THE MUSCULAR MECHANISMS OF DIGESTION. 297 



These peristaltic movements of the oesophagus may, like those of the in- 

 testine, be seen after removal of the organ from the body ; and, indeed, may 

 continue to appear, upon stimulation, for an unusual length of time. They 

 may, therefore, be carried out by the muscular elements, with or without the 

 help of the nervous elements imbedded in them, apart from any action of 

 the central nervous system. Nevertheless, in the living body, the move- 

 ments of the oesophagus seem to be in a special way dependent on the central 

 nervous system ; the contractions are not started and carried out by the 

 walls of the tube alone, and so transmitted from section to section in the 

 walls of the tube itself; but afferent impulses started in the pharvnx and 

 passing to the medulla oblongata, give rise to reflex efferent impulses which 

 descend along nervous tracts to successive portions of the organ. If the 

 oesophagus be cut across some way down, or if a portion of the middle region 

 be excised, stimulation of the pharynx will produce a peristaltic contrac- 

 tion, which travelling downward will not stop at the cut or excision, but 

 will be continued on into the lower disconnected portion by means of the 

 central nervous system. And it is stated that ordinary peristaltic contrac- 

 tions of the lower part of the oesophagus can be readily excited by stimu- 

 lation of the pharynx, but not by stimuli applied to its own mucous mem- 

 brane. In the reflex act which thus brings about the peristaltic contraction 

 of the oesophagus the afferent nerves are those of the pharynx, viz., the 

 superior laryngeal nerve and pharyngeal branches of the vagus, branches 

 of the fifth, and in some animals, at least, branches of the glosso-pharyngeal, 

 but chiefly the first ; and oesophageal movements can easily be excited by 

 centripetal stimulation of the superior laryngeal. -The centre lies in the 

 medulla oblongata, being a part of the general deglutition centre ; and effer- 

 ent impulses pass along fibres of the vagus, reaching the upper part of the 

 oesophagus by the recurrent laryngeal nerves, and the lower part through 

 the oesophageal plexus of the vagus. Section of the trunk of the vagus 

 renders difficult the passage of food along the oesophagus, and stimulation 

 of the peripheral stump causes oesophageal contractions. 



The force of this movement in the oesophagus is considerable ; thus in 

 the dog a ball, pulling by means of a pulley against a weight of 250 

 grammes, has been found to be readily carried down from the pharynx to 

 the stomach. 



At the junction of the oesophagus with the stomach the circular 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 prevent 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. 



232. Movements of the stomach. While the object of the oesophageal 

 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 

 insuring 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, dis- 

 tinguish between what we may call the " churning " and the " propulsive " 

 movements of the stomach. 



