Lungs and thorax bones. Coloured 3D computed tomography (CT) scan of normal lungs and thorax bones seen from the rear. The lungs (blue) are enclosed by the ribs. Twelve pairs of ribs circle the chest. They are attached at one end to the spine (backbone, down centre). The shoulder blades (scapulas, upper left and right) are blades of bone to which several back muscles attach. Each scapula is seen articulating with the head of its humerus (upper arm bone) and its clavicle (collar bone) to form the shoulder joints.