454 DIGESTION 



the waves, so that several of these are always seen on the stomach at 

 the same time. They sometimes become so pronounced in the pyloric 

 region, especially in a half-empty stomach, that they appear almost to 

 obliterate the cavity. They always stop at the pylorus, never going on 

 to the duodenum. The rate of recurrence of the waves varies somewhat 

 in different animals, being about six per minute in the cat and about 

 three in man. Their initiation does not seem to depend on the presence 

 of acid in the gastric contents, for, when food is introduced into the 

 stomach, they do not wait for the gastric contents to become acid in 

 reaction (see page 482). Nevertheless, acid does seem somewhat to stim- 

 ulate the depth and frequency of the waves, and they recur oftener with 

 carbohydrate than with fatty food. 



The pressure in the stomach contents — the intragastric pressure — is 

 low and constant at the cardiac end and fairly high and variable in the 

 pyloric end (in the former from 6 to 8 cm. of water, and in the latter 

 from 20 to 30). Constancy of pressure in the cardiac end indicates 

 that the stomach wall must adapt itself very promptly to the amount of 

 food in the organ. The higher and more variable pressure in the pyloric 

 end is, of course, due to the peristaltic waves, and it is interesting to note 

 that it is sufficient to propel the gastric contents through the pylorus for 

 several centimeters into the duodenum. 



The Effect of the Stomach Movements on the Food 



This has been studied: (1) by dividing the food into portions that 

 are differently colored and, after some time, killing the animal, freezing 

 the stomach and making sections of it (see Fig. 157) ; (2) by mak- 

 ing little pellets of bismuth subnitrate with starch and observing their 

 behavior under the x-rays; or (3) by removing samples of the stomach 

 contents by means of a stomach tube (Rehfuss tube) inserted so that 

 its free end lies in either the cardiac or the pyloric region. By the 

 first of the above methods it has been found that the first mouthfuls 

 of food lie along the greater curvature, where they form a layer over 

 which that subsequently swallowed accumulates, with the last por- 

 tions next the cardia. The pepsin and hydrochloric acid of the car- 

 diac end. therefore, act soonest on the first swallowed portion of a 

 meal, and the more recently swallowed central masses are not affected 

 by the secretions for some time, so that opportunity is given for the 

 saliva mixed with the food to develop its digestive action. 



As has been shown by removing the stomach contents with a tube at 

 various periods after feeding with starchy food, considerable amylolysis 

 may occur for some time. "When separate samples are removed in this 

 way from the eardiae and pyloric parts, it has been found that after 



