DIGESTION 320 



character and in every way is similar to the amylolytic 

 action of diastase in seeds. The slightly alkaline character 

 of saliva is necessary for the activity of ptyalin. It docs not 

 function in strong alkalies or acids. No other constituents 

 are acted upon in the mouth and not all of the starch is 

 rendered soluble. Food material, now united into a moist 

 ball, is swallowed and passes into the stomach where the 

 next change takes place. 



(b) Gastric Digestion. — In man, horses, and pigs, there is 

 but one stomach, but in the ruminants, like cattle and sheep, 

 there are four stomachs, or at least four compartments to the 

 stomach. Animals of this type "chew the cud," and food 

 passes from the mouth to the first and second compartments 

 of the stomach, is then forced back into the mouth for further 

 mastication, and then after swallowing is passed finally 

 through the third stomach into the fourth for final digestion. 

 The repeated mastication of food by these animals merely 

 serves to completely comminute the food and thoroughly 

 prepare it for digestion. In this way such animals are able 

 to digest fibrous material to a much greater extent than 

 other animals, like the horse. They can digest more crude 

 fiber and cellulose in this way because these insoluble food 

 constituents are so thoroughly separated and ground up 

 that bacteria and possibly the digestive juices can act on 

 them successfully. In the following discussion of gastric 

 digestion, it will be understood that the processes described 

 apply in the case of ruminants to the fourth stomach only. 



After the food reaches the stomach it is mixed with the 

 gastric juice, which is secreted by glands in the walls of the 

 stomach and is poured out when the food reaches the stomach. 

 The process of excretion of gastric juice is partly one of 

 response to a mechanical stimulus due to the contact of 

 food with the stomach; partly to psychic impulse caused by 

 the sight or odor of food; and partly in response to nerve 

 impulses when food is masticated. The gastric juice is 

 thoroughly incorporated with the food by to and fro move- 

 ments of the stomach. Gastric juice is a clear, colorless 

 liquid with a distinctly acid reaction, due to the presence of 

 about 0.2 per cent, hydrochloric acid. It consists largely of 



