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FOOD AND DIGESTION 369 



the presence of a mucin-like body, which, however, does not 

 yield sugar on boiling with an acid and which contains phos- 

 phorus. It is precipitated by acetic acid, but the precipitate 

 is soluble in excess. It is therefore a nucleo-protein. In 

 some animals a certain amount of mucin is also present. 

 (Chemical Physiology.) 



Inorganic Constituents. The most abundant salt is calcium 

 phosphate. Phosphate of iron is present in traces. Sodium 

 carbonate, calcium carbonate, and sodium chloride are the other 

 chief salts. 



2. Flow of Bile. The bile, when secreted by the liver cells, 

 may accumulate in the bile passages and gall bladder to be 

 expelled under the influence of the contraction of the muscles 

 of the ducts or of the pressure of the abdominal muscles upon 

 the liver. The flow of bile into the intestine thus depends 

 upon 1st, The secretion of bile; 2nd, the expulsion of bile 

 from the bile passages. It is exceedingly difficult to separate 

 the action of these two factors. The flow of bile in the human 

 subject has now been studied in several cases in which the 

 surgeon has had to make a fistula into the gall bladder through 

 which all the bile secreted escaped and could be collected. 



The flow of bile begins in intra-uterine life before the 

 twelfth week, and it continues without intermission through- 

 out the whole of life, even during very prolonged fasts. 



The taking of food increases the flow of bile, and the extent 

 to which it is increased depends largely on the kind of food 

 taken. In the dog a protein meal has the most marked effect, 

 a fatty meal a less marked effect, and a carbohydrate meal 

 hardly any effect. The increased flow of bile following the 

 taking of food does not reach its maximum till six or nine 

 hours after the food is taken, and some observers have found 

 that the period of maximum flow is even further prolonged. 



Pavlov found in dogs, in which a biliary fistula had been 

 made leaving the opening of the bile duct in the mucous mem- 

 brane of the intestine, that an expulsion of bile follows the 

 taking of food and the secretion of pancreatic juice, and 

 Starling finds that the flow of bile is increased by the in- 

 jection of secretin. It thus tends to run parallel with the flow 



of pancreatic juice. 



24 



