180 DIGESTION 



pharyngeal plexus coming from the vagus, the glosso-pharyngeal, and 

 the sympathetic. 



The process of swallowing presents for consideration continuous 

 series of muscular movements of a complex nature. They are one of the 

 best examples of a highly developed and perfectly coordinated neuro- 

 muscular mechanism. The tongue arranges the bolus of food in the 

 middle of its back surface and then tips up anteriorly. The bolus of 

 food then, partly drops and is partly squeezed backward between the 

 tongue's dorsum and the hard palate and between the anterior pillars of 

 the fauces. Meanwhile the soft palate has been raised and the posterior 

 pillars have approximated, the uvula closing what little opening remains. 

 This prevents the regurgitation of the food-mass into the mouth or into 

 the nasal fossae. By action of the upper pharyngeal muscles the pharynx 

 is raised to meet the descending bolus. The larynx is also raised and 

 at the same time closed above by the drawing forward of its posterior 

 boundaries. As an extra safe-guard against drops of liquid falling into 

 the lungs, the vocal cords are approximated at the same time. To the 

 same end, and even more importantly, respiration is entirely suspended 

 (reflexly by way of the glosso-pharyngeal nerve) during the entire swal- 

 lowing process. Thus, the bolus of food drops into the grasp of the con- 

 strictors. These rapidly force it in the line of least resistance down- 

 ward, where the inferior constrictors push it onward into the muscular 

 esophagus. In the latter tube it moves at first rapidly and then more 

 slowly by typical peristalsis. With a phonendoscope the thud made by 

 the bolus, dropping into the stomach can be often heard. 



In the case of a large bolus this process of swallowing from the tongue 

 to the stomach requires as much as six seconds. About five of these 

 seconds are consumed in the esophagus. As Meltzer showed, when the 

 mass of food swallowed is small or a liquid the process is simpler. 



Deglutition appears to have a controlling center in the medulla oblon- 

 gata apparently not far from the vagal center and closely related to the 

 center of respiration. Deglutition is essentially a reflex process, but it 

 may be initiated by the will. The act cannot be carried out completely 

 however, unless there is some substance passing over the mucous mem- 

 branes to reflexly actuate the apparatus. One cannot swallow several 

 times in quick succession because after the first or second swallow- 

 ing there is no saliva to furnish afferent nervous impulses. These 

 nervous influences pass inward by branches of the superior maxillary 

 of the fifth, of the superior laryngeal, of the tenth, and of the ninth cranial 

 nerve. The efferent or motor nerves of deglutition are the twelfth, fifth, 

 ninth, tenth, and eleventh. 



The Stomach. This viscus consists of two parts, functionally rather 

 distinct namely, the fundus and the antrum. The inlet of the stomach 

 above is the cardia; its outlet below is the pylorus. Both are guarded 

 by valves which consist of thickenings of the circular muscular fibers. 

 The wall of the stomach is composed of four layers. The muscular 

 part of the gastric wall consists of two complete and one incomplete 



