130 DIGESTION 



Movements of the Stomach 



As food enters it, the stomach expands in such a way 

 that the intragastric ])ressure is not raised. That is to say, 

 the stomach does not behave as though its walls were of 

 elastic. The pressure upon its contents is the same what- 

 ever the degree of dilatation. How this remarkable effect 

 is brought about is not known. The increase in circumfer- 

 ence is more than can be accounted for by an elongation 

 of the muscle fibres. It is therefore beheved that these 

 shde over one another in some way, the layers becoming 

 fewer. 



The stomach fills up from the pylorus to the cardia. A 

 certain amount of air taken in at each deglutition is always 

 present in the fundus. 



The stomach is divided physiologically into two parts : 

 the distal part, which undergoes peristaltic contraction, and 

 the proximal part or cardiac sac, which has no rhythmic 

 movement but which exerts a constant tonic contraction 

 upon its contents. Owing to the quiescence of the cardiac 

 sac and to the mucinous nature of the food, some con- 

 siderable time elapses before the food is permeated by the 

 gastric juice. During this period, which may last as long 

 as an hour, the hydrolysis of starch by jityalin continues 

 undisturbed, being stopped only when the ferment is 

 killed by the HCl. 



Soon after the intake of food peristaltic waves appear 

 at the transverse band, travelling towards the pylorus, 

 about three waves occurring per minute. The seat of origin 

 of the waves shifts gradually backwards till it reaches the 

 middle of the body of the stomach. Each peristaltic wave 

 kneads deeper into the stomach as it proceeds, and as it 

 approaches the pylorus the longitudinal muscles contract 

 with the circular. The sudden increase in pressure thus 

 caused and the narrowness of the advancing ring causes 

 the food to be driven partly through the pylorus if the 

 sphincter allows, and partly backwards through the ring 



