690 



THE THORAX. 



The pericardium is a fibro-serous membrane, and consists, therefore, of two layers ; 

 an external fibrous and an internal serous. 



The fibrous layer is a strong, dense, fibrous membrane. Above, it surrounds 

 the great vessels arising from the base of the heart, on which it is continued in 

 the form of tubular prolongations, which are gradually lost upon their external 

 coats; the strongest being that which incloses the aorta. Below, it is attached 

 to the central tendon of the Diaphragm ; and, on the left side, to its muscular 

 fibres. 



The vessels receiving fibrous prolongations from this membrane are the aorta, 

 the superior vena cava, and the pulmonary arteries and veins. As the inferior 

 cava enters the pericardium, through 'the central tendon of the Diaphragm, it 

 receives no covering from the fibrous layer. 



Fig. 351. Front View of the Thorax. The Ribs and Sternum are represented 

 in Relation to the Lungs, Heart, and other Internal Organs. 



The serous layer invests the heart, and is then reflected on the inner surface 

 of the pericardium. It consists, therefore, of a visceral and a parietal portion. 

 The former invests the surface of the heart, and the commencement of the great 

 vessels, to the extent of two inches from thejj origin ; from these it is reflected 

 upon the inner surface of the fibrous layerTlining, below, the upper surface of the 

 central tendon of the Diaphragm. The serous membrane incloses the aorta and 

 pulmonary artery in a single tube; but it only partially covers the superior and 

 inferior venae cavae, and the four pulmonary veins. Its inner surface is smooth 



