926 THE ORG Ays OF CIRCULATION 



sides of tlie pericardial sac; this is tlie irreat sinus of the })ericardiuni. The infe- 

 rior vena cava rt'ccivcs a very iiii]i«rfc(t covering. 



The vestigial fold of the pericardium is a doul»ling of the serous layer which 

 passes hetween the left pulmonary artery ahove and left superior i)uhnonary vein 

 below. It contains, besides some fatty and areolar tissue, the shrunken remains of 

 the left superior vena cava. It is connected aliove with the left superior intercostal 

 vein, and below with the left auricle and oblique vein of Marshall, — these veins 

 with the coronary sinus iiaving originally formed ])ortions of the left ujjper cava. 



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

 the sterno-jiericardiac ligaments, the left triangularis sterni muscle, the internal 

 mammary vessels, the anterior margins of the pleural sacs and lungs, and the ster- 

 num. Laterally, it is overlapped by the lungs with their pleural sacs, and it is in 

 relation with the i)hrenic nerves and their accompanying vessels. Posteriorly, it is 

 in relation with the o?so]>hagus, and pneumogastric nerves, the descending aorta, 

 the thoracic duct, and vena azygos major. 



On opening the pericardium the following structures may ])e observed: the 

 greater part of the right and a portion of the left ventricle with the interventric- 

 ular sulcus, the right auricle and right auricular appendix (the latter overlaji- 

 })ing the root of tlie aorta), the auriculo-ventricular suk-us; the first portion of the 

 aorta with the superior vena cava on its right side, and the pulmonary artery 

 at first overlying and then passing to its left side, with the tip of the left auricular 

 appendix. If the heart be raised upwards and to the right, its posterior surface 

 is seen to consist of the greater part of the left ventricle, and the remaining por- 

 tion of the right, the interventricular sulcus dividing the two. Lying transversely 

 above the ventricle in the auriculo-ventricular groove is the coronary sinus, 

 receiving some of the cardiac veins, and also the oblique vein of Marshall from 

 the back of the left auricle, the roots of the pulmonary veins, and the very small 

 portion of the inferior cava above the diaphragm, may also l)e noticed. 



The two ventricles rest in about ecjual ])roportions on the central tendon of the 

 diaphragm. The main branches of the coronary arteries of the heart occupy the 

 grooves. 



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

 oesophageal, and bronchial branches of the thoracic aorta and from the internal 

 manmiary and phrenic arteries. 



THE HEART 



The heart — enclosed in the pericardium — occupies the greater ]iart of the middle 

 mediastinal space. It is a somewhat flattened, cone-shaped, hollow, muscular 

 organ. 



Position. — In the adult the heart lies obliquely behind the lower two-thirds of 

 the sternum, projecting considerably to its left side. . Its base is directed slightly 

 upwards and backwards, and to the right; its apex downwards and forwards, and 

 to the left. The base corresponds to the sixth, seventh, and eighth thoracic 

 vertebrae; and the apex to the chest wall on the left side, between the fifth and 

 sixth rib cartilages. 



It may be mapped out on the chest wall in the following manner: a line drawn 

 across the sternum about the level of the lower borders of the second costal carti- 

 lages, passing half an inch to the right and one inch to the left of the sternum, will 

 indicate the jiosition of its base, from which the great vessels arise. 



The apex, as l)efore ol)served, strikes the chest wall between the fifth and sixtli 

 rib cartilages on the left side, at a spot about a couple of inches (50 mm.) below 

 the nipple, and one inch (25 mm.) to its sternal side, or three and a quarter inch(>s 

 (81 nnn. ) from the middle line of the sternuni, in the fifth interspace. 



The lower border is formed l)y the right ventricle and rests on the central 

 tendon of the diai)hragm. It is defined by a line curving downwards, connnencing 

 at the apex, and crossing close to the sterno-xiphoid articulation and ternnnatiug 

 at the right edge of the sternum near its junction with tlie sixth cartilage. 



The lateral borders may l)e completed ])y drawing curved lines upwards from 



