THORACIC CAVITY 73 



areolar tissue of the mediastinum, called the superior and 

 inferior sterno-pericardiac ligaments, connect the anterior surface 

 of the fibrous sac to the upper and lower ends of the body of 

 the sternum respectively, and (2) in the region of the sternal 

 extremity of the left fifth costal cartilage, where the left pleura 

 retreats somewhat towards the left side, and the pericardium 

 comes into direct relation with the sternum and the left trans- 

 versus thoracis muscle. That portion of the pericardium is 

 the so-called bare area. It is usually of small extent, and 

 frequently it does not extend beyond the margin of the 

 sternum, but it is of importance because through it the surgeon 

 may attempt to tap the pericardium when the sac is distended 

 with fluid. 



Each lateral wall of the pericardium is in relation with 

 the corresponding mediastinal pleura, the phrenic nerve and 

 the pericardiaco-phrenic vessels intervening (O.T. comes nervi 

 phrenici). The posterior surface lies anterior to the descend- 

 ing aorta and the oesophagus medially, whilst laterally it is 

 supported posteriorly by the lungs and pleurae (Fig. 21). At 

 the junction of the upper parts of the lateral and posterior 

 surfaces, on each side, two pulmonary veins enter the peri- 

 cardium and receive sheaths from its fibrous wall. 



Dissection. When the relations and prolongations of the 

 fibrous pericardium have been studied, the two anterior flaps 

 already made in the lateral walls of the sac (see p. 44) should be 

 connected together and converted into one large anterior flap. 

 This can be done by a transverse cut, passing across the median 

 plane just above the diaphragm. The large triangular flap thus 

 formed should be thrown upwards towards the apex of the 

 pericardium. 



The Serous Pericardium. The serous pericardium is a 

 closed and invaginated sac which lines the inner surface of 

 the fibrous sac and envelops the heart and the roots of the 

 great vessels passing to and from the heart. 



The uninvaginated portion of the wall of the serous sac, 

 which lines the inner surface of the fibrous sac, is called the 

 parietal layer, and the invaginated portion, which envelops the 

 heart and more or less covers the roots of the great vessels, 

 is the visceral portion. The inner surface of the sac is lined 

 by a flat endothelium, which, during health, is smooth and 

 glistening. The parietal and visceral layers are separated, 

 during health, merely by a thin layer of serous fluid, which 

 prevents friction between the two surfaces as they move over 



