CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 377 



fibres usually remain in a more or less permanent condition of 

 tonic or obscurely rhythmic contraction, more particularly when 

 the stomach is full of food, and thus serve as a sphincter to pre- 

 vent the return of food from the stomach into the oesophagus. 

 Upon the arrival of the bolus of food at the end of the oesophagus, 

 the centre for this sphincter is inhibited and the orifice is thus 

 opened up. Possibly the patency of the orifice is still further 

 secured by a contraction of the longitudinal muscular fibres which 

 radiate from the end of the oesophagus over the stomach. 



221. Movements of the Stomach. While the object of the 

 cesophageal movement is simply to carry the swallowed bolus with 

 all due speed to the stomach, and while the intestinal movement 

 has, in like manner, simply to carry the intestinal contents 

 onward, the twisted course of the looped path ensuring all the 

 mixing of the constituents of the contents that may be necessary, 

 the movements of the stomach have a double object : on the one 

 hand to provide an adequate exposure of the contents of the 

 dilated chamber to the influence of the gastric juice, and on the 

 other to propel the partially digested food, when ready, into the 

 duodenum. We may accordingly distinguish between what we 

 may call the " churning " and the " propulsive " movements of the 

 stomach. 



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

 said, longitudinal, circular and oblique, fall into a condition which 

 we may perhaps speak of as an obscure tonic contraction. The 

 whole stomach is small and contracted, its cavity is nearly obli- 

 terated, 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 contents 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 feeble 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. 



