122 THE HUMAN MECHANISM 



small peptids and amino-acids. A similar thing is true of the 

 carbohydrate enzymes they have no action on starch nor on 

 dextrines (p. 105), but disaccharides (that is, sugars formed 

 by the chemical combination of two simple sugars, as dipep- 

 tids are combinations of two amino-acids) are readily split 

 into their component simple sugars. Cane sugar (sucrose) 

 and milk sugar (lactose) are two carbohydrate foods which 

 belong to the disaccharides ; a third is maltose, which is the 

 stage in the cleavage of starch preceding the final separation 

 into its component molecules of grape sugar (dextrose). 

 These inverting enzymes insure the complete cleavage of the 

 larger carbohydrate molecules into their component sugars, 

 precisely as erepsin insures the complete cleavage of the 

 large protein molecule into its component amino-acids or 

 smaller peptids. 



Another most important character of the intestinal juice 

 is its large content of alkaline salts, especially sodium car- 

 bonate (soda). Two processes constantly occurring in the 

 intestine produce acid ; these are (1) the splitting of the 

 fats into fatty acids and glycerin by lipase and (2) the bac- 

 terial decomposition of carbohydrates and (to some extent) 

 of proteins. The sodium carbonate of the intestinal juice, 

 which, it will be remembered, is being secreted along the 

 entire length of the intestine, neutralizes these acids arid so 

 maintains the reaction of the contents at an approximately 

 neutral point. This reaction is most favorable for the action 

 of the enzymes present. The combination of sodium carbon- 

 ate with fatty acids, moreover, forms soaps, which are more 

 readily soluble than the fatty acids. In this way no doubt 

 the products of fat digestion are more promptly absorbed 

 than would otherwise be the case. 



30. Action of the muscular coat of the small intestine. 

 The object of the movements of the intestine is not the 

 grinding down of the food into smaller masses, but, in the 

 first place, the agitation of the digesting mixture so that, on 



