DIGESTION AND RESORPTION 95 



horse is the coecum and colon. Before the feed reaches these, 

 however, it has been acted upon by the amylases of the saliva 

 and the pancreatic juice and its starch and soluble carbohy- 

 drates pretty thoroughly extracted. Consequently, the meth- 

 ane production of the horse is substantially at the expense 

 of the crude fiber of his feed, although if starch for any reason 

 escapes digestion and reaches the ccecum it is doubtless also 

 attacked by the bacteria. 



133. The disaccharids. At first thought, it would seem 

 that the carbohydrates of this group need no digestive change, 

 since they are already soluble and diffusible and seemingly ready 

 to pass into the circulation. But while this is true, they are 

 not assimilable by the organism. Disaccharids are nowhere 

 found in the normal body fluids and if injected into the circu- 

 lation in any considerable amount are voided in the urine. In 

 other words, the disaccharids are treated in the organism as 

 foreign substances which the cells cannot use. 



In the small intestine the disaccharids are inverted, that is, 

 hydrolyzed to monosaccharids. Cane sugar taken in the food 

 appears to be inverted to some extent by the acid of the gastric 

 juice, but the principal action is by the inverting enzym sucrase 

 of the intestinal juice, which splits up the cane sugar into dex- 

 trose and levulose. Similarly, the maltose resulting from the 

 digestion of starch is split up by the maltase of the intestinal 

 juice, yielding dextrose, while lactose, at least in suckling animals, 

 is split up by lactase into dextrose and galactose. These in- 

 versions appear to take place to a considerable extent in the 

 epithelial cells lining the intestines, and this seems to be the 

 normal method of assimilation of lactose in many mature ani- 

 mals. The epithelial cells are also stated to convert levulose 

 and galactose into dextrose. 



Finally it should be added that the sugars, like other carbo- 

 hydrates, may undergo the methane fermentation in the first 

 stomach of ruminants. 



The digestion of fats 



134. Emulsification. As already indicated, the digestion 

 of fats includes two processes, namely, emulsification and 

 saponification, effected chiefly by the action of the bile and 



