Heart , a hollow muscular organ, the centre of the circulatory apparatus, situated within the cavity of the chest, giving origin to the arteries and receiving the termination of the veins. In the human species it is conical, with its base upward and backward, on the median line, from the fourth to the eighth dorsal vertebra, and its point directed downward, forward, and to the left, reaching nearly to the level of the sixth costal cartilage. In the human subject its length is about 5 in., its breadth about 3 1/2 in., and its average weight a little less than 10 oz. Its size in any particular individual corresponds very nearly with that of the closed fist. In man and all the warm-blooded vertebrate animals the heart is double, that is, it consists of two lateral halves, right and left, separated by an impervious partition; the right half being destined to receive the venous blood returning from the general circulation and send it to the lungs; the left half receiving the arterialized blood from the lungs and sending it into the arterial system, to be distributed throughout the body.

The left half is considerably the thicker, more muscular, and more powerful; the impulse required to propel the blood through the general circulation being greater than that needed to carry it through the vessels of the lungs. Each half consists of two cavities communicating with each other, called respectively the "auricle" and the " ventricle." The auricle is the smaller and thinner, receiving the blood directly from the veins, while the ventricle is the larger and stronger, receiving the blood from the auricle and discharging it into the corresponding artery. Between the auricle and ventricle, on each side, is an opening, the "auriculo-ventricular orifice," through which the blood passes from the former to the latter. From the right ventricle originates the pulmonary artery, going to the lungs, and from the left ventricle the aorta, the main trunk of the arterial system. The heart is so placed within the chest that the right auricle and ventricle are situated rather upon its anterior, and the left auricle and ventricle rather upon its posterior portion; so that in a front view the right side of the organ comes most prominently into notice.

The great arteries also arise from the base of the heart in such a way that the commencement of the pulmonary artery is in front and a little to the left, that of the aorta rather behind and to the right. Thus the two streams of blood, arterial and venous, in passing through the heart, cross each other in an obliquely spiral direction; the venous blood, which enters the right auricle, passing out by the pulmonary artery on the left, and the arterial blood, which enters by the left auricle, passing out by the aorta on the right. The structure of the heart, as already intimated, is essentially muscular. Its muscular fibres, which form by far the greater part of its mass, belong to the striped variety, resembling in this respect the fibres of the voluntary muscles. They are distinguished from the latter by two peculiarities: their smaller size, and the fact that, although generally arranged in a parallel direction, they frequently anastomose with each other, thus forming a kind of muscular network, and no doubt affording mutual points of support at the time of their contraction. The general arrangement of the muscular fibres of the heart is spiral and circular. They originate principally from a strong fibrous ring surrounding the auriculo-ventricular orifices.

Thence the fibres destined to form the walls of the auricles pass upward and encircle these cavities with a variety of interlacing bands, some of which pass across the intervening septum and thus connect the two auricles by mus-cular fibres common to both. The fibres of the anterior portion of the right ventricle also pass obliquely downward across the interventricular septum, and wind spirally round the apex of the left ventricle. The deep-seated fibres of each ventricle are still more strongly spiral and even nearly circular in direction, like the transverse fibres of the intestine, so that when they contract like the fingers of a closed hand, they nearly obliterate the cavity of the ventricles. At their termination they again run upward, and are attached to the auriculo-ventricular ring from which they originated, or, by fleshy columns, to the tendinous chords of the ventricular valves. Besides its muscular tissue, the heart is covered externally by a serous and fibrous membrane, the pericardium, and is lined internally by another serous membrane, the endocardium; the latter of which is continuous with the lining membrane of the blood vessels.

At each of the great orifices of the heart are membranous valves, composed of a thin and flexible but strong fibrous tissue, so arranged as to allow the blood to pass freely onward in its course, but to prevent its flowing back in the opposite direction. Those situated at the auriculo-ventricular orifices are the ventricular valves; those at the orifices of the great arteries are the arterial valves. Each set, on the right and left side, has also received a special name corresponding to certain peculiarities of form or position. The right ventricular valve is composed of a broad fibrous sheet with three main points or divisions, whence it is called the tricuspid valve. Its floating edges are easily displaced from the auricle toward the ventricle, thus allowing the blood a free passage in this direction; but on being forced backward, from the ventricle toward the auricle, its free edges come in contact with each other and are firmly held in this position by a number of tendinous chords running from its edge and under surface to the inner wall of the ventricle below, which are now put upon the stretch. Thus the passage from the ventricle to the auricle is completely closed, and it can be opened only in a forward direction.

The left ventricular valve is similar to the right, except that it consists of only two main pointed sheets, like the two halves of a bishop's mitre; it is therefore called the mitral valve. It is supported in position by tendinous chords, as in the former instance (fig. 3). At each of the arterial orifices are three valves, having the form of semilunar bags, with their concavities toward the wall of the artery to which they are attached, and their convexities toward the cavity of the ventricle. Thus when the stream of blood passes from the ventricle into the artery, the valves are flattened against the wall of the vessel, and offer no resistance to its current; but when distended by a backward impulse they fill with blood, and, their edges coming in contact, they close the orifice of the ventricle in this direction. Both sets of arterial valves, from the similarity of their figure, are called semilunar valves; but they are also known as the pulmonary or the aortic valves, according to the particular vessel in which they are situated.

