4 86 TEXT-BOOK OF PHYSIOLOGY 



3. The formation of urea. 



4. The conjugation of products of protein putrefaction. 



The Elaboration of Bile. The physical properties and chemical com- 

 position of the bile have already been considered (page 195). The character- 

 istic salts of the bile, sodium glycocholate and taurocholate, do not pre-exist 

 in the blood, and therefore must be formed by the liver cells out of materials 

 derived from the blood of the intralobular capillaries. The antecedents of 

 the bile salts, glycocoll and taurin, are crystallizable nitrogenized compounds, 

 and known chemically as amido-acetic and amido-ethylsulphonic acids. 

 Their chemic composition indicates that they are derivatives of the proteins, 

 though the intermediate stages in their production are unknown. The 

 origin of the cholalic acid with which they are combined is equally obscure. 

 The bile salts as they are found in the bile are produced however in the liver 

 cells by metabolic activity. 



The primary coloring-matter of the bile, bilirubin, has been shown to 

 be a derivative of hematin, a product of the disintegration of hemoglobin. 

 It is supposed that the liver cells bring about this change by combining water 

 with hematin, with the abstraction of iron. The product thus formed is bili- 

 rubin, which is excreted, while the iron is for the most part retained in the 

 liver cells. 



Cholesterin is a waste product derived largely from the nerve-tissue. 

 It is brought to the liver and simply excreted by the cells. The remaining 

 constituents of the bile, water and inorganic salts, are secreted here in the 

 same way as in all other glands. 



When once formed, the liver cells discharge these various compounds 

 into the channels by which they are surrounded; they then pass into the open 

 mouths of the bile-ducts at the periphery of the lobules. Under the increas- 

 ing pressure which arises from the secretion and accumulation of bile, this 

 fluid flows from the smaller into the larger bile-ducts, and finally is emptied 

 either directly into the intestine or into the gall-bladder, where it is stored 

 until required for digestive purposes. The secretion of bile, as observed 

 by means of a biliary fistula, is continuous and not intermittent, though the 

 rate of flow is subject to considerable variation. 



The liver cells, as far as the secretion of bile is concerned, appear to be 

 independent of the nerve system. Their activity, however, is stimulated by 

 the increased blood-supply which arises during digestion- in ^consequence of 

 the dilatation of the intestinal vessels, since it is at this period that the rate 

 of discharge is the greatest. The same results have been shown by experi- 

 ment. Thus, division of the splanchnic nerves is followed by an increased 

 discharge of bile, apparently due to the dilatation of the portal vessels; stimu- 

 lation of their peripheral ends is followed by a decreased discharge of bile 

 in consequence of the contraction of the portal vessels. The bile salts appear 

 to be the most efficient stimulants to the activity of the liver cells, for their 

 administration and absorption is followed by an increase not only in the 

 amount of water, but of the inorganic salts and other solid constituents as 

 well. 



The flow of bile from the bile capillaries to the main hepatic duct, though 

 primarily dependent on differences of pressure, is aided by the contraction 

 of the muscular walls of the bile-ducts and the inspiratory movements of the 

 diaphragm. Any obstacle to the discharge of bile leads to its accumulation, 



