298 THE TISSUES AND MECHANISMS OF DIGESTION. 



When the stomach is empty all the muscular fibres, as we have said, 

 longitudinal, circular, and oblique, fall into a condition which we may per- 

 haps speak of as an obscure tonic contraction. The whole stomach is'small 

 and contracted, its cavity is nearly obliterated, and the mucous membrane, 

 owing to the predominance of the circular coat, is, like the lining membrane 

 of an empty artery, thrown into longitudinal folds. As more and more food 

 enters the stomach all the coats become relaxed, with the exception of the 

 pyloric sphincter, which remains at first permanently closed, and the less 

 marked cardiac sphincter, which merely relaxes from time to time at each 

 act of swallowing. No sooner, however, do the coats thus become relaxed 

 than they set up obscure rhythmical peristaltic contractions, giving rise to 

 the "churning" movements. These movements have been described as of 

 such a kind that the contents flow in a main current from the cardia along 

 the greater curvature to the pylorus, and back to the cardia along the lesser 

 curvature, subsidiary currents mixing the peripheral portions of the con- 

 tents with the more central ; it may be doubted, however, whether any such 

 regularity of flow is marked or constant, and it is not easy to see by what 

 combination and sequence of contractions in the three coats, longitudinal, 

 circular, and oblique, such a regular flow can be produced. But in any 

 case, by such rhythmical contractions the food and gastric juice are 

 rolled about and mixed together. These churning movements are fee- 

 ble at first, even though the stomach be filled and distended by a large 

 meal rapidly eaten ; they become more and more pronounced as digestion 

 proceeds. 



Before digestion has proceeded very far the " propulsive " movements 

 begin. These occur at intervals, and are repeated at first slowly but after- 

 ward more rapidly. Each movement consists in a contraction of the circu- 

 lar muscular fibres more powerful than any taking part in the churning move- 

 ments, and leading to a circular constriction which, beginning apparently at 

 about the obscurely defined groove which marks the beginning of the antrum 

 pylori, travels down toward the pylorus, propelling the food onward. This 

 movement is accompanied or rather preceded by a relaxation of, that is to 

 say, in all probability an inhibition of the permanent contraction of, the 

 sphincter pylori itself* in order that the gastric contents may pass into the 

 duodenum. But the occurrence of this relaxation is determined by the na- 

 ture of the gastric contents ; for if the propulsive movement drives large 

 undigested pieces toward the pylorus, the sphincter is apt to close again, the 

 result of which is that the undigested morsels are carried back into the main 

 body of the stomach. 



The combined effect, then, of the churning and of the propulsive move- 

 ments is, after a certain part of the meal has been reduced to a thick fluid 

 condition somewhat resembling pea soup and often called chyme, to strain 

 off this more fluid part into the duodenum, and to submit the remaining still 

 solid pieces to the further action of the gastric juice. 



As digestion proceeds, more and more material leaves the stomach, winch 

 is thus gradually emptied, the last portions which are carried through being 

 those parts of the food which are least digestible, and any wholly indigesti- 

 ble foreign bodies which happen to have been swallowed ; the latter may 

 perhaps never leave the stomach at all. The presence of food leads to the 

 development of the movements ; but evidently it is not the mere mechanical 

 repletion of the organ which is the cause of the movements, since the stomach 

 is fullest at the beginning when the movements are slight, and becomes 

 emptier as they grow more forcible. The one thing which does increase 

 pari passu with the movements is the acidity, which is at a minimum when 

 the (generally alkaline) food has been swallowed, and increases steadily on- 



