THE DIGESTION OF FOOD 121 



mitted a number of these bodies to the action of several 

 diastatic ferments, show that "this variation reaches such 

 a degree that, under precisely the same conditions, cer- 

 tain of the starches require eighty times as long as others 

 for complete solution." The potato starches appear to be 

 acted upon much more rapidly than those from the 

 cereal grains. 



178. Digestibility of cellulose and gums. Other car- 

 bohydrates, cellulose and hemicelluloses, such as pento- 

 sans, galactan mannan, and related bodies, show great 

 variations in digestibility according to their source, 

 these variations ranging in observations by Swartz from 

 to 100 per cent. The extent to which these latter sub- 

 stances disappear from the alimentary canal appears 

 to be dependent on their susceptibility to attack by 

 bacteria. 



179. Digestibility of the fats. The actual extent of 

 the digestion and absorption of the fats or oils is also not 

 definitely known. If we were to accept the figures given 

 for ether-extract in tables of digestion coefficients as 

 applying to the real fats, we would believe that their 

 digestibility varies from less than one-third to the total 

 amount. It is unfortunately true that these coefficients 

 mean but very little. The ether-extract from some foods 

 is only partially fat or oil, as we have seen, and the inac- 

 curacy of a digestion trial is still further aggravated by 

 the presence in the feces of bile residues and other bodies 

 which are soluble in ether, so that the difference between 

 the ether-extract in the ingested food and that in the 

 feces does not give accurate information as to what has 

 happened to the actual fats. It seems very probable that 

 pure vegetable fats and oils and all mixed animal fats are 

 quite completely absorbed. 



