ALIMENTATION AND DIGESTION 



119 



acid of the chyme is transformed into the hormone secretin. 

 This is absorbed into the blood and chemically excites the 

 pancreas to secrete. 



The secretion of bile by the liver is continuous, although 

 it is greater at one time than at another. Circular muscle 

 fibers at the mouth of the 

 bile duct close the opening 

 into the intestine when bile 

 is not needed there ; at such 

 times the bile secreted accu- 

 mulates in the gall bladder. 

 During active digestion the 

 mouth of the bile duct re- 

 mains open and the bile flows 

 immediately into the intestine. 



Little is known of the fac- 

 tors determining the secretion 

 of intestinal juice, but it prob- 

 ably is continuously secreted, 

 at least so long as food is in 

 the intestine. Thus each con- 

 signment of chyme from the 

 stomach receives its share of 

 pancreatic juice and bile soon 

 after it enters the duodenum, 

 and then subsequently re- 

 ceives continuous additions of 

 intestinal juice as it is passed 

 along the intestinal tube by 

 the action of the muscular 

 coat presently to be described. 



27. The pancreatic juice is a strongly alkaline liquid and 

 consequently, when mixed with the acid chyme, neutralizes 

 most, if not all, of the hydrochloric acid of the chyme. Thus it 

 happens that while the food in the stomach is strongly acid, 



FIG. 58. Longitudinal section of the 

 small intestine 



The submucous coat consists of con- 

 nective tissue and contains the larger 

 blood vessels from which the mucous 

 and muscular coats are supplied with 

 blood 



