DIGESTION 303 



far the process has only occupied one-tenth of a second. After 

 several seconds, the food, or the portion which still remains in 

 the oesophagus, is forced through the cardiac sphincter into the 

 stomach by the arrival of the tardy peristaltic contraction of 

 the oesophageal wall (Kronecker and Meltzer). Two sounds may 

 be heard in man on listening in the region of the stomach or 

 oesophagus during deglutition of liquids, especially when, as 

 generally happens, they are mixed with air. The first sound 

 occurs at once, and is due to the sudden squirt of the liquid along 

 the gullet ; the second, which is heard after a distinct interval 

 (about six seconds), is caused by the forcing of the fluid through 

 the cardiac orifice of the stomach by the contraction of the oeso- 

 phagus. 



There are certain peculiarities which distinguish this peri- 

 staltic movement of the oesophagus from that of other parts of 

 the alimentary canal. It is far more closely related to the central 

 nervous system, and, unlike the peristaltic contraction of the 

 intestine, can pass over any muscular block caused by ligature, 

 section, or crushing, so long as the nervous connections are intact. 

 But division of the oesophageal nerves causes, as a rule, stoppage 

 of oesophageal movements ; although an excised portion of the 

 tu be retains its vitality for a long time, and may, under certain 

 circumstances, go on contracting in the characteristic way after 

 removal from the bdoy. Stimulation of the mucous membrane 

 of the pharynx will cause reflex movements of the oesophagus, 

 while stimulation of its own mucous membrane is ineffective. 

 From these facts we learn that although the oesophageal wall 

 may possess a feeble power of spontaneous peristaltic contraction, 

 yet this is usually in abeyance, or at least overmastered by central 

 nervous control ; so that impulses discharged as a ' fusillade ' 

 from successive portions of the vagus centre, and travelling down 

 the oesophageal nerves, excite the muscular fibres in regular 

 order from the upper to the lower end of the tube. 



Nervous Mechanism of Deglutition. The centre for the 

 whole involuntary stage (both pharyngeal and oesophageal) lies 

 in the upper part of the medulla oblongata. When the brain is 

 sliced away above the medulla, deglutition is not affected ; but 

 if the upper part of the medulla is removed, the power of 

 swallowing is abolished. In man, disease of the spinal bulb 

 interferes far more with deglutition than disease of the brain 

 proper. 



Normally, the afferent impulses to the centre are set up by the 

 contact of food or saliva with the mucous membrane of the pos- 

 terior part of the tongue, the soft palate and the fauces, the 

 nerve-channels being the superior laryngeal, the pharyngeal 

 branches of the vagus, and the palatal branches of the fifth 



