THE MECHANISMS OF DIGESTION 489 



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 cardiac and pyloric parts, it has been found that after 

 half an hour the contents of both have about the same percentage of 

 sugar, but that for some time after this interval the cardiac contents 

 contain considerably more sugar than the pyloric. Later the percentages 

 of sugar again become about equal, no doubt on account of diffusion. 

 The diastatic action in the fundus is finally brought to an end when 

 the contents become completely permeated by the hydrochloric acid. 

 In this connection it is worthy of note that the addition of hydrochloric 



Fig. 157. Section of the frozen stomach (rat) some time after feeding with food given in three 

 differently colored portions. (From Howell's Physiology.") 



acid up to the point of neutrality greatly accelerates the rate of diastatic 

 digestion. 



As the outer layers of food in the stomach become partly digested on 

 account of the action of the pepsin and hydrochloric acid, the food is 

 slowly pressed into the active right half of the stomach, where by the 

 action of the peristaltic waves it is moved on to the pyloric vestibule. 

 By observing the x-ray shadows cast by two pellets of bismuth subni- 

 trate it has been noted by Cannon that, as the peristaltic wave approaches 

 a pellet, it causes it to move forward more rapidly for a short distance, 

 but soon overtakes it and in doing so causes the pellet to move back a 

 little towards the fundus. This backward movement is less than the 

 forward movement, so that after the wave has passed, the position of 

 the pellet is a little forward of that which it would have occupied had 

 there been no wave. The behavior of the pellet, and, therefore, of the 

 stomach contents, is very like that of a cork floating at the edge of the 

 sea; as each wave approaches, it hurries the cork on a little, but after 



