The heart [cor] is a hollow organ principally composed of muscle, the myocardium. It is lined internally by endocardium which is continuous with the intima of the blood-vessels. Externally, it is covered by the epicardium, a serous membrane continuous with the serous lining of the pericardium. The form of the heart, when removed from the body without previous hardening, is that of a fairly regular truncated cone. The base [basis cordis] is poorly circumscribed but corresponds, in a general way, to the area occupied by the roots of the great vessels and the portion of the heart-wall between them. The base of the heart is held in position (Not necessarily fixed, for during systole the base performs a greater excursion than does the apex.) chiefly by the great vessels, which are attached to the pericardium; the remainder of the organ is capable of free movement within the pericardial cavity. The interior of the heart is longitudinally divided, into right and left cavities, by a septum passing from base to apex. Each cavity is subdivided into an atrium [atrium cordis] and a ventricle [ventriculus cordis], the former receiving the ultimate venous trunks and the latter giving rise to the main arteries. Thus the left atrium receives the four pulmonary veins, and the right atrium the superior and inferior vena cava and the coronary sinus; the aorta issues from the left ventricle and the pulmonary artery from the right. The ventricles, which constitute the major portion of the heart, may be recognized by their very thick walls. The atria have thinner walls and are less capacious than the ventricles; projecting from each is a diverticulum or auricle [auricula cordis]. The auricles (which receive their name from their resemblance to dog's ears) partially embrace the roots of the pulmonary artery and aorta.

The heart is formed by the blending in the median fine 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 the coelomic cavity. The blended endothelial tubes form the endocardium. The splanchnic mesoderm in relation to the endocardium becomes the myoepicardium, and the double layer connecting the heart dorsally and ventrally with the somatic mesoderm becomes the (temporary) dorsal and ventral mesocardia. The somatic mesoderm of the heart region becomes the pericardium.

Heart. 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 3.25 in.) from the median line. The base (above) corresponds to a line (A) drawn from a point 1 cm. (0.4 in.) below the second left chondro-costal articulation, and 3 cm (1.2 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 will he over the center of the pulmonary ostium, and the lower two-thirds of it (approximatively will overlie the main axis of the tricuspid ostium. The aortic ostium is immediately to the left of the above line with its center 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 all.

The pericardium follows the heart closely. The upper end (apex) in a subject can extend 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.

The right atrium (atrium dextrum) is in the right part of the base of the heart. It is irregularly cuboidai in shape with the apex forming an anteriorly directed ear-shaped portion called the auricle of the right atrium (auricuh dextra).

The pericardium is a cone-shaped, fibro-serous sac which surrounds the heart and contains a small amount of fluid [liquor pericardii]. Its apex is above at the root of the great vessels, and its base below, adherent to the diaphragm. Its connection with the diaphragm is in part to the central tendon and in part to the muscle, especially on the left side. It consists of an outer fibrous layer and an inner serous layer. The virtual space between the serous pericardium and the epicardium is commonly called the pericardial cavity.

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