338 FOOD AND DIGESTION 



is to assist in the emulsification of the major part, a process which is favorably 

 influenced by the bile. The proper emulsification of fat is a necessary pre- 

 liminary to its absorption, for when in disease the entrance of the pancreatic 

 juice and of the bile to the intestine is interfered with, the feces contain a 

 great excess of fat. 



Some recent experiments, however, tend to prove that the entire fat of the food is 

 changed in the intestine into fatty acids and glycerin; that the fatty acids are entirely, 

 or in part, changed to soaps; and that these soaps, or mixture of soaps and free fatty acids, 

 are absorbed in solution. The chief facts favoring this view are that: (i) The action of 

 steapsin is sufficiently rapid to allow the saponification of a full fatty meal within the 

 ordinary period of digestion; (2) histological examination has never shown that fat par- 

 ticles can pass into a columnar cell, and none have ever been found in the broad striated 

 border of the cell; (3) the fat globules found in columnar cells after a fatty meal grow 

 steadily larger as the period of absorption' progresses, indicating that they are deposited 

 from solution; (4^ the fatty acids are easily soluble in bile solutions, and the solubility of 

 the soaps is greatly increased by the presence of bile. The fat constituents, according to 

 this theory, are recombined in the columnar cells to form neutral fats. 



Conditions which Influence the Action of the Pancreatic Enzymes. 

 The various pancreatic enzymes are influenced by heat, by the presence of 

 an excess of digestion products, etc., in the same way as ptyalin and pepsin. 

 Pancreatic enzymes act in a neutral but best in an alkaline solution. The 

 trypsin, strange to say, is quickly destroyed -by the alkaline solution (Bayliss 

 and Starling). The pancreatic juice offers the special case of a secretion of 

 proenzyme which is stable in alkaline solution until acted on by enterokinase, 

 and the amount of kinase present will, therefore, markedly influence the 

 amount of digestion of proteid per unit of time. 



The Secretions of the Liver. The liver, the largest gland in the 

 body, situated in the abdomen on the right side chiefly, is an extremely vascu- 

 lar organ, and receives its supply of blood from two distinct sources, viz., 



44* 



L.L. 



FIG. 269. The Liver from Below and Behind. L. S., Spigelian lobe; L. C., caudate lobe; 

 L. (?., quadrate lobe; R. L., right lobe; L.L., left lobe; g. bl., gall-bladder; v.c.i., inferior vena 

 cava; u.f., umbilical fissure; f.d.v., fissure of the ductus venosus; p, portal fissure with portal vein, 

 hepatic artery and bile-duct. (Wesley, from a His model.) 



