The Digestive System 143 



A few hours after a meal, the waves start up near the entrance of 

 the esophagus and sweep over the entire stomach. The food is gradu- 

 ally moved along the stomach by these waves. 



The stomach, and particularly the fundus, is to a great extent an 

 organ for the temporary storage of food. The first food entering 

 moves against the wall, forming a more or less solid mass. The next food 

 which enters forms a layer against this first mass. Gradually there 

 is a whole series of layers of food. That portion in the center is not 

 affected by the gastric juice for a considerable period, and starch di- 

 gestion by ptyalin is continued. The peristaltic waves move the food 

 along toward the pyloris which is tightly closed. This forces the 

 semiliquid food backward, resulting in movements best described as 

 "churning." These movements continue for a period of time which 

 varies with different individuals and with the nature of the food. After 

 some time, the semiliquid mass of food and gastric juice passes from the 

 stomach through the pyloric valve and into the small intestine. It ap- 

 pears that the opening of this valve is dependent upon the consistency 

 of the food and the strength of the peristaltic waves. 



Water remains in the stomach for only ten to twenty minutes, 

 while carbohydrates move out usually within three hours. Proteins 

 take a bit longer, and fats are the slowest of all. A large amount 

 of fat in a meal will delay the emptying of the stomach, giving rise to 

 the feeling of being "very full." 



Two types of movements are visible in the small intestine. The 

 peristaltic waves occur along the entire length and are responsible for 

 moving the food along. Each wave progresses for a few inches and 

 then dies out. Other movements of a churning nature, segmentation 

 movements, occur in short segments of the intestine. The food thus 

 is moved along slowly by the peristaltic wave and then churned by the 

 segmental movements. Occasionally a large peristaltic wave passes a 

 longer distance, carrying the food for a greater length. These are the 

 peristaltic rushes. These two types of movements effectively mix the 

 food with the digestive juices and move it slowly along toward the 

 colon. It takes from three to six hours for food to traverse the entire 

 length of the small intestine. 



In general, the movements of the large intestine are similar to the 

 ones of the small, that is, there are both peristaltic waves and churning 

 movements. The material entering the large intestine is very liquid, 

 and one of the prime functions of the large intestine is to absorb excess 

 water. This is important in the prevention of excessive water loss, 



