DIGESTION 245 



much more forcible and active than those of the ftmdus. 

 It is here that the food is mixed with the gastric juice, 

 rubbed down to a more or less fluid consistency, and 

 gradually expelled into the duodenum. The mechanism 

 by which this takes place is as follows (Cannon) : Whilst 

 food is present in the stomach constriction waves are 

 seen continually coursing over the antrum towards the 

 pylorus. The fundus meanwhile serves as an active 

 reservoir for the food, and squeezes out its contents 

 gradually into the pyloric portion. The stomach is 

 emptied by the formation, between the fundus and 

 antrum, of a tube (' pre-antral ' or ' middle ' portion, in 

 Fig. 16), along which constrictions pass* at regular 

 intervals of fifteen to twenty seconds. The contents of 

 the fundus are pressed into the tube, and the tube and 

 antrum slowly cleared of food by the waves of con- 

 striction. The food in the pyloric portion is first pushed 

 forwards by the running wave, and then by pressure of 

 the stomach wall is returned backwards through the ring 

 of constriction, being thus thoroughly mixed with gastric 

 juice. Finally, when the solid food has been thoroughly 

 triturated by the constrictions, the pylorus opens and 

 allows the contents of the antrum to escape. 



It will be readily understood from this description that 

 the stomach is much more likely to be affected by 

 mechanical injuries at its pyloric end than at the fundus, 

 and it is perhaps for this reason that this portion is most 

 often the seat of an ulcer. Seeing, too, that the pressure 

 is highest at this end of the organ, it is important in 



* The waves of constriction can be seen very well, in cases of 

 congenital pyloric stenosis, coursing from left to right. 



