204 APPLIED ANATOMY. 



When the internal mammary artery is seen lying beneath, it is to be either ligated 

 or drawn to one side. The triangularis sterni muscle is either incised or drawn to 

 the outer side along with the edge of the left pleura. The pericardium can then be 

 lifted with forceps and incised and the drainage-tube introduced. 



THE HEART. 



In size the heart is somewhat larger than the clenched fist. It measures 12.5 

 cm. (5 in.) in length, 7.75 cm. ($/4 in.) in width, and 6.25 cm. (2^ in.) in thick- 

 ness. Its weight in the adult male is 250 to 300 Gm. (8 to 10 oz. ), in the female 

 it is 60 Gm. (2 oz. ) less. 



It lies enclosed in its pericardium in the middle mediastinum between the 

 sternum (from the upper edge of the third costal cartilage to the sternoxiphoid artic- 

 ulation) in front, and the bodies of the fifth, sixth, seventh, and eighth thoracic ver- 

 tebrae behind. Laterally it reaches from two centimetres to the right of the sternum 

 nearly to the left nipple line. On each side of it are the lungs, from which it is 

 separated by the pleurae and pericardium with the phrenic nerves between. Above 

 are the great vessels and below it rests on the central tendon of the diaphragm. 



In shape the heart resembles an acorn, the atria (auricles), forming the upper 

 right portion and the ventricles the lower left portion. It lies with its right side 

 resting on the diaphragm and its apex pointing forward and to the left. 



For convenience one speaks of a base, an apex, a right border, a lower border, 

 and a left border. 



OUTLINES OF THE HEART. 



The base of the heart is opposite the upper border of the third costal cartilage. 

 It is here that the superior vena cava ends and the aorta begins. It extends from 

 1.25 cm. (}4 in.) to the right of the sternum to 2.5 cm. (i in.) to the left of the 

 sternum. 



The right border of the heart extends from 1.25 cm. (Yz in.) to the right of the 

 sternum at the upper border of the third costal cartilage in an outwardly curved line 

 to the junction of the seventh rib and the sternum. In the fourth interspace it may 

 reach 2.5 cm. (i in. ) beyond the right edge of the sternum. 



The lower border passes from the seventh right chondrosternal junction across 

 the sternoxiphoid joint outward in the fifth interspace to the apex beat, which is 4 

 to 5 cm. ( i Yt in. to 1 2/^ in. ) below and to the inner side of the nipple and about 

 8.75 cm. (3^ in.) to the left of the median line. This marks the extreme left limit 

 of the heart. In children the apex is higher it is in the fourth interspace. In old 

 people it is lower. 



The left border arches upward from the apex beat, as just given, in an inward 

 and upward direction to 2.5 cm. (i in.) to the left of the sternum at the upper border 

 of the third costal cartilage. 



The atrio- (auriculo-) ventricular groove or line of junction between the atria 

 (auricles) and ventricles runs from the sixth right chondrosternal junction upward 

 and to the left to the third left chondrosternal junction. The atria lie above and to 

 the right of this line and the ventricles below and to the left. 



The right atrium (auricle) and right ventricle lie anteriorly and the left atrium 

 and left ventricle lie posteriorly. In the right atrioventricular groove runs the right 

 coronary artery. As it lies on the anterior portion of the heart it is liable to be injured 

 in stab-wounds and give rise to fatal bleeding, as may also the interventricular 

 branch of the left coronary artery as it passes down near the left border of the heai t 

 between the right and left ventricles. 



The Portion of the Heart Uncovered by Lung-tissue. When the lungs are 

 distended the right lung covers the heart to the median line. The left lung leaves 

 the median line at the level of the fourth costal cartilage and curves outward and 

 downward to about the apex beat in the fifth interspace, 2.5 cm. to the inner side of 

 the nipple line. At this point a small piece of the lung, the lingula, sometimes 

 curves around in front of and below the extreme tip of the heart. As the air leaves 

 the lungs they retract and their anterior borders hardly reach the edges of the sternum. 



