970 A MANUAL OF ANATOMY 



left anterior pulmonary plexus ; (2) branches to the left auricle of 

 the heart ; and (3) the left coronary plexus. 



Coronary Plexuses. — ^These are two in number, right and left. 



The right coronary plexus is formed by branches from (i) the 

 superficial cardiac plexus, and (2) the right half of the deep cardiac 

 plexus. It accompanies the right coronary artery, and furnishes 

 branches to the right auricle and right ventricle of the heart. 



The left coronary plexus is formed by branches which are derived 

 chiefly from the left half of the deep cardiac plexus, but it also 

 receives a few branches from the right half. It accompanies the 

 left coronary artery, and furnishes branches to the left auricle and 

 left ventricle of the heart. 



Ganglia are met with in the coronary plexuses, and in the course 

 of the fibres which supply the walls of the auricles. They are also 

 present on the fibres which supply the walls of the ventricles in 

 the region of the auriculo- ventricular groove, but nowhere else. In 

 the heart of the calf the nerves are easily recognised beneath the 

 visceral layer of the pericardium, as they pass across the muscular 

 fibres in an oblique manner. 



Heart. — ^The heart is a hollow, nmscular organ, which, enclosed 

 within the pericardium, is situated in the middle mediastinum, 

 where it lies obliquely between the two lungs. It is conical in shape, 

 and is free to move within its pericardial sac, except at the base, 

 where it is connected with the great bloodvessels. Its relation to 

 the thoracic wall, during life, is influenced by posture and by the 

 respiratory movements. When a person lies upon the left side, 

 or when the prone position is assumed, the organ is more intimately 

 related to the thoracic wall than in the opposite postures ; and 

 during inspiration it is less intimately connected with the thoracic 

 wall than during expiration. 



General Relations and Topography. — ^The heart lies obliquely 

 behind the lower three-fourths of the body of the sternum. About 

 two-thirds of the organ are contained in the left half of the thoracic 

 cavity, and about one-third in the right half. The base is directed 

 upwards, backwards, and to the right, and lies opposite the 

 bodies of the middle four thoracic vertebrae, namely, the fifth, 

 sixth, seventh, *and eighth. The apex is directed downwards, for- 

 wards, and to the left, and, during life, it strikes the thoracic wall 

 in the fifth left intercostal space, i| inches below the left nipple, 

 and about J inch within the left mammillary line. This point 

 represents the apex-beat, and is about 3J inches from the median 

 line of the sternum. The antero-superior surface, which is convex, 

 lies behind the lower three-fourths of the body of the sternum and 

 the corresponding costal cartilages, right and left, namely, the 

 third, fourth, fifth, and sixth, more particularly those of the left 

 side. This surface is encroached upon by the pleurae and the thin 

 anterior margins of the lungs. Opposite the cardiac notch on the 

 anterior margin of the left lung there is a small portion which is 

 uncovered by lung, unless during deep inspiration, and this cor- 



