120 THE FEEDING OF ANIMALS 



Some proteins are pratically all digested by artificial 

 methods, and probably are in natural digestion. It is 

 known definitely that protein is much more completely 

 dissolved from some foods than from others. That of 

 milk and meat is practically all digestible, that of some 

 grains very largely so, while with the coarse foods quite a 

 large proportion escapes solution. Whether this is due to 

 differences in the characteristic protein compoimds of the 

 various foods is not quite determined. The fact that 

 highly fibrous materials show the lowest proportion of 

 digestible protein suggests as an explanation that the 

 nitrogen compounds of plant tissue are so protected by the 

 cell-walls that they escape the full action of the digestive 

 juices. It is certain, however, that the protein in plant 

 tissue is less fully digested than that from milk and 

 meat products. 



176. Digestibility of the carbohydrates. — ^In the case 

 of the carbohydrates, our knowledge of the relative sus- 

 ceptibility of the individual compounds to enzym action 

 is more definite. First of all, the necessary 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 hydrolized, first 

 by submitting them in an artificial way to the action of 

 various diastatic ferments, and, second, by discovering 

 a complete absence of starch or its products in the normal 

 feces. Under normal conditions the unprotected starches, 

 like the sugars, are completely digestible. 



177. Starches unlike in rate of digestibility. — ^Digesti- 

 bility must be considered, however, from the standpoints 

 both of rapidity and of completeness. As to the former 

 factor, starches from unlike sources exhibit some remark- 

 able differences. Investigations by Stone, who sub- 



