r 



THE MECHANICS OF DIGESTION 1007 



solid food of different color and found it later on arranged in concentric 

 strata in the vicinity of the esophageal orifice. The material ingested 

 first was pushed downward and outward toward the gastric wall, 

 while that eaten last, occupied the central extent of this space. This 

 slow whirling about of the food serves to bring its different portions 

 into more intimate contact with the walls of the fundus and, therefore, 

 also with the gastric juice. Eventually, when even its innermost mass 

 has been completely acidified, the action of the ptyalin ceases, while that 

 of the pepsin begins. As far as the mechanical function of the fundus 

 is concerned, it may therefore be said that this gastric segment acts 

 merely as a reservoir for the digestive tract. Considerable amounts of 

 food may be stored in it which are then 

 fed, hopper-like, to the pyloric mill for 

 mechanical and chemical reduction. 

 This function does not require an un- 

 usual muscular activity, because gravity 

 and the pressure exerted by the food as 

 it slowly oozes through the relaxed car 

 diac sphincter, no doubt suffice to force 

 it toward the sphincter antri pylori. 

 Later on in the course of gastric diges- 

 tion, its walls contract more forcibly in Fig. 6I2.— Section of Frozen 

 1 , , -A. 1 J J. Stomach OF Rat During Digestion 

 order to empty its more dependent por- ^^ g^^^ ^^ Stratification of 



tions, a regurgitation of the food being Food Given at Different Times. 



prevented at this time by the closure of iGrutzner.) 



the cardiac sphincter. But the pressure 



present in this compartment at the height of digestion, rarely exceeds 



6 to 8 cm. of water. ^ 



The mechanical conditions existing in the pyloric end of the 

 stomach, differ widely from those just described. Food having been 

 received, a band-like constriction appears at the sphincter antri 

 pylori which gradually increases in depth until the fundus has been 

 completely shut off from the pylorus. This circular constriction then 

 moves slowly toward the pyloric sphincter, where it arrives about 20 

 seconds later. Some time before it disappears another one develops 

 and progresses in the same direction. In this way, a number of peri- 

 staltic waves are produced which force the food toward the pylorus, 

 whence it recoils along the wall toward its starting point. As many 

 as three of these waves may be observed at one time. Thus, one may 

 just be disappearing at the pylorus, while another is still at some dis- 

 tance from it, and a third is just forming at the antrum. Although the 

 intensity and frequency of these waves vary with the time of gastric 

 digestion, they usually recur at intervals of about 10 seconds (cat) 

 and invariably proceed from the fundus toward the pylorus. Anti- 

 peristaltic movements occur only under pathological conditions. 



iKelling, Zeitschr. fiir Biol., xliv, 1903, 161; and Schlippe, Deutsch. Arch, 

 klin. Med., Ixxvii, 1903, 450. 



