RELATIONS OF HEART AND PERICARDIUM 523 



The serous layer is smooth and ghstening and consists of connective tissue, rich in elastic 

 fibres, covered by endotheUum. It lines the interior of the fibrous layer and is continuous with 

 the epicardium or serous covering of the heart. The reflexion of the serous layer from the heart 

 to the fibrous layer of the pericardium occurs at both the arterial and venous attachments of the 

 heart. At the arterial attachment a simple tube of epicardium is reflected along the pul- 

 monary artery and aorta. At the venous attachment the serous layer is reflected from the front 

 of the pulmonary veins on the left, and from the front of these and from the roots of the venae 

 cavse on the right. This reflexion is separated above from that around the aorta and pulmon- 

 ary artery (figs. 424, 436). Around the lower margin of the left lower pulmonary vein (fig. 436) 

 and the root of the inferior vena cava, this reflexion is continuous with an arched reflexion from 

 the back of the atria (figs. 424, 436). The latter reflexion forms a pocket posterior to the atria 

 which is sometimes called the oblique sinus of the pericardium. 



Between the reflexions of the epicardium at the arterial and venous attachments of the heart 

 there is a dorsal communication between the right and left sides of the pericai'dial cavity. This 

 is the transverse sinus of the pericardium [s. transversus pericardii]; it passes behind the aorta 

 and pulmonary artery and in front of the superior cava and left atrium. 



During early embryonic life the sinus transversus is closed by the dorsal mesocardium 

 (see p. 527). The primitive ventral mesocardium, which divides the right and left sides of the 

 pericardial cavity ventraUy, is lost very early. 



The ligament of the left superior cava [lig. venae cavse sinistrae] (figs. 423, 436) 

 is a doubling of the serous layer which passes between the left pulmonary artery 

 above and the left superior pulmonary vein below. It contains, besides some 

 fatty and areolar tissue, the shrunken remains of the left superior vena cava. 

 It is usually connected above with the left superior intercostal vein by means of a 

 small tributary of the latter. Passing from its lower end to the left end of the 

 coronary sinus is the small vena obliqua atrii sinistri (oblique vein of Marshall). 

 The root of the left superior intercostal (and the adjacent part of the left innom- 

 inate) vein; the vein passing from the super or intercostal to the lig. venae 

 cavse sinistrae; the oblique vein of the left atrium, and the coronary sinus all 

 represent parts of the embryonic left vena cava superior. 



Relations. — In front of the pericardium are found the thymus gland or its remains, areolar 

 tissue, the sterno-pericardial ligaments, the left transversus thoracis muscle, the internal 

 mammary vessels, the anterior margins of the pleural sac and lungs, and the sternum. Laterr 

 ally, it is overlapped by the lungs with their pleural sacs, and it is in contact with the phrenic 

 nerves and their accompanying vessels. Posteriorly, it is in relation with the oesophagus and 

 vagus nerves, the descending aorta, the thoracic duct and vena azygos, and the roots of the lungs. 

 Below it is separated by the diaphragm from the stomach and the left lobe of the fiver. 



Vessels. — The arteries of the pericardium are derived from the pericardiac, oesophageal, 

 and bronchial branches of the thoracic .aorta and from the internal mammary and phrenic 

 arteries. 



RELATIONS OF THE HEART AND PERICARDIUM TO THE 



THORACIC WALL 



Heart (fig. 437 A and B). — The base of the heart corresponds posteriorly to the fifth, 

 to the ninth thoracic vertebra). Anteriorly the apex is in the fifth intercostal space, 7.5 to 

 8 cm. (3 to 3i in.) from the medan line. The base (above) corresponds to a line (A) drawn 

 from a point I cm. (* in.) below the second left chondro-costal articulation, and 3 cm (1^^ in. 

 from the median line, to another point (the same distance from the median line) 1 cm. above 

 the right third chondro-sternal articulation. The marge acutus, or lower border corresponds 

 to a line (B) drawn from the apex through the xiphi-sternal articulation, to a point on 

 the sixth costal cartilage 2 cm. to the right of the median line. The right border of the heart 

 may be indicated approximately by a fine (slightly convex to the right) joining the right ends 

 of A and B. The left border corresponds to a fine (slightly convex to the left) joining the left 

 end of A to the apex. 



If a line be drawn from the upper margin of the left third chondro-sternal articulation to the 

 right edge of the sternum in the fifth intercostal space, the upper end of the line wiU He over the 

 centre of the pulmonary ostium, and the lower two-thirds of it (approximately) will overlie 

 the main axis of the tricuspid ostium. The aortic ostium is immediately to the left of the above 

 line with its centre at the left edge of the sternum opposite the third space. The mitral ostium 

 is very largely behind the third left interspace; its upper end is behind the third cartilage, its 

 lower behind the left margin of the sternum opposite the fourth cartilage and space. 



Of the ostia of the heart, the pulmonary is nearest the anterior thoracic wall, the aortic is 

 slightly in advance of the mitral, and the tricuspid is the deepest of aU. 



The pericardium follows the heart closely. The upper end (apex) in the subject used in 

 preparing fig. 437 extended up, behind the sternum, to the lower margin of the first costal 

 cartilage on the right and the upper margin of the second on the left. 



MORPHOGENESIS OF THE HEART AND PERICARDIUM 



The heart is formed by the blending in the median line of two longitudinal endothelial 

 tubes lying ventral to the fore-gut of the early embryo. Each tube is partially surrounded 

 laterally by the splanchnic mesoderm which forms a septum between the right and left sides of 



