The Digestive System. 39 



fore, it is readily seen that the quantity varies with 

 amount of water in the feed, in fact on the dryness of 

 the food, not on the starch to be acted upon. Its action 

 is more mechanical (i. e., for the purpose of mixing, 

 softening, etc.) than chemical. The food is thus ren- 

 dered easy of shaping into a ball (bolus), in which shape 

 it is passed backward and swallowed. This fluid also 

 aids in tasting foods, and keeps the mouth and teeth 

 clean and moist; great quantities of it descend into the 

 first stomachs of cattle and sheep, the fluid being secreted 

 as soon as food is eaten or masticated. 



Swallowing (deglutition) is quite a complicated pro- 

 cess, although seemingly simple. It may be divided into 

 three stages, during the first of which the food is under 

 the control of the will and the food is passed back into 

 the pharynx; in the second stage through the pharynx 

 it is then beyond the control of the will, being only 

 brought back by coughing; in the third stage the bolus is 

 carried over the larynx into the gullet, and by the mus- 

 cular contractions of the walls of this tube is finally 

 ejected into the stomach. As the latter stages of swal- 

 lowing are involuntary the process known as reflex action 

 is gone through as follows: A nervous stimulus is sent 

 to the brain, the result of the presence of the bolus, and 

 a message is sent back from the brain to the muscles of 

 the pharynx and gullet directing the swallowing of the 

 bolus. A horse may swallow thirty boli in fifteen min- 

 utes if very hungry, from ten to twelve if the edge has 

 been taken off his appetite. Immediately behind the 

 mouth is the cavity already spoken of as the pharynx; 

 this cavity is common to the digestive and respiratory 



