DIGESTION OF THE CARBOHYDRATES 233 



ing the admixture of pancreatic juice with the foodstuffs, are again 

 seen to be clear and free from fat, while immediately below the new 

 point of entry of the bile-duct into the intestine active absorption 

 of fat is evidenced. Moreover, partial or total occlusion of the bile- 

 ducts through catarrhal conditions or tumors is not uncommon, and 

 this invariably leads to very defective absorption of fat, a large 

 proportion of unabsorbed fat passing into the feces, even when the 

 discharge of pancreatic juice into the intestine has not been interfered 

 with at all. 



Two separate factors are therefore essential for the proper absorp- 

 tion of fats, namely the bile and the pancreatic juice. The fat-split- 

 ting enzyme, Lipase, is contained in the pancreatic juice and not in the 

 bile. The essentiality of the bile in this process arises not from any 

 power of digesting fats which it possesses itself, but from the facili- 

 tation of the digestion of fats by pancreatic juice which it brings about. 



The fats differ from the other nutritive constituents of the diet in 

 their insolubility in water. The enzyme, lipase, which accomplishes 

 their digestion is, however, not only soluble in water, but secreted and 

 poured into the intestine in a watery medium. To secure contact of 

 these substances of diverse solubilities some special mechanism is 

 required and this is supplied by the Emulsification of the fat, partly 

 by the alkaline carbonates contained in the pancreatic juice and the 

 bile, but especially by the Bile-salts, sodium glycocholate and tauro- 

 cholate. 



By the action of alkalies and alkaline salts upon partially hydrolyzed 

 fat containing a little fatty acid, Soaps are formed, by combination 

 of part of the base in the alkaline salt with the fatty acid. 



Na 2 C0 3 + CnHssCOOH = NaHCOs + CnHfcCOONa 

 Sodium carbonate Stearic acid Sodium bicarbonate Sodium stearate. 



The presence of a small amount of soap facilitates the formation 

 of an emulsion of fat with water because the soap tends to collect in 

 a film at the surfaces of the oil-droplets and impedes their coalescence 

 into larger drops. The concentration of the soap at the surfaces 

 of the droplets is brought about by the fact that they lower the Sur- 

 face-tension of the oil-water interface, so that, a film having once 

 been formed, if a discontinuity should appear in it, the surface of 

 oil which is exposed will have a greater tension than the surface of 

 soap which covers the remainder of the droplet. The effect of this is 

 to cause the exposed surface, where the film is broken, to contract 

 more forcibly than the remainder and thus pull the edges of the film 

 together again. 



The emulsification of the fats in the foodstuffs is thus initiated by 

 the alkaline carbonate in bile and pancreatic juice, forming soaps 

 with the trace of fatty acid arising from lipolytic action of the gastric 

 juice. The emulsifying-power of the soaps is, however, far inferior 

 to that of the bile-salts, which reduce the tension of the oil-water 



