The integration of the bulbus duodeni into the aboral closure mechanism of the stomach.

The m. sphincter pyloricus does not represent an isolated ring muscle (sphincter). Rather, it is a concentration of muscle bundles deriving from the antrum musculature and woven into the aboral sphincter complex of the stomach. The pyloric canal can be regarded as a narrow gastric segment capable of only a restricted degree of dilatation. Myoarchitectonicly, the prepyloric antrum, the pyloric region, and the bulbus duodeni together belong to the aboral closure mechanism of the stomach (motor unit). If the bulbus duodeni occupies the whole of the pars superior, this sphincter mechanism extends as far as the superior duodenal flexure. In histological section the tunica muscularis first exhibits a polarized character aborally from the bulbus (or below the superior duodenal flexure). Angioarchitectonicly, too, the pars superior is fundamentally different from subsequent sections of the duodenum since it is vascularized from two sides, i.e., the vascularization here is still of a typically gastric character. There is no vascular cuff surrounding the pylorus.