FOOD AND DIGESTION 51 



aided by the mylohyoid muscles, crowds the food against 

 the roof of the mouth and, exerting pressure from before 

 backwards, forces the food under the soft palate, which 

 it drives upward to protect the back entrance of the nos- 

 trils, and over the epiglottis, which is pressed down over 

 the air passage (larynx) and the food is driven within 

 the grasp of the pharynx and esophagus over which a 

 wave of contraction passes from above downward until 

 the bolus is forced into the stomach. 



Stomach Movements. The walls of the stomach, when 

 empty, are in close contact, like any other empty bag. 

 When food enters it separates these walls only in pro- 

 portion to the bulk of the food, and closely adheres to 

 them. Hence, when the next mouthful is swallowed, it 

 does not come into direct contact with the stomach, but 

 with the coating of food which has already lined that 

 organ. Successive deposits of food will form successive 

 layers, millers' law of "first come first served" deter- 

 mining the order of digestion. It follows that the hydro- 

 chloric acid of the stomach, essential to the action of 

 pepsin but fatal to that of ptyalin, does not necessarily 

 come in contact with any but the first food swallowed; 

 and that the action of saliva on starch may continue for 

 an indeterminate time in the stomach. This result is 

 further promoted by the peculiar action of the stomach 

 muscle, which does not communicate a churning move- 

 ment to all the food contained in the organ, but seems 

 to divide into two sets of activity, one confined to the 

 large or cardiac end of the stomach and the other to the 

 small, pyloric, extremity. At the large end there is lit- 

 tle muscular action, this portion of the organ acting as 

 a reservoir for the undigested food; while, about the 

 middle of the stomach, the waves of contraction start 

 which force the food toward the pyloric end, mix it with 



