234 DIGESTION AMD ASSIMILATION OF THE FOODSTUFFS 



interface a great deal more than soaps do, and the alkaline salts of the 

 pancreatic juice, unaided by the sodium glycocholate and taurocholate 

 of the bile, are unable to bring about sufficiently rapid and complete 

 emulsification to permit digestion and absorption to occur with the 

 necessary rapidity to ensure total utilization of the fats in a meal. 

 Furthermore the bile-salts, in some way which is not yet fully under- 

 stood but which also probably arises from reduction of surface-tensions, 

 facilitate the Absorption of the fatty acid and glycerol by the intestinal 

 epithelium after the digestion of the fat has been completed. 



The emulsification of the fats enormously enhances the area of fat 

 and, therefore, the number of fat-molecules which are exposed simul- 

 taneously to the action of lipase. Thus one cubic centimeter of oil 

 floating upon the top of water in a test-tube which is one centimeter in 

 diameter will be in contact with the water. over an area of 0.785 square 

 centimeters. The same volume of oil, broken up into ten thousand 

 droplets and distributed through the water would expose a surface of 

 no less than a hundred square centimeters to contact with substances 

 dissolved in the water. Hence the droplets formed by emulsification 

 are rapidly eroded and finally consumed and converted into fatty 

 acids and glycerol by the Lipase in the pancreatic juice. 



The carbohydrate and the proteins are broken up by the digestive 

 ferments and the intestinal epithelium into their simple constituents, 

 and these are absorbed and carried to the liver as such, to be subse- 

 quently distributed therefrom over the body. The fats, on the con- 

 trary, reach the blood through the lymphatic circulation without 

 preliminary elaboration or reassortment by the liver. Corresponding to 

 this fact we find that they are thrown into the circulation, not in the 

 form of their simple components, but in the comparatively elaborate 

 form of Neutral Fat. The fatty acid and alcoholic radicals of the 

 original fat are, in fact, quantitatively recombined within the brief 

 period of their passage through the substance of the intestinal epithe- 

 lium, and the work of digestion is completely undone again before the 

 fat appears in the lacteals. 



It was inevitable that the appreciation of this fact by investigators 

 should suggest the question whether and to what extent the preliminary 

 hydrolysis of fats by lipase is essential. If the hydrolysis of fat is 

 completely reversed within so brief a period and distance, is the hydrol- 

 ysis itself a necessary prerequisite to absorption? 



Much investigation has been devoted to this problem, and as a 

 result we are in possession of a variety of results arising out of different 

 methods of attack. These results indicate that notwithstanding the 

 fact that the hydrolytic splitting of the fats is so temporary it is never- 

 theless essential. Thus fats which are not hydrolyzed by lipase, and 

 cholesterol esters, waxes and hydrocarbon oils which simulate fats in 

 their solubilities and other physical characteristics, but are not decom- 

 posed by lipase, are not absorbed to any measurable extent. Even 

 when vaseline or liquid petrolatum are administered in emulsions, 



