322 ESSENTIALS OF PHYSIOLOGY. 



pigment produced is excessive and the iron in the liver cells is increased. 

 Finally, when a solution of haemoglobin is injected into the blood 

 stream, there is an increased production of bile pigment. 



The bile pigments undergo bacterial decomposition in the large 

 intestine with the formation of stercobilin, the pigment of the fseces. 

 Some of the latter is reabsorbed, and appears in the urine as a chromo- 

 genic substance, urobilinogen, from which urobilin is formed by 

 oxidation. Urobilin itself is identical with stercobilin, and it occurs 

 in the urine in pernicious aneemia and other diseases in which destruc- 

 tion of red blood corpuscles is excessive. 



THE FUNCTIONS OF THE BILE. 



It has already been pointed out that bile is not a digestive juice 

 in the proper sense of the term. It is said to contain a weak amylo- 

 lytic enzyme, but the action of this ferment is quite insignificant. 

 Nevertheless the bile exercises important functions in connection with 

 the digestive process. (1) The acid metaprotein and proteoses result- 

 ing from the gastric digestion of proteins are precipitated by the bile 

 salts in the duodenum. This conversion of a fluid or semi-fluid 

 material into the solid condition will retard its progress along the 

 intestine and allow more time for the action of the pancreatic juice. 

 (2) The bile salts act as a "co-enzyme" to each of the principal 

 ferments of the pancreatic juice, that is, they increase the rate of the 

 digestive process without themselves taking any active part in it. In 

 the presence of bile salts the power of the pancreatic amylase to 

 hydrolyse starch is doubled, and the proteolytic power of trypsin is 

 similarly increased, while the action of lipase upon fats is quadrupled. 

 The adjuvant action of bile in digestion is due to the property which 

 the bile salts possess of reducing surface tension, as well as to their 

 property of dissolving fatty acids and soaps. By the reduction of 

 surface tension the contact of enzyme with substrate is promoted, 

 and this is of* especial value in facilitating the access of lipase to oily 

 fluids. (3) Bile promotes the absorption of the products of digestion, 

 this property also being due to the bile salts. Free fatty acids are 

 brought into solution, and in this form are more adapted for passing 

 through the epithelial cells of the intestinal villi. Moreover, these 

 cells have their surface tension lowered, and are thus made more 

 permeable by all the products of digestion. Lecithin and cholesterol, 

 which are held in solution in the bile, also play a part in promoting 

 absorption, but the precise way in which they act is not understood. 

 The importance of the presence of bile for the digestion and absorption 

 of fat is shown by the fact that when bile is prevented from entering 



