DIGESTION 425 



opportunity only for limited hydrolysis even by the amylase. For this 

 reason chemical changes in the mouth are probably neither extensive 

 nor important. 



After food is mixed with salivary amylase and swallowed, hydrolysis 

 of starch continues in the stomach until the pH drops to about 4 on 

 admixtiu'e with hydrochloric acid. While the pH is decreasing over 

 a period depending on the quantity and nature of the material eaten, 

 enzymatic action may continue for 15 to ^O minutes and convert 60 

 to 75 per cent of the starch to maltose. 



GASTRIC DIGESTION 



Voluntary initiation of swallowing carries the food over the epiglot- 

 tis and into the upper portion of the esophagus (oesophagus, gullet). 

 From this point the processes are involuntary, starting with a peri- 

 staltic wave induced by contraction of circular muscles in the esopha- 

 gus. This wave motion is preceded by a relaxation, permitting ready 

 movement of the swallow of food and extending to the cardiac sphinc- 

 ter at the junction of esophagus and stomach. The cardiac sphincter 

 is a muscular valve keeping the stomach closed and must be relaxed 

 for food to pass. Gastric digestion then begins in the stomach system 

 but takes various forms depending on the species. The three different 

 types of interest here are the single-sac stomach characteristic of most 

 mammals, including man, the gastric system of the ruminants, and 

 that of birds. 



The Single-sac Stomach 



Figure 17-1 represents a typical single-sac stomach and shows a large 

 segment connected to the esophagus and often called the cardiac 

 portion. At the other end the pyloric portion or pylorus is joined to 

 the duodenum by way of the pyloric valve or sphincter. This valve 

 controls the flow of materials from the stomach and prevents regurgi- 

 tation from the intestine back into the stomach. The large central 

 zone making up part of the cardiac portion is called the fundus and 

 possesses cells secreting the hydrochloric acid and pepsin of gastric 

 juice. 



The empty stomach is collapsed because of contraction of muscle 

 fibers in the walls. When the bolus of food reaches the cardiac portion 

 of the stomach, just enough expansion occurs to accept the volume 

 presented and to distribute the mass over the wall area. As more 



