762 



Comparative Animal Physiology 



HORMONES AND GASTROINTESTINAL COORDINATION 



In the mammal the integration of the activities of various parts of the di- 

 gestive system in deahng with ingested food is in part endocrine in char- 

 acter. ^'*' ^^^' ^'^' ^^ The major hormones and their general sites of formation and 

 regions of action are shown diagrammatically in Figure 288. 



The pyloric mucosa on appropriate stimulation secretes into the blood a 

 hormone, gastrin. Non-innervated, transplanted gastric pouches are caused 

 to secrete in response to the presence of food in the stomach of the animal. 

 For a long time gastrin was believed to be identical with histamine, but it is 

 now known that histamine-free extracts of the pyloric mucosa will stimu- 

 late secretion of a highly acid gastric juice with very little peptic activity and 

 simultaneously stimulate pancreatic secretion. By fractional precipitation the 

 gastric stimulant, gastrin, has been separated from the activator of the pan- 

 creas; the latter resembles very closely the secretin obtained from the duo- 

 denal mucosa. Gastrin and gastric secretin appear to have protein-like prop- 

 erties. 



^ALLBLADDEW 



Fig. 288. Diagrammatic representation of the sources and points of action of the 

 principal hormones operating in gastrointestinal coordination. 



The duodenal mucosa quantitatively stimulates the pancreas to liberate 

 its digestive juice through action of the hormone secretin. It was formerly 

 thought that secretin stimulated mainly the secretion of water and bicar- 

 bonate by the pancreas and that the production of enzymes was nervously 

 controlled, but no consistent data were obtained to support this hypothesis. 

 The variable concentrations of enzyme in pancreatic juice that have been 

 reported to be produced in response to secretin injections appear to find 

 their explanation in terms of the presence of a second hormone in the crude 

 extract. One fraction stimulates the secretion chiefly of water and inorganic 

 salts from a denervated pancreas. A second fraction will stimulate pancreatic 

 enzyme secretion. The active factor of this latter fraction has been termed 

 pancreozymin. 



