THE CARDIAC SPHINCTER 125 



rapidity down to the lower end of the oesophagus, the 

 completion of its journey into the stomach being performed 

 more slowly. The fluid thus reaches the stomach before 

 the wave of contraction which it has initiated while in the 

 pharynx. This wave follows in the wake of the fluid, 

 and serves to propel any remnants into the stomach. 



The oesophageal contraction is dependent upon the 

 discharge from the medulla of a succession of impulses 

 which travel down the vagi. This is shown by the fact 

 that the wave is interrupted by section of these nerves, 

 but not by section of the oesophagus itself. But this wave 

 of vagal origin is not the only form of contraction met 

 with in the oesophagus. The tube is divided into two parts 

 by differences in its muscular layer. There is an upper 

 region, where the muscle is striated, and a lower region, 

 where it is unstriated. When some days have elapsed 

 after section of the vagi, the unstriated part develops 

 the power of responding to pressure of food within it by 

 undergoing peristaltic waves. These waves, which are 

 quite independent of any voluntary act, eventually succeed 

 in conveying the food from the lower end of the oesophagus 

 into the stomach. The part played by these waves under 

 normal conditions is described below. 



The Cardiac Sphincter 



Normally the cardiac sphincter is closed, but it opens 

 on the approach of an oesophageal wave. When closed 

 the tonicity of the muscle is not great, for it can easily 

 be opened passively. It also opens on shght increase in 

 the intragastric pressure. The part which the vagus plays in 

 controlling the cardiac sj)hincter is complex, for on stimula- 

 tion this nerve causes increased tonus followed by relaxation. 



But although the sphincter is normally closed it opens 

 rhythmically, and allows regurgitation of food into the 

 lower part of the oesophagus. From here the food is 

 returned to the stomach by a peristaltic wave originating 

 in the unstriated part of the oesophagus. This wave is 



