490 



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, the origin of the biliary salts, the biliary pigments, 

 cholesterol and lecithin, have already been considered (see page 196). 



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. See page 196. 



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, 

 a rise of pressure beyond that of the capillary blood-vessels, and a reabsorp- 

 tion by the lymph-vessels of the bile constituents. . After their discharge into 

 the blood from the thoracic duct the pigments are deposited in part 

 in various tissues, giving rise to the phenomena of jaundice, and in part are 

 eliminated in the urine. 



The Production of Starch (Glycogen) and Sugar (Glycose or Glucose) . 

 In 1857 Bernard discovered the fact that the liver normally during life 

 produces a substance, analogous in its chemic composition to starch and 

 known as liver starch or animal starch. This substance can be obtained by 

 the following method: Small pieces of the liver of an animal recently killed, 

 preferably after a meal rich in carbohydrates, are placed in acidulated boiling 

 water for a few minutes; then rubbed up in a mortar with sand, again boiled, 

 after which the proteins are removed by filtration. The filtrate thus obtained 

 is opalescent and resembles a solution of starch. The starch may be 

 precipitated from this solution with alcohol. It may subsequently be ob- 

 tained free by drying, when it presents itself as a white amorphous powder, 

 soluble in hot or cold water. Chemic analysis shows that it consists of 

 C fl H 10 O 5 , or a multiple of it. 



When either the original solution obtained by boiling or a solution of 



