98 THE FEEDING OF ANIMALS 



and maltose, chiefly the latter. The agent which is active 

 in causing this change is an enzym (see Par. 128), to 

 which the name ptyalin has been given, and which is 

 always present in the saliva of man and of some animals. 

 It is classed among the diastatic ferments, and has an 

 ofBce similar to that of diastase in the germination of 

 seeds, viz., the transformation of starch into a sugar. 

 With man this change begins in the mouth and continues 

 in the stomach until the food becomes so acid that the 

 ferment ceases to act, for ptyalin is inactive except in an 

 alkaline medium. There is yet no reason for concluding 

 that with herbivora the saliva is as important in car- 

 bohydrate digestion as with man. Different observers 

 differ in opinion as to the diastatic value of saKva with 

 farm animals. 



The saliva also moistens the food, which is a most 

 important office, for it is a necessary preparation to the 

 act of swallowing. The saliva is not the same from the 

 different glands, that from the parotid being watery with 

 no mucin and that from the other glands being rich in 

 mucin and therefore very viscid. The former serves 

 chiefly to moisten the food while the latter aids in swal- 

 lowing. 



136. Quantity of saliva excreted. — ^With large rumi- 

 nants, the quantity of saliva required is large, as is evi- 

 dent when we remember that an ox or cow may consume 

 in one day twenty-foin- pounds of very dry hay and grain, 

 and that rumination goes on much of the time while the 

 animal is not eating. It is estimated that oxen and 

 horses secrete from eighty-eight to one hundred and thirty- 

 two pounds daily, an apparently enormous quantity 

 of liquid for organs no larger than the salivary glands 

 to supply. The extent and character of the secretion of 



