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The mediastinum, sometimes split into “media stinum” or hyphenated “media-stinum,” is the internal body cavity behind the middle of the sternum, or chest bone. It’s traditional to divide it into two areas: the lower posterior, and the upper superior. The superior mediastinum, or upper thorax for common reference, is of particular medical interest because the structures of the human anatomy contained therein generally represent the main connectors between the head and body. These structures are loosely organized and held together by soft connective tissue.
The whole of the body cavity, often also called the interpleural space, is bordered in front by the sternum and in back by the vertebrae, or backbone. A relatively narrow space, it is bordered on each side by the lungs. It starts at the top, from the thoracic opening at the base of the neck, and extends down to the large diaphragm muscle that operates the lungs.
The demarcation between the posterior and superior mediastinum is somewhat arbitrary, but widely agreed. This division is a horizontal plane traversing the bottom edge of the T4, or fourth upper thoracic vertebra, with the bottom edge of the manubrium sterni. The latter is the enlarged, quadrangular shaped bone of the uppermost sternum. The first two rib bones are connected to it.
This plane neatly clips the top edge of the heart. One of the most important structures within the superior mediastinum is therefore the aorta, a massive blood vessel that arches above the heart and downward to the entire body. It carries oxygenated blood. Other equally important tubular structures include the trachea, or windpipe, and the esophagus for food.
Most of the nerves connecting the head and body are bundled into the spinal cord within the backbone. The brain requires, however, a more direct connection for some critical organs or bodily functions. Some of these nerves, including the cardiac nerve to control the heart, pass through the superior mediastinum. The phrenic nerve connects to the diaphragm muscle which controls breathing. The vagus nerve collects all of the sensory nerves originating from most of the body’s other internal organs to advise the brain of their functioning state.
The thoracic duct, which passes through the entire mediastinum cavity, is the largest vessel of the human lymphatic system. Its branching, body-wide network of vessels containing a clear liquid called lymph is not unlike the human circulatory system. Working in concert with the blood in its network, the two systems constitute the body’s immune system to filter and combat invasive, potentially harmful agents such as viruses. One of the few completely self-contained organs within the superior mediastinum is the thymus. Connected to the lymphatic system, its only known function is to produce so-called T-cells, the most potent combatants of the immune system.