The Chest cavity was a part of Human biology enclosed by the Ribcage.
When Jack Harkness accidentally released nanogenes from a Chula warship, they started to convert the Human race into how Jamie's body was when he was killed, by fusing a gas mask with their face, and creating head trauma on the left of their brain, a gash on the back of his right hand, and a partial collapse of the chest cavity. The Ninth Doctor noticed this within a few seconds of examining a body with those symptoms. (TV: The Empty Child)