638 PHYSIOLOGY OF DIGESTION AND SECRETION. 



down by the first swallow waits in this case for the arrival of the suc- 

 ceeding wave before entering the stomach. 



Nervous Control of Deglutition. The entire act of swallowing, 

 as has been said, is essentially a reflex act. Even the comparatively 

 simple wave of contraction that sweeps over the esophagus is due 

 to a reflex nervous stimulation, and is not a simple conduction of 

 contraction from one portion of the tube to another. This fact was 

 demonstrated by the experiments of Mosso,* who found that after 

 removal of an entire segment from the esophagus the peristaltic 

 wave passed in due time to the portion of the esophagus left on the 

 stomach side, in spite of the anatomical break. The same experi- 

 ment was performed successfully on rabbits by Kronecker and 

 Meltzer. Observation of the stomach end of the esophagus in this 

 animal showed that it went into contraction two seconds after the 

 beginning of a swallowing act whether the esophagus was intact or 

 ligated or completely divided by a transverse incision. The afferent 

 nerves concerned in this reflex are the sensory fibers to the mucous 

 membrane of the pharynx and esophagus, including branches of the 

 glossopharyngeal, trigeminal, vagus, and superior laryngeal division 

 of the vagus. Artificial stimulation of this last nerve in the lower 

 animals is known to produce swallowing movements. Several 

 observers have attempted to determine the precise area or areas 

 in the pharyngeal membrane from which the sensory impulses 

 liberating the reflex normally start. According to Kahn,f the 

 most effective areas from whose stimulation the reflex may be 

 produced vary in location in different animals. In the rabbit the 

 reflex is originated most easily by stimulation at the entrance to 

 the pharynx the soft palate along the line extending from the 

 posterior edge of the hard palate to the tonsils (superior maxil- 

 lary branch of trigeminal); in the dog irritation of the posterior 

 pharyngeal wall is most effective (glossopharyngeal nerve); in 

 monkeys the area is approximately as in rabbits, that is, in the 

 region of the tonsils. The motor fibers concerned in the reflex 

 comprise the hypoglossal, the trigeminal, the glossopharyngeal, 

 the vagus, and the spinal accessory. For an act of such complexity 

 and such perfect co-ordination it has been assumed that there is a 

 special nerve center, the swallowing or deglutition center, which has 

 been located in the medulla at the level of the origin of the vagi. 

 There is little positive knowledge, however, concerning the existence 

 of this center as a definite group of intermediary nerve cells, after 

 the type of the vasoconstrictor or respiratory center, which send 

 their axons to the motor nuclei of the several efferent nerves con- 

 cerned. As in the case of other complicated reflex acts, we can only 



* Moleschott's "Untersuchungen," 1876, volume xi. 



f Kahn, " Archiv f. Physiologie, " 1903, suppl. volume, 386. 



