58 PHYSIOLOGY FOR DENTAL STUDENTS. 



glossopharyngeal, and the respiratory movements stop in what- 

 ever position they may have been in at the time. 



This inhibition of the oesophagus is indeed a most important 

 part of the process when liquid or semi-liquid food is swallowed 

 By the contraction of the mylohyoid muscle, fluids are quickly 

 shot down the distended oesophagus, at the lower end of which, 

 on account of the cardiac sphincter being closed, they accumulate 

 until the arrival of the peristaltic wave which has meanwhile 

 been set up by stimulation of the pharynx. If the swallowing is 

 immediately repeated, as is usually the case in drinking, the 

 oesophagus remains dilated because peristalsis is inhibited, and 

 the fluid lies outside the cardiac orifice until the last mouthful 

 has been taken. 



These facts have been revealed by listening with a stethescope 

 to the sounds produced by swallowing, and by observing with an 

 X-ray lamp the shadows produced along the course of the 

 oesophagus when food, impregnated with bismuth subnitrate, is 

 taken. When a solid bolus is swallowed, one sound is usually 

 heard, but with liquid food there are two, one at the upper end, 

 due to the rush of the fluid and air and the other at the lower 

 end (heard over the epigastrium) some four or six seconds later, 

 due to the arrival here of the peristaltic wave with the accom- 

 panying opening of the cardiac sphincter and the escape of the 

 fluid and air into the stomach. Sometimes, as when the person 

 is in the horizontal position, this second sound may be broken up 

 into several, indicating that, unassisted by gravity, the fluid does 

 not so readily pass through the sphincter. The X-ray shadows 

 yield results in conformity with the above. After swallowing 

 milk and bismuth, for example, the shadow falls quickly to the 

 lower end of the oesophagus and then slowly into the stomach. 

 When the passage of a solid bolus is watched by the X-ray 

 method, its rate of descent will be found to depend on whether 

 or not it is well lubricated with saliva ; if not so, it may take as 

 long as fifteen minutes to reach the stomach ; if moist, but from 

 eight to eighteen seconds. 



The Act of Vomiting. This is usually preceded by a feeling 

 of sickness or nausea and is initiated by a very active secretion 



