THE PERICARDIUM. 353 



the sternum. The fibrous layer is continued above for some distance along tTie large 

 blood-vessels in the form of tubular prolongations, which become gradually lost upon 

 their external coats. The superior vena cava, the four pulmonary veins, the aorta, 

 and the right and left divisions of the pulmonary artery, receive investments of this 

 kind. 



The serous layer of the pericardium not only lines the fibrous layer, but, like 

 other serous membranes, is reflected on the surface of the viscus which it invests. It 

 has, therefore, a visceral and a parietal portion. The parietal portion adheres firmly 

 to the fibrous membrane. It is reflected and becomes continuous with-the visceral 

 portion along the great vessels, about 1 to 1 J inches from the base of the heart. For 

 this distance the serous membrane encloses the aortic and pulmonary arterial trunks in 

 a common tubular sheath, so that a passage, the transverse sinus of the pericardium, 

 is formed between these vessels in front and the auricles of the heart behind (fig. 

 305, 5, 5). It is reflected also upon the superior vena cava (o), and on the four pul- 

 monary veins (p,p'), and forms a deep recess behind, between the entrance of the right 

 and left veins into the left auricle. The inferior vena cava (c') receives only a very 

 scanty covering of this membrane (3, 2), inasmuch as that vessel enters the right 

 auricle almost immediately after passing through the diaphragm, and is only 

 partially surrounded by a reflection of the pericardium in the narrow interval between 

 these parts. None of the vessels, indeed, joining the hdart, with the exception of 

 the aorta and pulmonary artery where they are united together, receive a complete 

 covering from the pericardium, or can be said to be entirely enveloped in the sac. 



When the left pulmonary artery and subjacent pulmonary vein are separated, a 

 triangular fold of the pericardium, the vestigial fold of Marshall, is seen between 

 them. It is from half to three-quarters of an inch in length, and from half to one 

 inch deep, and is formed by a duplicature of the serous layer, including areolar and 

 fatty tissue, together with vessels and nerves. It also encloses a vestige of the left 

 superior vena cava (duct of Cuvier) existing in early embryonic life, in the form of 

 a small fibrous band which may often be traced from the left superior intercostal 

 vein above the pulmonary artery downwards to the side of the left auricle, where it 

 is lost in a narrow streak coursing round the root of the lower left pulmonary 

 vein. 



Beneath the serous layer of the pericardium there are more or less developed ac- 

 cumulations of fat, especially in the grooves of the heart, surrounding the cardiac 

 vessels and nerves, along the sharp margin of the right ventricle, around the arterial 

 trunks, and sometimes on the surface of the diaphragm. One small mass often 

 causes a transverse projection of the serous membrane on the right side and front 

 of the ascending aorta about an inch above its origin, marking the upper limit of 

 the right auricular appendix. Minute villiform processes are also common, occurring 

 singly or in tufts chiefly along the edges of the auricles. 



The pericardium is in relation in front and behind with the anterior and posterior 

 mediastina and their contents. Anteriorly also it is covered by the pleurae and to 

 some extent by the lungs, except below, where it approaches the surface in the 

 angular space to the left of the lower part of the sternum. At the sides it is in 

 contact with the phrenic nerves, as well as with the pleurae and their contained 

 viscera. Its relations to the diaphragm and great vessels have been already 

 noticed. 



In structure the serous layer of the pericardium agrees with that of serous mem- 

 branes generally, being formed of connective tissue containing a network of elastic 

 fibres, blood-vessels and lymphatic vessels. 



Vessels and nerves. The parietal pericardium is supplied with blood by small offsets 

 of the descending thoracic aorta, by the superior phrenic and pericardial branches of the 



