CHEMISTRY OF DIGESTION AND NUTRITION. 321 



and to the blood-vessels, the lymphatics, and the bile-ducts. The histology of 

 the liver Lobule, and the relationship of the portal vein, the hepatic artery, and 

 the bile-duct to the lobule, must be obtained from the text-books upon histol- 

 ogy and anatomy. It is sufficient here to recall the fact that each lobule is 

 supplied with blood coming in part from the portal vein and in part from the 

 hepatic artery. The blood from the former source contains the soluble prod- 

 ucts absorbed from the alimentary canal, such as sugar and proteid, and these 

 absorbed products are submitted to the metabolic activity of the liver-cells 

 before reaching the general circulation. The hepatic artery brings to the liver- 

 cells the arterialized blood sent out into the systemic circulation from the left 

 ventricle. In addition, each lobule gives origin to the bile-capillaries which 

 arise between the separate cells and which carry off* the bile formed within 

 the cells. In accordance with these facts, the physiology of the liver-cell falls 

 naturally into two parts — one treating of the formation, composition, and physi- 

 ological significance of bile, and the other dealing with the metabolic changes 

 produced in the mixed blood of the portal vein and the hepatic artery as it flows 

 through the lobules. In this latter division the main phenomena to be studied 

 are the formation of urea and the formation and significance of glycogen. 



Bile. — From a physiological standpoint, bile is partly an excretion carrying 

 off" certain waste products, and partly a digestive secretion playing an import- 

 ant role in the absorption of fats, and possibly in other ways. Bile is a con- 

 tinuous secretion, but in animals possessing a gall-bladder its ejection into the 

 duodenum is intermittent. For the details of the mechanism of its secretion, 

 its dependence on nerve- and blood-supply, etc., the reader is referred to the 

 section on Secretion. Bile is easily obtained from living animals by establishing 

 a fistula of the bile-duct or, as seems preferable, of the gall-bladder. The 

 latter operation has been performed a number of times on human beings. In 

 some cases the entire supply of bile has been diverted in this way to the ex- 

 terior, and it is an interesting physiological fact that such patients may con- 

 tinue to enjoy fair health, showing that, whatever part the bile takes normally 

 in digestion and absorption, its passage into the intestine is not absolutely 

 necessary to the nutrition of the body. The quantity of bile secreted during 

 the day has been estimated for human beings of average weight (43 to 73 kilo- 

 grams) as varying between ;j<>»> and 800 cubic centimeters. This estimate is 

 based upon observations on cases of biliary fistula. 1 Chemical analyses of the 

 bile show that, in addition to the water and salts, it contains bile-pigments, 

 bile-acids, cholesterin, lecithin, neutral fats and soaps, sometimes a trace of urea, 

 and a mucilaginous nueleo-albumin formerly designated improperly as mucin. 

 The last-mentioned substance is not formed in the liver-cells, but is added 

 to the bile by the mucous membrane of the bile-ducts and gall-bladder. The 

 quantity of these substances present in the bile must vary greatly in different 

 animals and under different conditions. As an illustration of their relative 



1 Copeman and Winston: Journal of Physiology, 1889, vol. x. p. 213; Robson : Proceedings 

 of the Royal Society, Loudon, L890, vol. 17, p. 499; Pfaff and Balch Journalof Experimental 

 Medicine, L897, vol. ii. p. 49. 

 Vol. t. 21 



