The thorax (also referred to as the thoracic crest or chest) was a division of a person's or animal's body that lay between the head and the abdomen. The heart and the lungs resided in the thoracic cavity, as well as many blood vessels. The inner organs were protected by the rib cage and the sternum. (citation needed • edit)
Having one thorax is but one of many anatomical characteristics of humanoid lifeforms. In Humans, the thorax is the region of the chest formed by the sternum, the thoracic vertebrae and the ribs, and extends from the neck to the diaphragm. In insects, the thorax is one of three main divisions of the creature's body, each of which is in turn composed of multiple segments. (citation needed • edit)
The thoracic assembly was a collection of cybernetic devices implanted within every Borg drone. Following the surgical removal of many of her implants, former Borg drone Seven of Nine no longer had access to the instruments that were part of her thoracic assembly. (VOY: "Revulsion")
In the revised final draft script of TOS: "Balance of Terror", chest pains were experienced by a Romulan centurion who had been injured in battle. He therefore found swallowing too difficult, which led the centurion to have to reject a medicine which a Romulan commander, his commanding officer, tried to get him to drink.