312 ESSENTIALS OF PHYSIOLOGY. 



control by the vagi and the sympathetic, the former having on the 

 whole a motor and the latter an inhibitory effect. 



The periodic opening of the pylorus has been experimentally shown 

 to be under the control of a local reflex mechanism, depending on the 

 reaction of the contents in the pyloric portion of the stomach and the 

 duodenum respectively. When the reaction in the duodenum is acid, 

 the pyloric sphincter remains tightly contracted ; if, on the other 

 hand, the reaction on the stomach side of the pylorus is acid and on 

 the duodenal side it is neutral or alkaline, the sphincter relaxes. Thus 

 when acid material has passed from stomach to intestine, the pylorus 

 remains closed until the duodenal contents have been neutralised by 

 the alkalies present in the bile and pancreatic juice. When neutralisa- 

 tion has taken place, a further quantity of the gastric contents is allowed 

 to pass through the pyloric opening. The precise nature of the reflex 

 by which this is effected is still uncertain. If a quantity of water is 

 drunk, it does not excite the secretion of gastric juice, the pyloric 

 sphincter is not stimulated to contract, and, if the stomach is empty, 

 the fluid begins to enter the duodenum within one or two minutes 

 of its being taken into the mouth. 



SECTION IV. 

 DIGESTION IN THE SMALL INTESTINE. 



. If an experimental meal is given to an animal in which a fistula has 

 been made just beyond the pylorus, it is found that food begins to pass 

 from the stomach into the intestine eight to twelve minutes after the 

 meal is taken. The rate of escape of the food from the stomach is 

 indicated in the following table : 



1st hour . . . 32'6 percent. 



2nd ... 17-9 



3rd . . 29-5 



4th . . . 1-87 



5th . . 6-66 

 6th 4-21 



If the material collected in this way is analysed, it is found that 

 67 per cent, of the nitrogen is in the form of proteose and peptone, and 

 that of the starch of the meal 21 per cent, has been converted into 

 dextrin and 4 per cent, into sugar. The whole of the protein and carbo- 

 hydrate of the meal is accounted for, no absorption of these substances 

 or of fat having taken place in the stomach. The mixture of semi- 



