92 NUTRITION OF FARM ANIMALS 



130. Hemicelluloses. What is true specifically of the 

 pentosans appears to hold also for the reserve carbohydrates 

 of the cell wall the so-called hemicelluloses (18) as a 

 whole. No animal enzyms are known which hydrolyze the 

 galactans, levulans, etc., or which break up their union, if it 

 exists, with the pentosans, but nevertheless these substances 

 disappear in part in the digestive tract of herbivora. Pending 

 more exact knowledge on this point, the assumption seems 

 warranted that they as well as the pentosans undergo bacterial 

 fermentation and yield substantially the same products. 



131. Starch. The first agent to act upon starch is the 

 ptyalin of the saliva (115). As is the case with the other 

 enzyms, ptyalin has never been isolated, but its effects and the 

 conditions governing its action have been extensively studied, 

 in part owing to the ease with which saliva can be procured. 

 The most important of these conditions are that ptyalin acts 

 most efficiently in the neighborhood of 40 C., that is, at about 

 blood temperature, in a neutral solution, while more than a 

 trace of free acid or alkali inhibits its action. To acids or al- 

 kalies combined with proteins, even though they show an acid 

 or alkaline reaction to ordinary indicators, ptyalin is much less 

 sensitive and it is also less sensitive to organic than to inorganic 

 acids. In brief, the action of ptyalin is inhibited by a very low 

 concentration of either hydrogen or hydroxyl ions. 



The action of ptyalin on starch consists of a succession of 

 cleavages and hydrations resulting in the formation of the various 

 dextrins (24) and finally of sugar. With cooked starch, the 

 action is fairly rapid ; upon raw starch ptyalin acts more slowly, 

 the rate varying somewhat with the kind of starch and being 

 apparently determined by the degree of resistance of the cellulose 

 envelope of the starch grains. Chemically, the action is analo- 

 gous to that of acids, but is less vigorous and is not carried so far. 

 The action of acids yields dextrose as a final product ; that of 

 ptyalin is usually stated to stop with the production of maltose. 1 



The action of ptyalin in the mouth is necessarily very brief. 

 In the stomach the feed comes into contact with the gastric 

 juice containing free hydrochloric acid. At first, this acid 

 combines with the proteins contained in the feed, but as soon 



1 Carlson and Luckhart (Amer. Jour. Physiol., 23 (1908-9), 149) state that both 

 ptyalin and amylopsin produce dextrose from starch. 



