The external and internal intercostal muscles are important respiratory muscles in humans, but their mechanical actions have been controversial. We used finite-element analysis based on anatomic and mechanical measurements in dogs to assess the action of the intercostal and other rib cage muscles in a model of an isolated canine rib cage. When intercostal muscle forces of either the internal or the external layer were applied in a single interspace, they pulled the adjacent ribs together, consistent with published observations in dogs. However, when the forces were applied in all interspaces, the external layer caused an inspiratory motion and the internal layer caused an expiratory motion, consistent with conventional understanding of intercostal muscle actions. Parasternal intercostal, levator costae, and transversus thoracis (triangularis sterni) muscle actions were also simulated. These muscles caused expected movements of the ribs and sternum. We conclude that the actions of intercostal muscles depend on the spatial extent of their activation. Their actions in a single interspace and in multiple interspaces can be observed and explained with three-dimensional finite-element models.