this difficulty exists. The proteins which are most important for the nutrition of the tissues are practically indiffusible. Therefore, it must be assumed that their presence in the lymph is due to filtration from the blood." The plasma in the capillaries is under a somewhat higher pressure than the lymph in the tissues, and this tends to squeeze the constituents of the blood, including the proteins, through the capillary walls. Waymouth Reid finds that absolutely pure proteins exert no osmotic pressure; the pressure observed is due to saline and other materials from which it is difficult to disentangle the proteins. Imbibition is the absorption of fluid by a solid body without resultant chemical changes in either. All colloids are subject to imbibition, which in many cases is impossible to distinguish from the processes of solution. As noticed before, the processes of osmosis and diffusion are most important from the physiological standpoint. There are aqueous solutions of various substances in the body separated from one another by membranes: the endothelial walls of the capillaries separating the blood from the lymph; the epithelial walls of the kidney tubules separating the blood and lymph from the urine; epithelium in all secreting glands; and lastly the wall of the alimentary canal separating the digested food from the blood vessels and lacteals. Hence, in problems such as lymph formation, the formation of urine and other excretions and secretions, and absorption of food, the laws which regulate the movements both of water and substances which are held in solution by water must be taken into account. Filtration. Osmosis is not the only force at work. Filtration must be considered that is, the forcible passage of materials through membranes, due to differences of mechanical pressure. That osmosis is not sufficient to account for the absorption of food is clearly shown by the fact which Reid has pointed out, namely, that if the living epithelium of the intestine is removed, absorption comes very nearly to a standstill, although from the purely physical standpoint removal of the thick columnar epithelium would increase the facilities for osmosis and filtra tion. Consequently, mechanical and physical factors supply no wholly adequate explanation of the observed facts of the absorption of food. There is some hitherto unexplained factor bound up in the characteristics of the living protoplasm of the epithelial cells themselves. If isotonic blood serum be introduced into the intestine the salts and water are at once absorbed, also the albumins, but more slowly. When this occurs the osmotic conditions are in balance and the pressure is greater on the side of the blood vessels so that absorption takes place with the actual expenditure of energy. The really important fact remains that the absorption through a living membrane is influenced by the membrane in ways that as yet have not been thoroughly made plain. This is the factor that determines the different rate of absorption and the so-called selective absorption in various regions of the alimentary canal. The object of this chapter is to show as far as possible how absorp tion of food in the body is carried on. No absolutely definite conclusion has been reached on this point, but the majority of those who have been engaged in physiological investigations and observations are of the opinion that it is a living and not a mechanical action. In a healthy body the skin as an origin of absorption may be excluded as a negligible quantity. The views of authorities on physiology have been largely drawn upon. BIBLIOGRAPHY ABDERHALDEN. Text-book of Physiological Chemistry. CANNON. The Movements of the Stomach Studied by Means of the Roentgen Rays, Am. J. Physiol., 1, 259. The Movements of the Intestines Studied by Means of the Roentgen Rays. Am. J. Physiol., 6, 261. Salivary Digestion in the Stomach, Am. J. Physiol., 9, 396. The Passage of Different Foodstuffs from the Stomach and through the Small Intestine. Am. J. Physiol., 12, 387. The Acid Control of the Pylorus, Am. J. Physiol., 20, 283. CHITTENDEN. The Nutrition of Man, chaps. 1 and 2. FISCHER. Physiology of Alimentation, 1907. FOLIN. A Theory of Protein Metabolism, Am. J. Physiol., 13, 117-138. GERHARDT. Ueber Darmfäulniss-Ergebnisse der Physiologie, 3, I, 107154, 1904. HAMMARSTEN. Text-book of Physiological Chemistry. HERTER. Chemical Pathology, 1902. Bacterial Infections of the Digestive Tract, 1907. HOWELL. Text-book of Physiology, 2nd ed., 1907. Text-book of Physiology, chaps. 47 and 48. LUSK. The Fate of the Amino Acids in the Organism, J. Am. Chem. Soc., 32, 671-680, 1910. OPPENHEIMER. Handbuch der Biochemie, vol. iii, part 2, 1909. SCHAEFER. Text-book of Physiology, 1898. SCHMIDT and STRASSBURGER. Die Faezes der Menschen im normalen und krankhaften Zustande, 2nd ed., 1905. STARLING. Recent Advances in the Physiology of Digestion, 1907. 153-168. CHAPTER VIII BACTERIAL DECOMPOSITION OF FOOD IN THE GASTROINTESTINAL TRACT DURING THE PROCESS OF 1 DIGESTION 1 ARTHUR I. KENDALL, B.S., PH.D., Dr.P.H. Experimental Observations of Bacterial Metabolism: Relation to Human Nature of Bacterial Metabolism; Anabolic and Catabolic Activity; Spe- Gastro-intestinal Bacteriology: In Normal Infants; Adolescents; Adults. Experiments to Determine the Nature of Bacterial Activity.-Direct observations relating to bacterial decomposition of food in the gastro-intestinal tract are few in number and, for the most part, these observations have been but incidental details in studies of material obtained from natural or artificially produced fistulæ in the human being or the experimental animal. As a general rule such material is small in amount, difficult to obtain and not collected primarily for bacteriological purposes. It is obviously impossible to reproduce experimentally, at the present time at least, those complex intestinal conditions in which alimentary enzymes, bacterial symbioses and antibioses, absorption of products of digestion and of bacterial excretion and other factors mutually react as in a normal digestive system. Relatively simple observations in vitro, where intestinal organisms are grown in the products formed from the digestion of mixtures of proteins and carbohydrates and fats by gastro-intestinal enzymes, are unrecorded. It will be seen, therefore, that available evidence relating to the bacterial decomposition of foods in the gastro-intestinal tract must be largely circumstantial; it is drawn chiefly from two sources,-first, the changes induced in sterile foods by various extracts of the gastro-intestinal tracts 1 A bibliography covering the subject matter presented herein will be found in Kendall's "Bacteriology, General, Pathological and Intestinal," published by Lea and Febiger, 1916. |