692 METABOLISM 



teals into the villi, and then by way of the lymphatics to the thoracic 

 duct. 



The chemical explanation of the absorption of fat is very different from 

 that formerly held by histologists who maintained that the fine particles of 

 emulsified fat in the intestine penetrate by a mechanical process through 

 the striated border of the epithelial cell into its protoplasm. The histologic 

 evidence for this view seemed very convincing, for fine fat globules can 

 readily be seen in the epithelial cells of the intestine after fatty food 

 has been taken, while they are absent during starvation. These par- 

 ticles seemed to have passed directly from the intestinal canal into the 

 epithelial cells because, when the fat was stained with characteristic fat 

 stains before feeding it to the animal, the globules in the epithelial cells 

 were found to be similarly stained. The supporters of this mechanistic 

 view of fat absorption maintained that the appearance of the stained fat 

 globules in the epithelial cells could not be explained in any other way 

 than by supposing that the fat globules had wandered unbroken into 

 the epithelial cells. Such a conclusion is, however, unwarranted, for the 

 stains that are soluble in fat are also soluble in soap, so that when the 

 fat splits up, the stain will remain attached to the soap and be carried 

 along with it into the intestinal epithelium. 



Absolute proof that the chemical theory is the correct one has been 

 supplied by a large number of experiments. The following may be 

 cited: (1) When the lymph flowing from the thoracic duct is examined 

 after feeding with fatty acids instead of neutral fat, it is found to contain 

 only neutral fat, indicating that a synthesis must have occurred between 

 glycerine and fatty acid during the absorption. The glycerine for this 

 synthesis is furnished from sources which will be described later. (2) 

 Wlien an emulsion made partly of neutral fats and partly of some hy- 

 drocarbon, such as albolene, is fed and the feces are examined for these 

 substances, it has been found that all the fat but none of the hydrocar- 

 bon is absorbed; the feces contain all of the albolene but none of the fat. 

 This experiment supplies very strong evidence against the mechanistic 

 theory, for microscopic examination of the above described emulsion 

 shows. the particles of neutral fat and hydrocarbon to be of exactly the 

 same size. (3) By examining the properties of the fatty substances in 

 the thoracic lymph collected during the absorption of such an emulsion 

 as that described above, nothing but neutral fat has been found present. 

 (4) Similar results are obtained when wool fat, which is an ester of 

 cholesterol and fatty acid, is fed. 



We may conclude that fatty substances which are insoluble in ivater or 

 can not be changed by digestion into substances (soap) that are soluble 

 in water y are not absorbed, however like fat they may be in other particulars. 



