THE MECHANICS OF DIGESTION 1009 



whether this organ possesses a muscular power sufficient to force its 

 contents through the pyloric orifice. Obviously, the latter is situated 

 at this time somewhat above the level of the general cavity of the py- 

 lorus. Moreover, when standing erect, the stomach assumes more 

 nearly the shape of a suspended stocking and allows the gases to escape 

 very freely, whereas, when lying down, the esophageal orifice assumes 

 a position somewhat below the level of the general gastric cavity and 

 entraps any gases that may have been formed. 



The Evacuation of the Gastric Contents. — The purpose of the peri- 

 staltic movements of the stomach is to mix the food with the gastric 

 juice, and to reduce it eventually into a liquid which is known as 

 the chyme. In this form, the gastric contents are then ejected through 

 the relaxed pyloric orifice into the duodenum. The muscular activity 

 which is required to accomplish this end is somewhat different from 

 that previously noted in the course of the formation of the chyme. 

 It consists essentially in a contraction of the horizontal and oblique 

 layers of muscle tissue which employ the cardia as a fixed point and 

 raise the fundus above the general level of the pylorus. Meanwhile, 

 the pylorus continues its peristaltic activity, and forces its contents to- 

 ward the pyloric orifice. Naturally, the chyme cannot escape as long 

 as this sphincter remains closed and must in this event be whirled 

 back along the sides of the gastric wall. 



Two reasons may be assigned for the continued closure of this 

 sphincter, namely: (a) the gastric contents still contain solid masses 

 which exert a mechanical influence upon the mucosa of this region, 

 and (6) the gastric contents have not as yet been sufficiently acidified.^ 

 Contrariwise, if the gastric contents have been thoroughly liquefied 

 and acidified, these mechanical and chemical stimulations cease and 

 allow the sphincter to relax. The chyme is then ejected into the duo- 

 denum, being here thrown against the upper surfaces of the valvulae 

 conni ventes which extend as transverse flaps partially across the lumen 

 of this passage. The presence of acid in the duodenum then effects 

 the closure of the pyloric orifice. Thus, the ejection of chyme is im- 

 mediately followed by a constriction of the sphincter until the acid 

 liquid in the duodenum has again been neutralized. The ejection of 

 chyme is then repeated. Consequently, it may be concluded that the 

 opening and closing of the pyloric sphincter is dependent upon the 

 physical condition of the gastric juice as well as upon the relative de- 

 grees of acidity in the cavities of the stomach and duodenum. Conse- 

 quently, the evacuation of the stomach is not a continuous act, but 

 takes place at intervals until its cavity has been completely emptied. 

 The nervous mechanism concerned in this reflex act, lies in the domain 

 of the plexus gastro-duodenalis. 



The Time of Evacuation of the Gastric Contents. — The preceding 

 discussion must show immediately that the time of evacuation of the 

 gastric contents is subject to considerable variations which depend 

 1 Hirsch, Zentralbl. fiir innere Med., 1901. 



