Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/735

Rh NUTRITION 677 and still more so in free acid ; but after a time, as more food is added, and as the first -coming food begins to be dissolved and absorbed, the proportion of pepsin is in creased. The movements of the stomach mix intimately the acid juice and the alkaline food ; the first effect of this is a neutralization of the mass, but, as the secretion of juice goes on for some time after the last portion of an ordinary meal has been swallowed, the mass becomes more and more acid. The amylolytic action of the swallowed saliva is gradually checked, and the still unchanged starch remains unchanged so long as it lingers in the stomach. Such saline and other dissolved matters as are still soluble in slightly acid solutions remain dissolved, and, with the water which is swallowed, may pass at once into the blood-vessels by absorption. Other salts, as the carbonates and phosphates of the alkaline earths, become dissolved in the presence of an acid. The gelatigenous tissues con nective tissue holding together in its meshes fibres of muscle, globules of fat, strands of nerve, &c. are dissolved. The skins or pellicles of the fat-vesicles suffer the same fate. The elastic sarcolemma is now thought to be similarly affected. As a consequence the proteid and fatty portions of the organs referred to escape and fall into smaller fragments. Muscular fibres readily split into fibrillse and disks, and are then in the very best physical condition for the free play of the gastric juice. The fat escapes, flows together to form larger globules, but is otherwise unchanged. Cane-sugar, which is of no use to the organism until it is converted into maltose or glucose, finds in some slight degree the condition for this conversion in the com bined action of the gastric juice and the gastric mucus, although we are still in great doubt as to where the greatest conversion takes place. The all-important change which the food undergoes in the stomach is, however, the change of its proteid element into the more diffusible form of peptones, about which it will be enough to say that it is not a mere solution of proteid in a dilute acid, although this may be the first step in the operation, but probably a decomposition of the original proteid molecule. The obvious effect of gastric digestion is to reduce the food to a grey pulpy mass called chyme, in which condition it passes into the duodenum along with more refractory or less perfectly digested portions of food such as may still remain in the stomach at the end of the normal period of gastric digestion. The presence of food in the stomach, as has been re marked, is of itself a stimulus to the secretion of bile and pancreatic fluid ; by the time, therefore, that the chyme passes into the duodenum, a considerable quantity of the fresh juices is prepared for it. Not only so ; the contact of the acid chyme with the duodenal membrane at once brings on a reflex contraction of the ducts and gall-bladder of the liver, by which a sharp stream of alkaline fluid is at once poured out, wherewith the chyme is drenched. So far as the mixture is made perfect and the point of neutraliza tion is reached, a precipitate of parapeptones and peptones is formed, carrying down with it the active pepsin. But the acidity of the chyme is not at once overcome ; not until the middle of the small intestine is reached does the acid reaction entirely disappear ; and we may therefore assume that a kind of exotic gastric digestion may go on in the parts of chyme which still remain acid. But when ever the reaction ceases to be strongly acid the pancreatic juice takes up the work of digestion. Proteids are changed into soluble peptones, and the conversion of starch into dextrin and maltose begins again with redoubled vigour. Fats are seized upon, resolved into glycerin and their fatty acids, and emulsified both by bile and by pancreatic fluid. The resulting fatty acids combine with the alkalies of the mass to form soaps, which in turn aid the process of emulsification. The chyme, which, from being grey, be came of a golden-orange colour when saturated with bile, acquires a decidedly cream-like appearance from the emul sion that is formed. It is still an interesting question whether pancreatic proteolysis ever, in health, goes beyond the formation of peptones, as it may certainly do in artificial digestion in the laboratory. Some leucin and tyrosin undoubtedly appear in the small intestine ; but, inasmuch as they are bodies which, if formed, would be rapidly absorbed, it is im possible to say whether any considerable amount of proteid matter suffers this fate, or whether leucin and tyrosin are formed, as it were, accidentally, from the too long staying of an excess of proteid in the alimentary canal. Leucin and tyrosin are never found in the freces. As the remnant of food passes down the intestine, changes allied to putrefaction invariably occur. Lactic acid is always to be detected in the small intestine, and the amount of it increases as the ileo-oecal valve is approached. Possibly the butyric acid fermentation likewise occurs as a constant, if not an essential, phenomenon of intestinal digestion. At least the gases of the small intestine always contain a small amount of hydrogen ; but if we are to gauge the butyric acid fermentation by the amount of hydrogen detected, we must assume it to be of very small proportions. We are so ignorant of the nature of the enteric or intestinal juice that we need not here speculate as to the changes in the remnant of food which the addi tion of it may bring about. Let it suffice to say that the intestinal contents pass through the ileo-crccal valve with none of the odour and little of the appearance of fsecal matter. While in the large intestine they become reduced in bulk, and approach a solid consistency by the abstrac tion of water from them. Their reaction becomes distinctly acid once more ; but now, from inward processes of putre faction and fermentation which were started already in the small intestine, putrefactive gases may arise, light car- buretted hydrogen, carbonic acid, sulphuretted hydrogen, nitrogen, and hydrogen. The faeces themselves are com monly acid ; besides the indigestible parts of food, such as horny matter and cellulose (the denser sorts at least), and the undigested but digestible overplus of starch, proteid, &c., they contain derivatives from the bile which cause the characteristic colour, and some final decomposition products of elements of food, such as indol. II. ABSORPTION or NUTRITIVE MATTERS INTO THE BLOOD. The complex processes of digestion result in the conver sion of insoluble and indiffusible food -stuffs into soluble and diffusible sugars and peptones. These, with the soluble saline matters, the finely-divided or emulsified fats and water, are (if we except the small quantity of soaps formed in the course of pancreatic digestion, and the small amount of soluble leucin and tyrosin evolved in the same process) the only contents of the alimentary canal capable of entering the organism from the outer world. They are not indeed the only soluble and absorbable bodies in the intestine ; a large part of the digestive juices themselves are reabsorbed, and may possibly do duty over again in their respective secretions. But these are not foods. How, it may now be asked, do these soluble or finely -divided substances pass the confines of the body 1 From what has already been said about the organs of digestion it will be evident that, from the stomach downwards, the alimentary cavity is separated from an infinite number of thin-walled vessels by a delicate layer of columnar epithelium and a filmy basement membrane. There seems at first sight to be no difficulty in understanding how water and dissolved and diffusible matters may pass these barriers with the greatest readiness by physical processes of diffusion which