526 ANGIOLOGY 





flattened cells resting on loose connective tissue. The heart invaginates the wall of the serous 

 sac from above and behind, and practically obliterates its cavity, the space being merely a 

 potential one. 



The fibrous pericardium forms a flask-shaped bag, the neck of which is closed by its fusion 

 with the external coats of the great vessels, while its base is attached to the central tendon and 

 to the muscular fibers of the left side of the diaphragm. In some of the lower mammals the 

 base is either completely separated from the diaphragm or joined to it by some loose areolar 

 tissue; in man much of its diaphragmatic attachment consists of loose fibrous tissue which can 

 be readily broken down, but over a small area the central tendon of the diaphragm and the 

 pericardium are completely fused. Above, the fibrous pericardium not only blends with the 

 external coats of the great vessels, but is continuous with the pretracheal layer of the deep cervical 

 fascia. By means of these upper and lower connections it is securely anchored within the thoracic 

 cavity. It is also attached to the posterior surface of the sternum by the superior and inferior 

 sternopericardiac ligaments ; the upper passing to the manubrium, and the lower to the xiphoid 

 process. 



The vessels receiving fibrous prolongations from this membrane are: the aorta, the superior 

 vena cava, the right and left pulmonary arteries, and the four pulmonary veins. The inferior 

 vena cava enters the pericardium through the central tendon of the diaphragm, and receives 

 no covering from the fibrous layer. 



The serous pericardium is, as already sta,ted, a closed sac which lines the fibrous pericardium 

 and is invaginated by the heart; it therefore consists of a visceral and a parietal portion. The 

 visceral portion, or epicardium, covers the heart and the great vessels, and from the latter is 

 continuous with the parietal layer which lines the fibrous pericardium. The portion which 

 covers the vessels is arranged in the form of two tubes. The aorta and pulmonary artery are 

 enclosed in one tube, the arterial mesocardium. The superior and inferior venae cavae and the 

 four pulmonary veins are enclosed in a second tube, the venous mesocardium, the attachment 

 of which to the parietal layer presents the shape of an inverted U. The cul-de-sac enclosed between 

 the limbs of the U lies behind the left atrium and is known as the oblique sinus, while the passage 

 between the venous and arterial mesocardia i. e., between the aorta and pulmonary artery in 

 front and the atria behind is termed the transverse sinus. 



The Ligament of the Left Vena Cava. Between the left pulmonary artery and subjacent 

 pulmonary vein is a triangular fold of the serous pericardium; it is known as the ligament of the 

 left vena cava (vestigial fold of Marshall) . It is formed by the duplicature of the serous layer 

 over the remnant of the lower part of the left superior vena cava (duct ofCuvier), which becomes 

 obliterated during fetal life, and remains as a fibrous band stretching from the highest left inter- 

 costal vein to the left atrium, where it is continuous with a small vein, the vein of the left atrium 

 (oblique vein of Marshall), which opens into the coronary sinus. 



The arteries of the pericardium are derived from the internal mammary and its musculo- 

 phrenic branch, and from the descending thoracic aorta. 



The nerves of the percardium are derived from the vagus and phrenic nerves, and the sympa- 

 thetic trunks. 



THE HEART (COR). 



The heart is a hollow muscular organ of a somewhat conical form; it lies between 

 the lungs in the middle mediastinum and is enclosed in the pericardium (Fig. 490). 

 It is placed obliquely in the chest behind the body of the sternum and adjoining 

 parts of the rib cartilages, and projects farther into the left than into the right 

 half of the thoracic cavity, so that about one-third of it is situated on the right 

 and two-thirds on the left of the median plane. 



Size. The heart, in the adult, measures about 12 cm. in length, 8 to 9 cm. in 

 breadth at the broadest part, and 6 cm. in thickness. Its weight, in the male, 

 varies from 280 to 340 grams; in the female, from 230 to 280 grams. The heart 

 continues to increase in weight and size up to an advanced period of life; this 

 increase is more marked in men than in women. 



Component Parts. As has already been stated (page 497), the heart is sub- 

 divided by septa into right and left halves, and a constriction subdivides each 

 half of the organ into two cavities, the upper cavity being called the atrium, the 

 lower the ventricle. The heart therefore consists of four chambers, viz., right and 

 left atria, and right and left ventricles. 



The division of the heart into four cavities is indicated on its surface by grooves. 

 The atria are separated from the ventricles by the coronary sulcus (auricula- 



