ROLE OF LYMPH AND BLOOD 3 



postulates that the triglycerides of the short-chain acids, such as tri- 

 butyrin, are completely hydrolyzed in the gut. 



Most of the discussion and experimentation on absorption has centered 

 on the triglycerides. Until recently, it has generally been accepted that 

 the phospholipids are partially or completely hydrolyzed in the gastro- 

 intestinal tract. This concept must now be moditied in view of convinc- 

 ing evidence that a considerable proportion of the phospholipid may be 

 absorlied without undergoing hyclrol3^sis. 



The mechanism of the absorption of fatty acids, when fed as such, is 

 controversial. According to the Lipolytic Theoiy, it was believed that 

 the free fatty acids were absorbed l>y the same mechanism as were those 

 given in combination as triglycerides. The only exception is that the first 

 step in the absorption process of the triglycerides, namely that of hydroly- 

 sis, is not ref[uired. On the other hand, Frazer presented some experi- 

 mental evidence to the effect that the pathway of absorption and distri- 

 bution of the fatty acids in the tissues dilTers according to whether tri- 

 glyceride fats or the corresponding free fatty acids are fed. For a more 

 complete discussion of the digestion and absorption of triglycerides, 

 phospholipids, and fatty acids, the reader is referred to Chapters II and 

 IV of ^^olume II, The Lipids. 



3. The Role of the Lymph and Blood in the Transport of 

 Triglycerides, Phospholipids, and Fatty Acids 



Information is fragmentary as to the mechanism by which the fat parti- 

 cles pass from the epithelial cells of the intestinal wall to within the in- 

 testinal villi during the active phases of fat absorption. These epithelial 

 cells lining the gut can store only a fraction of the total lipid which may be 

 taken in with a single normal meal; it is therefore necessary for the fat 

 droplets in these cells to be transferred to the lymph stream as rapidly as 

 possible, to assure that absorption be a continuous process. 



There are several theories as to how fat travels from the interior of the 

 cell into the villus. The theor%^ of Schafer postulates that the leucocytes 

 cany out this transport by engidfing the fat droplets and then disgorging 

 the fat into the lacatels. Heidenhain suggests that the contraction of proto- 

 plasm accomplishes the transfer of the fat droplets to the lymphatics. 

 A third theoiy (Loevenhart) postulates that the neutral fat is hydrolyzed 

 at each cell boundary, and that the fatty acids and glycerol are resynthe- 

 sized into neutral fat after passage through the cell membrane. 



There are two pathways for transporting fat from the intestinal wall to 

 the liver and tissues. The first of these in\-oh-es the route via the lacteals 



