In producing realistic, animatable models of the human body, we see much to be gained from developing a functional anatomy that links the anatomical and physiological behavior of the body through fundamental causal principles. This paper describes our current Finite Element Method implementation of a simplified lung and chest cavity during normal quiet breathing and then disturbed by a simple pneumothorax. The lung model interacts with the model of the chest cavity through applied forces. The models are modular, and a second lung and more complex chest wall model can be added without disturbing the model of the other lung. During inhalation, a breathing force (corresponding to exertion of the diaphragm and chest wall muscles) is applied, causing the chest cavity to expand. When this force is removed (at the start of exhalation), the stretched lung recoils, applying pressure forces to the chest wall which cause the chest cavity to contract. To simulate a simple pneumothorax, the intrapleural pressure is set to atmospheric pressure, which removes pressure forces holding the lung close to the chest cavity and results in the lung returning to its unstretched shape.