The Mediastinum-The Pericardium

The Mediastinum

The mediastinum is defined as ‘the space which is sandwiched between the two pleural sacs’. For descriptive purposes the mediastinum is divided by a line drawn horizontally from the sternal angle to the lower border of T4 (angle of Louis) into superior and inferior mediastinum.

The inferior mediastinum is further subdivided into the anterior in front of the pericardium, a middle mediastinum containing the pericardium itself with the heart and great vessels, and posterior mediastinum between the pericardium and to lower eight thoracic vertebrae (Fig. 22).

The Pericardium

The heart and the roots of the great vessels are contained within the conical fibrous pericardium, the apex (TOP) of which is fused with the adventitia (THE OUTERMOST CONNECTIVE TISSUE COVERING OF ANY ORGAN OR VESSEL) of the great vessels and the base with the central tendon of the diaphragm.

Anteriorly it is related to the body of the sternum, to which it is attached by the sternopericardial ligament, the 3rd–6th costal cartilages and the anterior borders of the lungs. Posteriorly it is related to the oesophagus, descending aorta, and vertebra T5–T8, and on either side to the roots of the lungs, the mediastinal pleura and the phrenic nerves.

The inner aspect of the fibrous pericardium is lined by the parietal layer of serous pericardium. This is reflected around the roots of the great vessels to become continuous with the visceral layer or epicardium. The lines of pericardial reflexion are marked on the posterior surface of the heart by the oblique sinus. (Fig. 23)  

The oblique sinus is bound by the inferior vena cava and the four pulmonary veins, which form a recess between the left atrium and the pericardium, and the transverse sinus which is between the superior vena cava and left atrium posteriorly and the pulmonary trunk and aorta anteriorly.