THE MECHANISMS OF DIGESTION 



487 



in their passage do not progress as an unvarying band of constriction but 

 exhibit alternately phases of increased and diminished tone phases of 

 contraction and relaxation. The period of gastric activity intervening 

 between a phase of complete relaxation and the reappearance of this 

 phase is termed a gastric cycle. In some types of gastric motility a 

 peristaltic wave takes but one gastric cycle to complete the journey from 

 its origin to the pylorus. Such a wave would show in consequence one 

 phase each of complete relaxation and maximal contraction. In other 

 types two, three or four gastric cycles may occur during the passage 

 of the wave. The waves therefore are spoken of as one, two, three or 



Fig. 155. Outlines of the shadows cast by the stomach at intervals of an hour each after feeding 

 a cat with food impregnated with bismuth subnitrate. (From Cannon.) 



four cycle types of peristalsis respectively. It follows from this that at 

 any moment the stomach will show one, two, three, or four waves accord- 

 ing to the particular type of peristalsis which is present. The four cycle 

 is the most common type ; the rapid one cycle type is very rare. 



The duration of a gastric cycle is from 2-3 seconds. The time required 

 for the passage of a four cycle type of peristaltic wave from its origin 

 to the pyloric sphincter therefore will be from 8-12 seconds. It rarely 

 exceeds 10 seconds. Following the cardiac analogy Cole divides the 

 gastric cycle into a systole and a diastole. In the former which occupies 

 7/10 of the cycle the constrictions produced by the peristaltic waves 

 are most marked. Diastole occupies 3/10 of the cycle and represents the 

 period during which relaxation is taking place. (Fig. 156, VII-X.) 



