The heart is a hollow muscular organ, surrounded by a membranous sac called the pericardium. It lies between the two pleurae of the lungs, and rests upon the cord-like tendon of the midriff, in the cavity of the chest.

It shape is conoidal, though it is somewhat flattened upon that side that rests upon the tendon of the diaphragm. Its apex inclines to the left side, touching the walls of the thorax between the fifth and sixth ribs. It measures about five inches and a half from its apex to its base, three and a half inches in the diameter of its base, and weighs about six or eight ounces. It contains four cavities, which perform two functions: that of receiving the blood and emptying the blood into the lungs, and that of receiving it again after it has been oxygenated, and distributing it throughout the vascular system. The receptacles are auricles, and the ventricles propel the blood to the lungs and through the body.

The auricle and ventricle of the right side receive and propel the venous blood into the lungs. The auricle and ventricle of the left side receive and propel the arterial blood throughout the system.

The blood circulates as follows: The ascending and descending vena cavae empty the blood (venous) into the right auricle; from here it passes to the right ventricle, through an opening protected by a valve, downwards; from the right ventricle it is propelled through the pulmonary artery, which divides into two branches, to the lungs; in the lungs it is oxygenated by the inspired air; it is then brought from the lungs, by four pulmonary veins, into the left auricle. The left auricle has an opening communicating with the left ventricle, protected by a valve opening downwards, and from the left ventricle it passes into the aorta, thence to be distributed throughout the body.

The right auricle is a cavity of irregular shape, somewhat oblong, and like a cube; anteriorly it has a convexity which is called its sinus; superiorly there is an elongated process resembling the ear of an animal, whence the term auricle. Its walls are thin, and composed of muscular fibres, which are called musculi pectinati, on account of their parallel arrangement, resmbling the teeth of a comb. The superior, and inferior vena cavae enter the auricle from behind. The elevation between the orifices is called the tuberculum Loueri. The coronary veins open into this cavity, and their orifices are protected by the valves of Thebesius. The opening to the ventricle is circular, and surrounded by a dense white line.

The right ventricle is a triangular cavity, with thick walls and of greater capacity than any other cavity of the heart. Its muscular structure is in the form of large fleshy bundles, called columnos carneas, from which proceed thin, white cords, called chordae tendineae, attached to the edge of the tricuspid valve. This valve is circular, having at its lower edge three spear-pointed processes, whence its name. It closes downwards, and prevents the blood from returning into the auricle, and, therefore, it passes out by the pulmonary artery. The valves protecting the orifice of the pulmonary artery open outwards, and are called the semi-lunar valves. They are formed by three half-moon-shaped folds of the lining membranes, and their use is to prevent the blood returning from the artery to the ventricle, when it dilates. Behind each valve is a pouch or dilatation, called the Sinus of Valsalva, into which the blood flows by its reflux tendency upon the dilatation of the ventricle, and thus these valves are closed. The pulmonary artery is of the same diameter as the aorta, but its walls are thinner. After its origin it curves upwards and backwards, and divides into two branches, the right of which is larger than the left, and passes under the arch of the aorta.

The left auricle is more concealed from its natural position than the right. The four pulmonary veins enter into it, which give it a quadrangular shape. Its walls are muscular and somewhat thicker than those of the right auricle. The partition between the auricles is not always perfect even in adult life.

The left ventricle forms by its cavity the apex of the heart; it is like a cone in shape. Its walls are thick, and its columnae carneae numerous, strong and projecting; the chordae tendinea are well developed, and attached to the bicuspid or mitral valve. This valve consists of two leaflets, one of which is much larger than the other. The contraction of the ventricle closes the valve, and the blood passes out by the aorta. The heart is supplied with blood by the right and left coronary arteries; the veins which accompany them empty by a common trunk into the right auricle.

It will thus be seen what a complex piece of machinery the human heart is, and how vital the organ must be. It will be apparent to every reader that the least interruption or derangement of its functional action is sure to be manifested upon the integrity of the general system. Any valvular derangement or breaking down of the septum between the auricles and ventricles will allow the commingling of arterial with venous blood, threatening death with asphyxia. Atrophy and hypertrophy interfere with the muscular action of the walls of the heart, and, in fact, it will be obvious from the complex character of the structural anatomy and the importance of the functional actions of the heart, that any disease assailing the organ is attended with danger.
 In circulation the contraction of all the cavities is followed by their dilatation. The contraction is called the systole; the dilatation, the diastole.   What is called the impulse of the heart occurs during the diastole. The heart's impulse is the shock communicated by its apex to the walls of the thorax, in the neighborhood of the fifth and sixth ribs. The imppulse is not the same as the arterial pulse. The heart emits two sounds, first and second, followed by an interval. The first are the longest. The following table shows the connection of the sounds of the heart with its movement.:--

First Sound. -- Second stage of ventricular diastole. Ventricular systole, and auricular  diastole. Impulse against the chest. Pulse in the arteries.

Second Sound. -- First stage of ventricular diastole.

Interval. -- Short repose, then auricular systole, and second stage of ventricular diastole,  etc.

Each cavity of the heart will hold about two fluid ounces, but it is probable that the ventricles do not entirely empty themselves at each stroke; they will therefore discharge about one and one-half ounces at each pulsation. Reckoning 75 pulsations to the minutes, there will pass through the heart in this time 112 ounces or 7 lbs. of blood. The whole quantity of blood in the human body is equal to about one-fifth of its weight, or 28 lbs. in a person weighing 140 lbs. This quantity would therefore pass through the heart once in four minutes, or about fifteen or twenty times an hour. It is very probable that circulation is much more rapid than this estimate. The number of contractions of the heart in a minute is about 70 or 75. The frequency of its action gradually diminishes from the commencement to the end of life. Just after birth it ranges from 140 to 130, in old age 65 to 50. Age, sex, muscular exertion, emotions, and temperament exert a controlling influence over the heart's action. In persons of sanguine temperament the heart beats more frequently than in those of the phlegmatic, and in the female sex more frequently than in the male. Its action is also increased after a meal, and by rising from a recumbent to a sitting or standing posture. The time of day also affects it; the pulse is more frequent in the morning, and becomes gradually slower as the day advances.

The pulse is always a sure index of health or disease. In inflammation and fevers the pulse is much more frequent than during health. When the vital powers decline it becomes frequent and feeble. In nervous affections with more oppression than exhaustion of the forces, the pulse is often remarkably slow.

The membrane lining the interior of the heart is called the endocardium, and the enveloping membrane on the exterior the pericardium.