The thorax 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.
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.
The thoracic assembly was a collection of cybernetic devices implanted within every Borg drone. Following the surgical removal of many of her implants in 2374, former Borg drone Seven of Nine no longer had access to the instruments that were part of her thoracic assembly. (VOY: "Revulsion")