372 EXAMPLES OF CHEMICAL CORRELATION 



between the acts involved in the intake of foodstuffs and the secretory 

 activity of the glands of the gastric mucosa is, however, as the classical 

 researches of Pawlow have shown, nervous in origin, and not chemical, 

 arising in part from reflexes arising from optical and olfactory stimuli 

 and in part from gustatory and tactile impressions. The detailed 

 consideration of their mechanism belongs therefore to the domain of 

 physiology rather than to that of biochemistry. The subsequent steps 

 in the process of digestion involve, however, a very remarkable series 

 of chemical correlations. 



During gastric digestion the pyloric sphincter remains closed, and it 

 opens to permit the discharge of the stomach contents only when the 

 digestion of the proteins has attained the stage of nearly complete 

 conversion into Proteoses or Peptones. The mechanism which regulates 

 the tone of the sphincter is nervous, but the stimulus which releases 

 the reflex dilatation is chemical, and consists of the presence in the 

 lower end of the stomach of foodstuffs containing a definite excess of 

 hydrogen ions. This is very clearly shown by the investigations of 

 Cannon, who found that the period which elapses before the first open- 

 ing of the sphincter and discharge of Chyme into the intestine, is pro- 

 portional to the quantity of substances in the food which are capable 

 of neutralizing acids. Thus, solutions of sugar or starches are retained 

 for but a brief time in the stomach, but the period of their retention 

 may be enhanced very greatly by admixture with substances which 

 neutralize free acids. Meat and other dietary constituents which 

 contain proteins on the contrary are retained for a relatively prolonged 

 period. 



When the acid chyme has been discharged into the duodenum in 

 sufficient quantity to induce a certain acidity of the contents of the 

 upper part of the small intestine, the pyloric sphincter again closes in 

 accordance with the general law governing the musculature of the 

 intestine, namely, that any localized stimulus causes relaxation below 

 and contraction above the stimulated point. 



When the Chyme is being discharged from the stomach through the 

 dilated pyloric sphincter, an augmented outflow of Pancreatic Juice 

 is already travelling down the pancreatic duct to meet it. The time- 

 relations of the production of the two digestive fluids, gastric juice and 

 pancreatic juice, is illustrated by the following data obtained by 

 Pawlow. 



Time after Gastric secretion Pancreatic secretion 

 partaking of food. after 100 grams of meat. after 600 c.c. of milk. 



1 hour 11.2 8.8 



2 ' 8.2 7.5 



3 ...... 4.0 22.5 



4 \ - ... 1 . 9 9.0 



5 ........... 0.1 2.0 



the maximal secretion of pancreatic juice coinciding, in time with the 

 moment when the maximal quantity of chyme is leaving the stomach. 



