The Digestive System 141 



however, that if all the nerves to the stomach are cut, gastric juice will 

 still be secreted. This shows that there is some other type of con- 

 trol involved in this secretion. It appears that the partially digested 

 portions of protein stimulate the secretion of another material, gastrin. 

 This gastrin stimulates further secretion by the gastric glands either 

 through the blood stream or perhaps simply by its presence in the 

 immediate neighborhood. 



The understanding of this dual control of gastric secretion was 

 made possible by a series of experiments which were begun by Pavlov, 

 a Russian physiologist. Pavlov designed the ingenious Pavlov pouch 

 which is an isolated section of the stomach attached to the abdominal 

 wall. An opening is made from this pouch through the body wall, mak- 

 ing possible the collection of the gastric juices. In this way it was 

 possible to study gastric secretion. 



Soon after the acid contents of the stomach enter the small intes- 

 tine, the flow of juice from the pancreas and liver begins. Thus the flow 

 of these juices seems to be in some way associated with the presence of 

 hydrochloric acid in the duodenum. This acid stimulates the produc- 

 tion of two special substances, secretin and pancreozymin, by the duode- 

 nal mucosa. These pass into the blood stream and initiate and main- 

 tain the flow of the pancreatic juice. Pancreozymin stimulates the 

 secretion of the pancreatic enzymes while secretin stimulates that of 

 the water and bicarbonates. The presence of the undigested fats in 

 the food mass brings about the secretion of another substance, chole- 

 cystokinin by the mucosa. Cholecystokinin likewise passes into the 

 blood stream, but its action is to stimulate the emptying of the gall 

 bladder. The flow of bile from the liver is apparently increased by 

 the secretin. A fourth substance, enterogastrone, also secreted by the 

 duodenal mucosa into the blood, inhibits the flow of gastric juice and 

 stomach movements. 



The initiation of the secretion of the intestinal juice apparently 

 is likewise due to a substance liberated into the blood by the duodenal 

 mucosa. This is enterocrinin which is secreted after the acid material 

 or chyme enters the intestine from the stomach. 



The interrelationships of these various secretions and their actions 

 are illustrated in Fig. 43. 



Movements of the Digestive Tract. — Once food is swallowed, its 

 movements through the digestive tract become automatic. The muscles 

 which cause these movements are all of the smooth type. There are 



