THE ABSORPTION AND USE OF FOODS 499 



will be considered in detail in a later paragraph, as will also the 

 relation of the liver to them. 



The Absorption of Fats. The result of fat digestion is to split 

 the fats to fatty acid and glycerin. It is believed that they are 

 taken up by the cells of the intestinal lining partly in this form; 

 but not wholly so, since free fatty acid in the presence of free 

 alkali, such as is furnished by the bile and pancreatic juice, reacts 

 with the alkali to form soap. That there is in the small intestine 

 a certain amount of soap formation cannot be doubted. The ad- 

 vantage of soap formation is one of increased solubility; fatty 

 acids are insoluble in water, soap quite soluble. There is reason 

 to believe, however, that only part of the fatty acid is combined 

 into soap, and that the remainder is absorbed, as stated above, 

 as fatty acid. This direct fatty acid absorption seems to be ef- 

 fected largely through the agency of the bile. It is known that 

 fatty acids are soluble in bile, and can thus be brought in solution 

 into contact with the absorbing cells; and a very common observa- 

 tion of physicians is that stoppage of the flow of bile into the in- 

 testine, as by occlusion of the bile-duct, is followed by an almost 

 complete failure of fat absorption. The glycerin part of the de- 

 composed fat is quite soluble in water and is doubtless absorbed 

 readily. 



After the absorbing cells of the intestinal wall have taken up 

 the fatty acid and glycerin, these are recombined within the cells 

 into fat. The presence of fat droplets in the absorbing cells can be 

 demonstrated microscopically. We know that the fat droplets are 

 not absorbed as such, but are formed after their constituents have 

 been separately taken up, because these fat droplets are always 

 observed in the part of the cells away from the intestinal cavity, 

 and never in the part next to it; also because we know that the 

 digestive splitting to acid and glycerin takes place, a meaning- 

 less process if not necessary to absorption. 



The fat finds its way into the circulation by way of the lymph- 

 channels of the villi, the lacteals, and the thoracic duct, entering 

 the blood-stream at the point of emptying of the thoracic duct in 

 the large vein of the shoulder. The fats alone, of all the food 

 stuffs, take this course, and we may suppose the difference to 

 mean that the liver has no special function to carry out in con- 

 nection with the fats as it has for carbohydrates and proteins. 



