DIGESTION 117 



are in a concentrated form. They constitute as much as 10 per 

 cent, of the pancreatic juice. Proteins are present, if the 

 juice be fresh. If it has stood for any length of time, they are 

 found as peptones. The digestive ferments of pancreatic juice 

 are the most powerful which are secreted into the alimentary 

 canal. 



Bile. In its most important functions the liver has no rela- 

 tion to digestion. It is a storehouse of absorbed food. This 

 organ will therefore be treated in a separate chapter. The 

 bile which the liver secretes into the alimentary canal has no 

 chemical action on any of the constituents of food, with the 

 exception of a feeble tendency to digest starch. Yet it is in 

 some degree accessory to digestion. Poured into the second 

 portion of the duodenum through an orifice common to the liver 

 and the pancreas, it mingles with the semi-digested food, or 

 " chyme," which, about two hours after a meal, passes through 

 the pyloric valve. Gastric digestion has converted the greater 

 part of the proteid constituents of the food into peptones or 

 intermediate stages. The proteoses or propeptones a name is 

 needed for the intermediate products of proteid digestion which 

 does not commit us to any theory as to their chemical con- 

 stitution are quickly peptonized by the pancreatic juice. But 

 portions of the proteins have escaped the action of gastric 

 juice, or have at most been affected by its acid only ; these 

 are precipitated by the bile-salts on the mucous membrane of 

 the small intestine, which is raised into projecting flanges for 

 the purpose of delaying the passage of the chyme, in order that 

 it may be thoroughly submitted to the digestive action of pan- 

 creatic juice. Bile-salts also favour the digestion of fat, and 

 its passage through the intestinal wall. The action of bile- 

 salts in spreading fats is well known to artists. Ox-gall is 

 smeared upon glass when it is desired to apply oil-paints to its 

 surface. When mixed with oil, it causes its emulsification, 

 or breaking up into microscopic globules. In the absence of 

 bile, but little fat passes into the lymph- vessels which convey 

 digested food from the intestine to the thoracic duct, and so 

 to the great veins of the neck. Its action is mechanical. It 

 favours the digestion of fats by rendering them easily amenable 

 to hydrolysis by pancreatic juice. 



Bile as secreted by the liver is a clear, limpid fluid of low 



