Complex sentence: At the mouth opening, it is continuous with the back of the oral cavity, passing downwards through the rear part of the mediastinum, through the diaphragm, and into the stomach.
Split into simpler sentences: At the mouth opening, it is continuous with the back of the oral cavity. It passes from the back of the oral cavity downwards and through the back part of the mediastinum, through the diaphragm, and into the stomach.