216 THE PRENTIPLES OF AGRICULTURE 



under the ears. They discharge their secretions 

 into the mouth. In grain- eating birds, similar 

 glands surround the crop,— an enlargement of 

 the gullet in the region of the neck. 



380. A ferment (ptyalin) in the saliva acts 

 on the starch in the food, causing it to chemi- 

 cally unite with additional water and become 

 transformed into sugar. Eaw starch is insoluble 

 in water, and cannot pass into the circulation ; 

 but the sugar formed from it is freely soluble, 

 can be readily absorbed into the blood, and 

 contributes to the activity, growth and nourish- 

 ment of the body. 



381. The ptyalin acts slowly on raw starch, 

 and much more rapidly on boiled starch, so that 

 cooking of vegetable food favors its digestion. 

 It acts best in the absence of acids. It is 

 less active when weak organic acids are present, 

 and its action is arrested in the stomach by the 

 free mm-iatic or hydrochloric acid. 



382. In animals with one stomach, therefore, 

 it is important that the food should be thor- 

 oughly masticated and saturated with saliva, and 

 not bolted whole, or imperfectly insalivated. In 

 ruminants (or cud-chewing animals), as cattle, 

 sheep and goats, the food is long delayed in the 

 drst three stomachs, in which any slight som*- 

 ness which may exist is due to mild organic 

 acids only ; and, therefore, there is ample 



