DIGESTION AND RESPIRATION 519 



first into the rumen, where it is temporarily stored, and the cellulose it 

 contains is acted upon by the enzyme cellulose, produced there by bac- 

 teria. The food then passes into the reticulum, is afterwards regurgitated 

 and the animal ruminates, or chews its cud. The thoroughly masticated 

 and partly digested food next passes to the omasum and finally into the 

 true stomach, or abomasum. It has been postulated that this complex 

 mechanism evolved in plains-dwelling animals to permit them to feed 

 hastily when exposed to predators and to chew their food later and more 

 leisurely in shelter. The mechanism also facilitates the digestion of 

 celkdose. Vertebrates cannot digest this carbohydrate without the aid 

 of micro-organisms living in the stomach or intestine since none of them 

 can synthesize the necessary enzyme, cellulase. 



219. The Stomach 



The stomach is a J-shaped pouch whose chief functions are the 

 storage and mechanical churning of food, and the initiation of the 

 chemical breakdown of proteins. Lampreys, lungfishes and some other 

 primitive fishes do not have a stomach, and the absence of this organ is 

 thought to have been a characteristic of the ancestral vertebrates. The 

 early vertebrates, like the lower chordates, were probably filter-feeders 

 that fed more or less continuously on minute food particles that could 

 be digested by the intestine alone. Presumably the evolution of jaws and 

 the habit of feeding less frequently and on larger pieces of food required 

 an organ for the storage and initial conversion of this food into a state 

 which could be digested further by the intestine. In most vertebrates 

 both mechanical and chemical digestion begins in the stomach. 



After food enters the stomach, the cardiac sphincter at the anterior 

 end of the stomach and the pyloric sphincter at the posterior end close. 

 Muscular contractions of the stomach churn the food, breaking it up 

 mechanically and mixing it with the gastric juice. This juice is very acid 

 and eventually stops the action of the salivary enzymes, but it may take 

 40 minutes or more before there has been sufficient mixing to accom- 

 plish this. During this period the salivary enzymes continue to function 

 and it has been estimated that they will break down about 40 per cent 

 of the starch into maltose. Gastric juice is secreted by tubular-shaped 

 gastric glands, which, in mammals, contain two types of secretory cells. 

 The chief cells secrete the enzyme precursor pepsinogen. The parietal 

 cells secrete hydrochloric acid, which is required for the conversion of 

 pepsinogen into the active enzyme, pepsin. It also makes the stomach 

 contents acid. Pepsin, with a very acid pH optimum (about pH 2.0), 

 hydrolyzes proteins to large polypeptides such as proteoses and pep- 

 tones. Other proteolytic enzymes secreted by the pancreas also attack 

 intact protein molecules but preferentially split peptide bonds adjacent 

 to certain amino acids. 



Pepsin is the most important enzyme in the gastric juice, but not 

 the only one present. Rennin is particularly abundant in the stomach 

 of young mammals, and causes the milk protein casein to coagulate so 

 that it will remain in the stomach long enough to be digested by pepsin. 



