DIGESTION 327 



This appears to be governed by the nature of the material 

 brought to the liver by the blood, and by the activity of the 

 liver cells. It is an example of function regulated by 

 chemical substances rather than by a nerve mechanism ; 

 although it is quite probable that these chemical substances 

 act through the rich terminal plexus of nerves which runs 

 throughout the liver. 



o. Mode of Secretion of Bile- — It has been seen that the 

 bile salts are actually formed in the liver-cells, and there is 

 ofood es'idence that the water of the bile is not a mere tran- 

 sudation but is the product of the living activity of these 

 cells. The pressure under which bile is secreted ma}' be 

 determined by fixing a cannula in the bile duct or in a 

 biliary fistula, and connecting it with a water manometer. 

 In man the pressure is as much as 20 to 80 mm. Hg, while 

 the pressure in the portal vein of the dog is only 7 to 16 mm. 

 Hg. Hence bile cannot be formed b}' a process of filtration. 



4. Nature and Functions of Bile. — Bile is not a secretion of 

 direct importance in digestion — (1) It has practically no 

 action on proteins or carbohydrates, and its action on fats is 

 merely that of a solvent. Pavlov maintains that it activates 

 the lipase of the pancreatic juice, and others have found 

 that it increases the activity of trypsin and possibly of 

 diastase, while its action on the surface tension of the 

 intestinal contents may favour the absorption of fat. It 

 may thus be considered as an adjuvant to the action of 

 pancreatic juice. (2) Its secretion in relationship to food 

 does not indicate that it plays an active part in digestion. 

 It is formed durino- intra-uterine life and durins: fastinsf, and 

 it is produced many hours after food is taken, when digestive 

 secretions are no longer of use in the alimentary canal. (3) 

 Digestion can go on quite well without the presence of bile 

 in the intestine, except that the fats are not so well absorbed. 

 (4) The composition of bile strongly suggests that it is a 

 waste product. The pigment is the result of the decompo- 

 sition of haemoglobin and the acids are the result of protein 

 disinteojration. 



