326 VETERINARY PHYSIOLOGY 



not yield sugar on boiling with an acid and which contains 

 phosphorus. 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). 



(6) 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, 

 and later 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 the 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 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 ettect, 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 delayed. 



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

 made leaving the opening of the bile duct in the mucous 

 membrane of the intestine, that an expulsion of bile follows 

 the taking of food ; and Starling finds that the flow of bile 

 is increased by the injection of secretin. It thus tends to 

 run parallel with the flow of pancreatic juice. 



Influence of Nerves upon the Flow of Bile. — (a) Expulsion 

 of Bile. — There is good evidence that nerve fibres pass to 

 the muscles of the bile passages and that they ma}- cause an 

 expulsion of bile by stimulating them to contract. 



(h) Secretion of Bile. — There is no convincinsf evidence 

 that nerve fibres act directly upon the secretion of bile. 



