It begins at a point between the fifth and sixth cervical vertebrae and descends along in front of the spine through the posterior mediastinal space, passes through the diaphragm, and entering the abdomen, terminates in the stomach wall at a point opposite the tenth dorsal vertebra. At its commencement it is placed in the median line and gradually inclines to the left as it passes forward to the esophageal opening to the diaphragm. The esophagus is from one-half to an inch in diameter. Arteries.—The arteries which supply the esophagus are the esophageal, which are branches from the aorta. Veins.—The esophageal veins empty into the ascending vena cava. =The Diaphragm.=—The diaphragm (a partition wall) is a dense, muscular, fibrous septum, placed obliquely across the trunk. It separates the thoracic from the abdominal cavity, forming the floor of the thoracic and the roof of the abdominal cavity. It is attached in front to the ensiform process of the sternum, on the sides to the inner surface of the cartilages and bony portions of six or seven inferior ribs, and behind it is attached to the lumbar vertebrae. The diaphragm has three openings, as follows: opening for the esophagus, opening for the aorta, opening for the ascending vena cava. The diaphragm is the principal muscle of respiration. The arteries which supply the diaphragm are the phrenic arteries. The phrenic veins receive the blood from the diaphragm. CHAPTER VIII. ORGANOLOGY.—Continued. =The Abdomen.=—The abdomen is the largest cavity in the body. It is oval in form, the extremities of the oval being directed upward and downward. To facilitate description, the abdomen is artificially divided into two parts: An upper and larger part, the abdomen proper. A lower and smaller part, the pelvis. These two cavities are not separated from each other, but the limit between them is a line drawn around the brim of the true pelvis. The abdomen proper differs from the other great cavities of the body, in being bounded for the most part by muscles and fascia. It varies in capacity and shape according to the condition of the viscera which it contains and in addition, it varies in form and extent with age and sex. Boundaries.—The diaphragm forms the dome over the abdomen, the cavity of the abdomen extending high into the bony thorax. The lower end of the abdomen is limited by the bones of the pelvis. In front and at the sides it is bounded by the lower ribs and abdominal muscles. Behind by the vertebral column and muscles. Regions.—For convenience of description of the viscera, the abdomen is artificially divided into nine regions. Thus if two circular lines are drawn around the body, the one at the extremities of the ninth ribs where they join the costal cartilages, and the other around the crest of the ileum, the abdominal cavity is divided into three zones. [Illustration: FIG. 22—The regions of the abdomen and their contents. (Gray)]