It's the upper part of the body wall. It is comprised of skeletal frame and also the musculatures and coverings. The skeleton of thoracic wall consists of a sternum, 12 pairs of ribs with the costal cartilages and the 12 thoracic vertebrae. These make the frame of the walls of the thoracic cavity. Throughout the sides and anteroposterior aspects, this frame is filled with the intercostal muscles between the ribs. The roof of thoracic cavity is the lung apices covered by the supra pleural membrane. The floor is made by the diaphragm. While expiration the diaphragm is relaxed and the dome is much prominent. This makes the volume of thoracic cavity lesser than the bony cage would suggest. The sternum has two symphysis; the upper manubriosternal joint and the lower xiphisternal joint which are non-movable.
The ribs are attached posteriorly to the vertebral column in 2 places by their heads and the tubercles. These are the costovertebral joints. Anteriorly they are attached to the costal cartilages. The upper 7 of these cartilages make costochondral joints with the sternum. The 8th, 9th and 10th join with each other to form interchondral joints. The 8th costel cartilage then joins with the 7th one and completes the costal margine. The 11th and 12th costel cartilages dont have any anterior articulation. So the 2 corresponding ribs are reffered to as the floating ribs. Some upper organs of the abdominal cavity are fully or partly covered by the ribs. These include the liver, spleen and the upper parts of kidneys and stomach.
Thoracic muscles lie in same morphological layers as in abdomen.