690 METABOLISM 



important function in the transference of substances through cell mem- 

 branes. When mixed with water it swells up by imbibition, and if crys- 

 talloids or other substances are dissolved in the water, a means is offered 

 for bringing water-soluble and fat-soluble substances into intimate con- 

 tact. 



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 l)ile 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. 



Eemoval 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 

 improves fat absorption. In the dog, although ligation of the pancreatic 

 duct apparently only slightly influences fat absorption, removal of the 

 pancreas itself greatly interferes with the process; from which fact some 

 observers have concluded that the pancreas, in addition to its external 

 secretion into the intestine, must produce an internal secretion into the 

 blood which has something to do with the efficient absorption of the 

 fat (Pratt, McClure and Vincent' 48 ). It is, however, improbable that such an 

 hypothesis is necessary, for it is very likely that the moribund condi- 

 tion into which an animal is brought by extirpation of the pancreas, 

 adequately accounts for the suppression of the fat-absorbing function. 



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 glycerine. If bile is examined, no 

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



