§4.112 DIGESTIVE GLANDS 127 



enterogastrone. Concentrated sugar solutions, which have been 

 shown to be capable of inhibiting gastric secretion when present 

 in the duodenum", probably act in the same way. "Curiously, 

 although gastric motor inhibition by acid in the duodenum 

 depends on the integrity of the vagi, the gastric secretory inhibition 

 apparently does not. This phenomenon deserves further study" 

 (Grossman, 1950). 



To summarize the situation in mammals: when food distends 

 the stomach, gastrin causes the secretion of acid in which diges- 

 tion starts. As this acid passes on into the duodenum, aided by 

 gastrin stimulating the stomach muscles to contract, it causes 

 SECRETIN to stimulate the flow of neutralizing alkali from the 

 pancreas, and bile from the liver. Acid in the duodenum also 

 stimulates the secretion of duocrinin w^hich causes a flow of 

 enzymes from Brunner's glands. At the same time, fats entering 

 the duodenum cause enterogastrone to inhibit acid secretion 

 and reduce muscular action in the stomach. Incidentally, fats 

 also stimulate the secretion of cholecystokinin which controls 

 the muscles of the gall bladder (§ 3.112). When the products of 

 protein digestion reach the duodenum from the stomach, they 

 cause PANCREOZYMIN to Stimulate the flow of enzymes from the 

 pancreas to continue the digestion of proteins in the duodenum. 

 When the resulting peptones reach the ileum, they cause entero- 

 CRININ to stimulate there the secretion of further digestive enzymes 

 in the succus entericus. It is therefore apparent that this series of 

 hormones acts independently of any nervous stimulation to produce 

 a co-ordinated series of changes in the gut, and especially in the 

 sequence of secretions poured into its lumen, so that the phases of 

 digestion follow one another in the right order and at the right time. 



These hormones have several peculiar features in common: 

 they are not stimulated by nerves but by direct mechanical or 

 chemical stimuli; they are kinetic and appear in this case to be 

 nerve-like, in that their actions are the same as those of the 

 parasympathetic nervous system in lower vertebrates ; they are all 

 secreted by isolated cells in the gut mucosa, since no discrete 

 endocrine glands are to be found, but their histological origin is 

 unknown (§ 2.132). If the cells are endodermal, they are unlike 

 those secreting any other kinetic hormones. 



