The thorax is a division of an animal's body that lies between the head and the abdomen. The heart and the lungs reside in the thoracic cavity, as well as many blood vessels. The inner organs are 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 the three main divisions of the creature's body, each of which is in turn composed of multiple segments.
The thoracic assembly is 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")