scraped from the interior of the mouth of a pig may be bathed in a solution of colouring matter and yet admit no trace of the colour into the substance of its cells so long as they remain alive. It is only when they cease to be living protoplasm that the physical processes of diffusion come into play and that the cell-substance takes up the colour. Guided by this simple observation we shall hesitate to assume, because we have water containing soluble sugars, salts, and peptones on one side of the epithelium of the alimentary tract, while on the other we have fluids differ ently constituted, that therefore we must needs have a process of transfusion tending to the passage of the dis solved substances from one side to the other. This &quot;absorption&quot; of matters, even simple saline solutions, at the surface of the intestine may be, and most probably is, a vital operation. Be the nature of the process what it may, it is easy to conceive how fluids at least arrive at the interior of the blood and the lymph capillaries of the alimentary mucous membranes. The case of the particles of emulsified fat presents more difficulty ; but even here there are only two possibilities open to us. Either the fine globules pass through the columnar cells, or they pass between them ; if they pass through them they must traverse the thick striated inelastic end of the cells in a manner which it is somewhat difficult for us to picture ; if they only pass between the cells it is difficult to explain why so many of the acutest observers have described the occur rence of fat-particles within the epithelial cells of recent preparations of intestinal mucous membranes. Once through the epithelial layer the absorbed matters pass into the blood-vessels or the lymphatics in some way the details of which are as yet mere matters of speculation, The axial lymphatic vessels of the villi the so-called lacteal radicles have a special interest, since villi are pre-eminently the absorbent rootlets of the body, As soon as they become filled, the layer of muscular fibres investing them contracts and empties the contents of the lacteal towards the deeper vessels of the mucosa. The subsequent expansion of the radicle will evidently favour the refilling of it. It is very disputable where absorption takes place, The filtrate obtained from the contents of the stomach yields very little peptones, the inference being that much has already become absorbed by the gastric capillaries, haemic or lymphatic. There seems to be no reason why water, salines, and dissolved sugars should not also be taken up at the same place. In the large intestine water at least is absorbed, for the contents become drier as they pass along the bowel. But it is the small intestine chiefly which is the scene of most extensive absorption of digested products ; here fats are demonstrably taken into the organ ism ; and here, doubtless, more than elsewhere, peptones, sugars, and other soluble bodies suffer the same fate. Blood-vessels and lymph-vessels are both sharers of the work, though to what extent each is employed is a matter of surmise rather than of observation. The lacteals cer tainly absorb the largest part of the fat, though, as the relative amount of fat in the portal vein increases after a meal, fat must aho be absorbed by the blood-capillaries, The material absorbed into the portal vein is at once submitted to the smaller circulation of the liver and the activity of the liver -cells ; that absorbed into the lacteals traverses many lymphatic glands before it reaches the thoracic duct and the general circulation at the root of the neck. Thus in both ways of absorption the raw material is immediately passed into certain organs before it reaches the common stock of blood. That changes are effected in the constitution of the just-received food-stuffs during their stay in these organs there is every reason to believe ; for, on the one hand, the blood issuing from the liver in the hepatic veins differs from the blood entering it by the portal vein ; and, on the other, the contents of the thoracic duct differ from the contents of vessels nearer the absorbent radicles, as, for example, the contents of the mesenteric lacteals. The nature of these changes must be discussed in the next section, III. CHEMICAL PKOCESSES IN THE TISSUES AND OEGANS OF THE BODY. While it is probable that the liver modifies the recently- digested raw material of food before it reaches the common stock of the blood, it is quite unknown what (or, indeed, if any) difference exists between the action of the liver on blood laden with raw material immediately after a meal and its action on blood traversing its capillaries during a fast. Does the liver exert an action on recently-imported fat, peptones, and sugar in any sense different from the action it exerts on fatty, albuminous, and sugary matters as they exist in the common stock of blood ? Or which is the same question do the fats, albuminous matters, and sugars of the portal blood differ at all from the fats, albuminous matters, and sugars of ordinary blood ? While this question remains unanswered it will be well to consider the liver as like any other tissue drawing on the common source of nutriment, the blood, for its own particular pur poses, and not as an organ akin to the special digestive organs, devoted to the elaboration of food for the benefit of the other tissues. It need not be so, however, with the lymphatic struc tures with which the chyle of the lacteals comes into con tact before it is poured into the blood, Chyle from the thoracic duct at its entrance into the veins is of course mixed with the general lymph of the body, the juice of the tissues which is collected in the lymph -vessels and carried back to the blood. It is a milky fluid which coagulates on standing, the clot of which after some time becomes tinted red at the surface from the presence of immature red corpuscles. The coagulum consists of fibrin resembling that of the blood, Other constituents of chyle are white corpuscles, oil-globules coated with albuminous matter, i.e., emulsified, and exceedingly fine fatty granules usually spoken of as the &quot;molecular basis of chyle.&quot; Chyle obtained from vessels nearer the intestines has very little fibrin, very few white corpuscles, and no red cor puscles, It is probable, therefore, that the raw matters of digested food are undergoing a process of manufacture into blood during their passage through the lymphatic glands to reach the thoracic duct. When once they arrive at the blood, the imported mate rials of food are lost beyond our power to follow them individually. The question now becomes one of the inter changes between the blood generally and the tissues. That such an interchange occurs there can be no doubt ; for if all food be withheld from an animal the tissues rapidly grow less in quantity, while the blood maintains a fairly constant composition. If such an animal be fed, the tissues regain their former weight, and may even store up an overplus of matters, while again the blood remains of approximately constant composition. The tissues can both take away from the blood and give to the blood such matters as are necessary. But the matters taken from the blood are not in the same form as the matters given up to the blood. When blood is made to circulate through living tissues of