384 A TEXTBOOK OF PHYSIOLOGY 



It is from the pigment of effete corpuscles that the bile pigments are 

 formed in the liver. In the intestine, the bile pigments are converted 

 by bacteria into the pigment stercobilin (similar to hydrobilirubin). 

 Most of this is excreted with the faeces, but some is absorbed into the 

 portal blood, in part to be excreted again in the bile, in part to give 

 rise to a urinary pigment urobilin. 



Lecithin and cholesterol have already been dealt with (Chapter VI.). 

 These substances are widely distributed in the tissues, and are of 

 great value to the organism. They are probably to be regarded both 

 as secretory and excretory products in the bile. It has been found 

 that the bile of animals which normally ingest much fat e.g., the polar 

 bear contains relatively more lecithin (and probably other phos- 

 phorus-containing bodies) than does the bile of other animals. Lecithin 

 and cholesterol play a part in the digestion of fat, helping the bile 

 salts in the solution of fatty acids and absorption of fat. 



The Inorganic Salts are chiefly sodium chloride, sodium carbonate, 

 and disodium hydrogen phosphate. There are also salts of iron (iron 

 phosphate), calcium, and magnesium. Manganese is also present in 

 minute quantity. 



The Mechanism of Secretion. While the secretion of the bile from 

 the liver is contimious, periods of greater and less activity occur. 

 Its entrance into the intestine is intermittent. The first increase in 

 flow is brought about directly the food begins to enter the duodenum ; 

 a second maximum is reached about six hours later. The bile secreted 

 in the periods between digestion is stored in the gall-bladder. Some 

 animals, however, such as the horse, elephant, donkey, mouse, have 

 no gall-bladder. The stimuli to the continuous secretion of bile are 

 at least three in number viz.: 



1. The hormone " secret in." 



2. The products of digestion. 



3. The reabsorbed bile salts. 



The flow in response to secretin is by no means so marked as that of 

 the pancreatic juice. It is stated that the products of digestion modify 

 the amount of bile excreted considerably, proteins exciting the biggest 

 flow, fats somewhat less, and carbohydrates little, if any, flow. In- 

 jected bile salts are known to act as cholagogues. The constituents 

 of the bile salts reabsorbed from the intestine probably play a similar 

 part, and stimulate an adequate secretion of bile to fill the gall-bladder 

 in the periods between active digestion. The gall-bladder and the 

 excretory passages receive nerve fibres from the vagus and sympathetic 

 nerves. The sympathetic are stated to be inhibitory, the vagus to 

 be motor, in nature. It is probably in virtue of these latter fibres 

 that bile is ejected from the gall-bladder in response to passage of food 

 through the pyloric orifice. 



The Functions of Bile. That the secretion of bile is continuous, 

 even during starvation, points to the fact that it is an excretion ; that 

 bile is poured into the beginning of the digestive 4ract, and not into 



