CHAP, i.] DIGESTION. 285 



oesophagus can be readily excited by stimulation of the pharynx, 

 but not by stimuli applied to its own mucous membrane. 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, branches of the fifth, and in 

 some animals at least branches of the glossopharyngeal, but chiefly 

 the first. The centre lies in the medulla oblongata, being a part 

 of the general deglutition centre ; and the efferent 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 

 plexuses over the root of the lungs and the stomach, to which the 

 vagus gives origin. Section of the trunk of the vagus renders 

 difficult the passage of food along the oesophagus, and stimulation 

 of the peripheral stump causes cesophageal contractions. The 

 force of this movement in the oesophagus is considerable; thus 

 Mosso found that in the dog a ball pulling by means of a pulley 

 against a weight of 250 grammes was readily carried down from 

 the pharynx to the stomach. 



The junction of the oesophagus with the stomach remains in a 

 more or less permanent condition of tonic or obscurely rhythmic 

 contraction, more particularly when the stomach is full of food, and 

 thus serves as a sphincter to prevent the return of food from the 

 stomach into the oesophagus. During the passage of the food 

 from the oesophagus into the stomach this sphincter becomes 

 relaxed, probably by a mechanism which will be described in 

 treating of vomiting. 



Movements of the Stomach. These are at bottom peristaltic 

 in nature, though largely modified by the peculiar arrangement of 

 the gastric muscular fibres. When food first enters the stomach, 

 the movements are feeble and slight, but as digestion goes on 

 they become more and more vigorous, giving rise to a sort of 

 churning within the stomach, the food travelling from the cardiac 

 orifice along the greater curvature to the pylorus, and returning by 

 the lesser curvature, while at the same time subsidiary currents 

 tend to carry the food which has been passing close to the mucous 

 membrane toward the middle of the stomach, and vice versa. At 

 the pyloric end strong circular contractions are set up, by which 

 portions of food, more especially the dissolved parts, but also small 

 solid pieces, are carried through the relaxed sphincter into the 

 duodenum. As digestion proceeds, more and more material leaves 

 the stomach, which is thus gradually emptied, the last portions 

 which are carried through being those matters which are least 

 digestible, and foreign bodies which happen to have been swallowed. 

 The presence of food then leads to the development of obscurely 

 peristaltic rhythmic movements, the stomach when empty being 

 contracted, but quiescent; but evidently it is not the mere 

 mechanical repletion of the organ which is the cause of the move- 



