Subdivision of trunk proper, which is demarcated from the neck by the plane of the superior thoracic aperture and from the abdomen internally by the inferior surface of the diaphragm and externally by the costal margin and associated with the thoracic vertebral column and ribcage and from the back of the thorax by the external surface of the posterolateral part of the rib cage, the anterior surface of the thoracic vertebral column and the posterior axillary lines; together with the abdomen and the perineum, it constitutes the trunk proper[FMA].