Why Digestibility Varies 123 



large amount of fiber present that they escape the 

 full action of the digestive juices. It is certain, anyway, 

 that the protein of young and tender tissues and of the 

 grains is more fully digested than that of the hays and 

 straws. 



In the case of the carbohydrates, our knowledge of 

 the relative susceptibility of the individual compounds 

 to enzym action is more definite. First of all, the nec- 

 essary modification of the sugars, which are already 

 soluble, is slight, and they are wholly digested. In the 

 second place, we have learned in two ways that the 

 starches are wholly transformed to diffusible compounds, 

 first by submitting them in an artificial way to the ac- 

 tion of various diastatic ferments, and, second, by dis- 

 covering a complete absence of starch or its products in 

 the feces of our domestic animals. In no case that has 

 come under the writer's notice has either starch or sugar 

 been found in the solid excrement. We can say, there- 

 fore, that under normal conditions the starches, like the 

 sugars, are completely digestible. 



Digestibility must be considered, however, from the 

 standpoints both of rapidity and of completeness. As 

 to the former factor, starches from unlike sources ex- 

 hibit some remarkable differences. Investigations by 

 Stone, who submitted 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 certain 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. 



