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.
Orientation of the heart
The apex of the heart [apex cordis] points forward, to the left and downward. The base is directed backward, to the right and upward. The longitudinal axis of the heart forms an angle of about 40° with the horizontal plane and also with the median sagittal plane of the body.
The long axis of the heart is therefore slightly more horizontal than vertical, and slightly more antero-posterior than transverse. The atria are posterior to rather than above the ventricles. To arrive approximately at the longitudinal axis, it is necessary to select the central point of the base. By cutting the vessels short in several hearts, hardened by formalin before removal, a point immediately to the left of the left lower pulmonary vein was selected in determining the data above given. A steel pin was passed through this point to the apex cordis, and the angles controlled by frontal and transverse sections of the thorax. Mention of angular measurements of the axis of the heart could be found only in the text-books of Testut and Luschka; the former gives 40° to the horizontal plane, the latter 60° to the mid-sagittal. Luschka's angle appear to be too large; but further investigation in this direction is desirable.
Size and weight
In the adult, the heart measures about 12.5 cm. (5 in.) from base to apex, 8.7 cm. (31 in.) across where it is broadest, and 6.2 cm. (24 in.) at its thickest portion. In the male, its weight averages about 312 gm. (eleven ounces), and in the female about 255 gm. (nine ounces). It increases both in size and weight up to advanced life, the increase being most marked up to the age of twenty-nine years. The proportion of heart-weight to body-weight is about 1:205 in the adult.
Exterior of the heart
In hearts which have been hardened by injection before removal from the body, the regularity of the heart-cone is disturbed by a well-marked triangular facet, imparted by contact with the diaphragm. This facet is the diaphragmatic surface [facies diaphragmatica], which is directed downward and slightly backward. It ends abruptly along a sharp margin extending from the apex toward the right. This margin is the margo acutus; it separates the diaphragmatic surface from the sternocostal surface. The other margin of the diaphragmatic surface is more rounded and shades gradually into the very wide margo obtusus, which passes almost insensibly into the sternocostal surface. The convex sternocostal surface [facies sternocostalis], directed forward and somewhat upward and to the right, is triangular and bounded below by the margo acutus. To the left, it goes over into the margo obtusus along a line extending from the apex of the heart to the root of the pulmonary artery. The margo obtusus corresponds to the rounded left side of the left ventricle.
The interventricular sulcus is a slightly marked groove indicating the separation of the ventricles upon the exterior of the heart. It lodges coronary blood vessels and a moderate quantity of fat which can be seen through the epicardium.
The anterior part of this groove, sulcus longitudinalis anterior, beginning posteriorly, runs obliquely over the upper part of the margo obtusus on to the sternocostal surface. Crossing the margo acutus to the right of the apex, it is continuous with the sulcus longitudinalis posterior upon the diaphragmatic surface. The diaphragmatic surface is formed about equally by the right and left ventricles, and the sterno-costal surface mainly by the right. Where the longitudinal sulcus crosses the margo acutus it produces a slight notch, the incisura (apicis) cordis.
The atria are separated externally from the ventricles by the sulcus coronarius. This is a horseshoe-shaped groove well marked below and laterally, and interrupted above by the roots of the pulmonary artery and aorta. It lodges the coronary sinus, smaller coronary vessels and fat.