THE INTAKE OF MATERIALS AND ENERGY 



69 



Digestion in the small intestine. The chyme is highly acid. Upon 

 its entry into the normally alkaline intestine, it produces two immediate 

 effects. The first is the production of pancreatic juice; the second the 

 entry into the intestine of a quantity of bile. These two reactions occur 

 in response to stimuli that are partly chemical and partly nervous. The 

 acid chyme, coming into contact with the cells of the intestinal mucosa, 

 causes a substance called secretin 1 to be liberated into the blood stream. 



esophagus 



hepatic ducts 



gall bla 



cardiac 



portion 



of stomach 



liver 



common 

 bile duct 



opening of 



common 



duct 



small intestine 

 (duodenum 



pyloric portion 

 of stomach 



pancreas 



pancreatic duct 



Fig. 4.4. Sectional diagram to show the functional relationships of liver and pancreas to 

 small intestine. 



Secretin is a hormone, 2 or chemical messenger. Carried rapidly to all 

 parts of the body by the blood, it reaches the pancreas and liver and 

 stimulates both these organs to secrete. 



1 The discoverers of secretin thought that it was represented in the mucosa cells 

 by an inactive precursor, prosecretin, and that the latter was changed into secretin by 

 the hydrochloric acid of the chyme. More recent studies, however, show that secretin 

 exists preformed in the mucosa cells, from which it can be extracted by water, alcohol, 

 and other solvents, as well as by acid. Secretin has been obtained in pure crystalline 

 form and proves to be a proteinlike substance. 



2 A hormone, or chemical messenger, is a substance which, secreted in one part of 

 the body, is capable of regulating the functions of other parts when carried to them by 

 the blood stream. 



