Behind the manubrium there is little or no lung, the space being occupied chiefly by the vessels, etc. The left innominate vein is most superficial, and lies just below the upper border of the manubrium. Then come the main branches of the aortic arch, then the trachea bifurcating at the manubrio-gladiolar junction, and, most deeply, the oesophagus. The highest portion of the aortic arch is about 1 inch below the upper border of the manubrium, and the course of the innominate artery is indicated by a line from the right sterno-clavicular articulation to the middle of the manubrio-gladiolar junction.
The junction of the manubrium and gladiolus sterni is marked by a transverse ridge, which can nearly always be felt, and which indicates the level of the cartilage of the second rib, the bifurcation of the trachea, and the disc between the fourth and fifth dorsal vertebrae. The two bones generally lie at an angle to one another, which is supposed to be particularly marked in pulmonary affections, and which is referred to as the angulus sternalis. They do not become firmly united until middle life, and dislocation occasionally occurs, the gladiolus and ribs being displaced forwards. Fracture of the sternum is uncommon, is generally transverse, and due to violent flexion forwards or backwards. It also occurs generally about the manubrio-gladiolar junction, and owing to the thick periosteal covering of the bone, displacement is generally slight. The bone has occasionally been penetrated by stabbing, and has been trephined with the object of reaching the mediastinum and pericardium, a similar operation having been suggested for ligature of the innominate.
As most segments of the gladiolus are developed from two lateral centres of ossification, want of union between these sometimes occurs, giving rise to wide median fissures of the bone, leaving the heart uncovered by bone, the condition occasionally being associated with ectopia cordis. In less severe cases an aperture may be met with in the gladiolus through which deep suppurative processes may pass to the surface, or superficial infections may gain access to deeper parts. Deep indentations also are met with in the sternum, resulting from regular pressure applied to it in certain trades, as joiners and bootmakers.