The Heart

The Heart is a four-chambered muscular organ situated in the thorax, in the space between the lungs, and resting on the diaphragm. Its function is to receive the blood from the veins and to propel it through the arteries.

Fig. 54. - The Heart.

The shape of the heart is that of a cone; it has an apex, a base, and two surfaces (inferior and antero-superior). A shallow groove runs round the heart transverse to its long axis, separating the upper auricular portion from the lower ventricular portion. The division of the upper portion into two auricles is only faintly marked, but a distinct groove divides the lower portion into two ventricles.

The heart is enclosed in a fibro-serous sac- the pericardium-which separates it from the surrounding organs. It rests on the diaphragm, the long axis pointing obliquely downwards, forwards, and to the left. On the surface of the body its position is marked by a quadrilateral area, the boundaries of which are as follows:

Right side - a line slightly convex outwards from the upper end of the third costal cartilage to the sixth, its greatest distance from the middle line being 1 1/2 inches.

Base - from the lowest point on the right side to the fifth intercostal space on the left side, 3 1/2 inches from the middle line. This point marks the position of the apex of the heart.

Left side - a line slightly convex outwards from the left extremity of the base line to the lower border of the second interspace on the left side, 1 inch from the mid-line.

Upper line - a line slightly convex downwards connecting the upper extremities of the right and left sides.

The Base of the Heart is formed by the auricles, and is directed upwards, backwards, and to the right. It lies opposite the sixth, seventh, and eighth dorsal vertebrae, separated from them by the oesophagus and descending aorta. It has six orifices- the superior and inferior venae cavae entering the right auricle, and the four pulmonary veins entering the left auricle.

The Apex of the Heart is formed entirely by the left ventricle. It is directed downwards, forwards, and to the left, and is separated from the anterior thoracic wall by the left lung and pleura.

The Inferior Surface is formed by the ventricular portion of the heart, and rests upon the central tendon of the diaphragm and the muscular part to the left of the tendon.

The Antero-Superior Surface is directed backwards, upwards, and to the left. It lies behind the area of the anterior thoracic wall mapped out above, and is divided into an upper and lower part by the auriculo-ventricular groove. At this point are the orifices of the pulmonary artery and aorta from the right and left ventricles respectively.

The Interior Of The Heart

Internally the heart is completely divided into right and left halves by a septum indicated on the outside by the longitudinal grooves. The transverse septum is not complete, as each auricle communicates with the ventricle below by a valvular opening.

The Auricles are almost cubicle in form and each possesses a well-marked process from its upper angle, which projects forwards on the outside of the heart, known as the auricular appendix. The walls are lined with a serous membrane- the endocardium - and are smooth except in the region of the appendix, where there are a few muscular fibres, known as the musculi pectinati. The right auricle receives posteriorly the superior vena cava above and the inferior vena cava below, the orifice of the latter being guarded by the rudimentary Eustachian valve. The left auricle resembles the right auricle, except that it has four openings in its posterior wall - the orifices of the pulmonary veins.

The Ventricles are conical in form and very muscular, the left ventricle being larger and thicker-walled than the right. The longitudinal septum is so placed that the apex of the heart is formed entirely by the left ventricle. The right ventricle is continuous with the right auricle, its base giving origin to the pulmonary artery. The right auriculo-ventricular orifice is guarded by the tricuspid valve, and the orifice of the pulmonary artery is also guarded by a valve composed of three cusps. The left ventricle is continuous with the left auricle, its base giving origin to the aorta. The left auriculo-ventricular orifice is guarded by the mitral or bicuspid valve, and the orifice of the aorta by the aortic valve composed of three cusps.

The cavities of the ventricles are lined with endocardium, and are smooth except for the muscle fibres, which are more numerous and more pronounced than those of the auricles. Each cavity can be divided into two - the body of the ventricle and the vestibule of the vessel to which it gives origin.

The muscles are of three kinds :

Columns Carneae - bundles of muscle-fibre raised in relief on the walls of the ventricle.

Papillary Muscles - which are attached at each end to the walls of the ventricles, but are free in the middle. These prevent over-distension.

Chordae Tendineae-thin bundles of muscle-fibre from the bases of the papillary muscles to the apices of the cusps of the valves.

The Valves of the Heart and large vessels are circular orifices with semilunar flaps of muscle (cusps) projecting inwards from the circumference. These meet in the middle and completely close the orifice. A rush of blood pushes these forward, and the cusps automatically close the orifice when the flow ceases. This keeps the blood flowing in one direction and prevents regurgitation.

The Pericardium is a fibro-serous sac surrounding the heart. It is attached to the diaphragm all round the inferior surface of the heart, and is prolonged upwards, being gradually lost on the great vessels. The inferior vena cava pierces the pericardium posteriorly.