the chemical properties of the gastric juice 

 and partly upon mechanical changes induced 

 by the churning action of the gastric mascu- 

 lature. 



The churning movements are confined 

 mainly to the pyloric part of the stomach, as 

 is shown in x-ray photographs of the stomach 

 during the digestion of a barium meal. The 

 barium compounds mixed with such a meal 

 are opaque to x-rays, and the barium-laden 

 chyme throws a shadow, outlining the stom- 

 ach contents on the photographic plate (Fig. 

 16-23). Thus it may be seen that during ac- 

 tive digestion the pyloric mill is very active, 



The Digestive System - 309 



while the body of the stomach remains more 

 or less passive. As digestion proceeds, how- 

 ever, the body gradually shrinks as it slowly 

 passes its contents into the actively churning 

 "pyloric mill." 



Essentially, the churning of the pylorus 

 represents a series of regularly recurrent 

 peristaltic waves that sweep toward the 

 pyloric valve. However, this valve remains 

 tightly closed until the chyme is thoroughly 

 liquid, and consequently the waves merely 

 sweep the food mass back and forth in the 

 "pyloric mill," until the valve begins to open. 

 When the chyme is thoroughly liquefied, the 



Fig. 16-23. X-ray photograph showing contraction waves in the human 

 stomach. Barium sulfate taken recently in some milk has made the stomach 

 contents visible in the photograph. Note that the peristaltic waves that churn 

 the food mass are most vigorous in the pyloric part of the stomach. A metal 

 rod marks the position of the backbone. (Couresty Roentgenology Staff, 

 Billings Hospital.) 



