ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 289 



dium silicate to rabbits and young dogs and was able to re- 

 cover the salt from the urine in exceedingly small but never- 

 theless distinct amounts. This means, of course, that the 

 colloidal sodium silicate diffused through the gastro-intestinal 

 wall. 



There is a difference, however, between asking whether fat 

 can pass as such through the intestinal mucous membrane 

 and whether under ordinary circumstances this is the way 

 in which it is absorbed. ' There seems to be little doubt that 

 even if fat can be absorbed as such, most of it passes through 

 the intestinal wall (and from tissue to tissue) in the form of 

 its soluble digestion products. As evidence of this^we may 

 quote the experiments of CoNNSTEiN. 1 If the essential change 

 necessary for the absorption of fats lay in their conversion 

 into an emulsion in the animal body, then it would be reason- 

 able to expect that a fine emulsion of one fat should be 

 absorbed as rapidly as an emulsion of any other fat provided 

 it were equally finely divided. An emulsion of lanolin ought, 

 therefore, to be absorbed as readily as an emulsion of butter- 

 fat. As an actual matter of fact, CONNSTEIN found that when 

 he fed a dog with a lanolin emulsion in water 97J percent of 

 the entire amount fed could be recovered from the faeces. 

 The formation of an emulsion from the fat is, therefore, only 

 of secondary importance in the absorption of this foodstuff. ' 

 The real reason why lanolin is not absorbed in the above ex- 

 periment lies in the fact that it is acted upon only exceedingly 

 slowly by the fat-splitting enzymes of the digestive tract, 

 and hence is not converted into the absorbable products of 

 fat digestion. 



The recognition by KASTLE and LOEVENHART, and inde- 

 pendently of them by HANRIOT, that the action of lipase is re- 

 versible, has altered entirely our conception of the mechanism 

 by which fat is absorbed from the intestinal tract. 2 The belief 



1 CONNSTEIN: Archiv fiir (Anat. u.) Physiol., 1899, p. 30. 



2 See p. 151, and LOEVENHART: American Journal of Physiology, 1902, 

 VI, p. 332. 



