This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The sternum consists of three pieces: the manubrium or presternum, gladiolus or mesosternum, and xiphoid cartilage or metasternmn. It is developed in two lateral halves. Should these fail to unite an opening is left in the bone through which the pulsations of the heart have been seen and felt. The junction of the first and second pieces of the sternum is opposite the second rib. The seventh is the last rib to articulate with the sternum directly.
The first and second pieces of the sternum are connected by a joint which persists to advanced age. The projection caused by this joint is called the angulus sterni or angle of Ludwig. Fractures pass either through this joint, opposite the second rib, or through the bone just below it. They are produced by both direct and indirect force. Usually the upper fragment is beneath the lower one. It is however more true to state that the lower fragment is displaced anteriorly. Any marked posterior displacement of the upper fragment would tend to press on the trachea and interfere with breathing; the trachea bifurcates opposite the joint. As the pleurae and lungs of the two sides almost or quite touch behind the second piece of the sternum, they may be wounded and emphysema may occur. The heart also may be wounded. Suppuration has followed these injuries, in which case it will be necessary to trephine the sternum to give exit to the pus. The necessity of avoiding wounding of the pleurae in such a procedure is evident, as it would be followed by collapse of the lung and empyema.