FAT METABOLISM 721 



it is often associated with carbohydrate molecules (galactose), forming the substance 

 known as cerebrin. It may therefore have some role to play in carbohydrate metabo- 

 lism. Some workers also attribute to lecithin an important function in the transference 

 of substances through cell membranes. When mixed with water it swells up by imbibi- 

 tion, and if crystalloids or other substances are dissolved in the water, a means is 

 offered for bringing water-soluble and fat-soluble substances into intimate contact. 



DIGESTION OF FATS 



A certain amount of fat, especially when it is in an emulsified condi- 

 tion, can be digested in the stomach by the lipase contained in the gas- 

 tric juice. Most of it, however, is digested in the small intestine, into 

 which as we have seen, it is gradually discharged suspended in the chyme. 

 For this intestinal digestion of fat both pancreatic juice and bile are nec- 

 essary. This is easily shown in the rabbit, in which the pancreatic duct 

 enters the intestine at a considerable distance below the bile duct. If the 

 mesentery is inspected during the absorption of fatty food, no fat in- 

 jection of the lymphatics will be noted between the bile and the pan- 

 creatic ducts but only below the latter. In the dog, in which both the bile 

 and the main pancreatic ducts enter the intestine at about the same level, 

 fat injection of the lymphatics starts at this point, but if the bile duct 

 (or rather the gall bladder) is transplanted at some distance down the 

 intestine, it will be found that the injection of the lymphatics with fat 

 occurs only below the new point of insertion of the bile duct. 



Removal of the pancreas interferes very materially with the absorption 

 of fat. In man, for example, absence of the pancreatic juice alone di- 

 minishes the absorption of fat by 50 or 60 per cent. If the bile is also 

 absent, the diminution amounts to 80 or 90 per cent, and in such cases, 

 as is well known, the administration of bile or pancreas powder greatly 



tproves fat absorption. This interference is no doubt mainly dependent 

 >n the secretion of pancreatic juice. Pratt, McClure and Vincent, 48 for 

 example, found that completely depancreated dogs could still absorb con- 

 siderable quantities of fat. On the other hand, the internal secretion of 

 the pancreas has a considerable influence on fat metabolism as is evi- 

 denced by the fact, observed by us, that there is usually 10-12 per cent 

 of total fatty acid in the livers of depancreated animals but only a small 

 amount after the animals have been treated with pancreatic extract 

 (insulin, page 717). 



As to the relative roles of pancreatic juice and bile in the digestion of 

 fat, we know of course that in the pancreatic juice there exists a lipolytic 

 enzyme, lipase, which, under suitable conditions has the power of split- 

 ting neutral fat into fatty acids and glycerol. If bile is examined, no 

 lipolytic enzyme will be found in it. It is entirely inactive on fat, but 



