THE MECHANICAL PHENOMENA OF DIGESTION 319 



pass over any muscular block caused by ligature, section, or crush- 

 ing, so long as the nervous connections are intact. But division 

 of the oesophageal nerves causes, as a rule, stoppage of cesophageal 

 movements; although an excised portion of the tube retains its 

 vitality for a long time, and may, under certain circumstances, go 

 on contracting in the characteristic way after removal from the body 

 (p. 790) . Stimulation of the mucous membrane of the pharynx wilt- 

 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 cesophageal 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 cesophageal) 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 posterior 

 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 nerve.* A feather 

 has sometimes been swallowed involuntarily by a reflex movement 

 of deglutition set up while the soft palate or pharynx was being 

 tickled to produce vomiting. Artificial stimulation of the central 

 end of the superior laryngeal will cause the movements of deglutition 

 independently of the presence of food or liquid; but if the central 

 end of the glosso-pharyngeal nerve be stimulated at the same time, 

 the movements do not occur. The glosso-pharyngeal is therefore 

 able to inhibit the deglutition centre, and it is owing to the action 

 of this nerve that in a series of efforts at swallowing, repeated within 

 less than a certain short interval (about a second), only the last is 

 successful. It is also through the glosso-pharyngeal nerve that 

 the respiratory movements are inhibited during deglutition. When 

 the central end of this nerve is stimulated, respiration is stopped 



* It appears that the most' influential reflex paths may differ in different 

 animals. In the rabbit, e.g., the reflex is set up by excitation of the trigeminal 

 fibres which supply the mucous membrane anterior to the tonsils, in the dog 

 and cat by excitation of the glosso-pharyngeal fibres in the posterior wall of 

 the pharynx, and in monkeys by excitation of the trigeminal branches dis- 

 tributed to the mucous membrane over the tonsils (Kahn). 