They are usually described in connection with the heart; but it is evident from what has been said above that they belong in reality rather to the arterial system. - The action of the heart consists in an alternate contraction and relaxation, by which at one instant it receives the blood from the veins and at another propels it into the arterial system. In this process the two auricles, right and left, contract simultaneously; and the two ventricles subsequently contract, also at the same instant with each other. Still the auricular and ventricular contractions are not distinctly and separately alternate with each other, to the same extent as the strokes of the two pistons of a force pump. The action of the heart appears rather to consist in a single continuous contraction, which begins at the auricle and thence runs forward to terminate at the ventricle. The ventricular action is much more powerful than that of the auricle; and of the two ventricles the left is so much the more important in size and strength, that mainly to the action of this part are due the changes in form and position of the heart at the instant of contraction.

The contraction of the ventricles is almost instantly followed by their relaxation; and while in this condition they are gradually filled by the blood flowing steadily into them from the veins and through the quiescent auricles. Then comes the contraction of the auricles, which completes the distention of the ventricular cavities; and this distention is at once followed by the vigorous contraction of the ventricles, discharging their blood in great abundance, to be followed again by a period of relaxation. This is the succession of the phenomena which present themselves when the heart is seen in activity during life. - At every ventricular contraction the substance of the heart becomes harder; it twists slightly upon itself from left to right; and its point strikes the walls of the chest, in the human subject, in the fifth intercostal space, a little to the left of the edge of the sternum. The induration of the organ at the moment of its activity is due to its muscular nature, since the same tension and momentary induration can be felt in any of the voluntary muscles in contraction. Such a muscle, when in activity, becomes swollen and at the same time harder to the touch; and the induration disappears when the muscle becomes relaxed.

That the induration of the heart really coincides in time with the contraction of the ventricle and the compression of the blood, may be easily shown by an experiment first performed by Harvey, namely, by thrusting a small metallic canula through the substance of the left ventricle into its cavity, upon which the blood is driven out from the external orifice of the canula in interrupted jets, each jet corresponding in time with the induration and tension of the substance of the heart. The movement by which the point of the heart strikes the walls of the chest produces a visible and tangible undulation of the integument at the fifth intercostal space. This is called the " impulse." The impulse is a valuable guide for the physician in many cases, as it is increased to an abnormal degree in cases of hypertrophy of the heart, and also displaced from its natural location when the organ is enlarged or from any cause altered in position. The heart is loosely attached, at its base, by means of the great vessels to the anterior surface of the spinal column; but its body and apex are nearly unconfined, being covered only by the pericardium, whose exterior is itself a serous membrane, being invested by the free surface of the pleura.

Consequently, while the base of the heart is generally fixed, the point of the organ may move in various directions when compressed or encroached upon by neighboring growths. When the heart is simply enlarged, its point is turned further toward the left side. When the pericardium is distended with an accumulation of serum, the point of the organ fails to reach the walls of the chest, and the impulse disappears altogether. When there is an accumulation of fluid in the left pleural cavity, the point of the heart may be turned completely over to the right of the sternum, so that the impulse may be felt on that side. There are also cases, though rare, in which, from a congenital malformation, the relative position of all the internal organs is reversed, the heart being placed on the right side, the liver on the left, the spleen and great pouch of the stomach on the right, etc. The abnormal position of the heart may be more easily distinguished during life than that of any other internal organ, owing to its very perceptible impulse against the walls of the chest. - The action of the heart is accompanied by two peculiar sounds, very audible when the ear is applied to the front part of the chest.

These sounds follow each other, in the healthy condition, with perfect regularity, and are both produced at each cardiac contraction. They are known as the first and second sounds of the heart. The first sound occurs at the instant of the ventricular contraction. It is distinct, but slightly prolonged, and is heard most plainly over the anterior and lateral surface and toward the apex of the organ. It is produced by the shutting back and sudden tension of the ventricular valves, combined, according to some authorities, with the muscular action of the heart's walls and the passage of the blood through its cavities. The second sound, which immediately follows the first, is shorter, but sharper and clearer. It is heard most distinctly over the sternum at the level of the third costal cartilage. Doubtless it is due exclusively to the shutting back of the arterial valves (aortic and pulmonary); since, in the first place, it is heard with remarkable clearness directly over the situation of these valves, and becomes fainter and less marked as the ear is moved further away; and secondly, it has been found by experiment upon the larger animals that if these arterial valves be hooked back by a curved needle introduced into the base of the great vessels, the second sound immediately disappears, but returns again as soon as the valve is liberated.

The character, regularity, position, and relative intensity of these sounds often become valuable signs to the medical man in determining the nature and progress of affections of the heart. - The pulsations of the heart follow each other, in every species of animal, with a certain degree of rapidity. Generally speaking, this rapidity is greater in the warm-blooded, less in the coldblooded animals. In species of the same class, the pulsations are more rapid in the smaller, less so in the larger. In the human subject the average rapidity of the cardiac pulsations, for an adult male, is about 70 a minute. They are more frequent, as a rule, in young children and in the female sex; and there are variations in this respect, within certain limits, in particular persons, owing to individual peculiarities of organization. Thus it would not necessarily be an abnormal sign to find in any particular person the habitual frequency of the heart's action from GO to G5, or from 75 to 80 a minute. As a general rule, the heart's action is slower and more powerful in fully developed and muscular persons, more rapid and feebler in those of slighter organization.

In all cases the heart is temporarily excited to increased frequency of action by unusual exertion or by nervous excitement; and its pulsations also become more rapid in many diseased conditions, particularly those of a febrile character. The heart's action is always purely involuntary, being entirely independent of the will. Its persistency is very remarkable. In the warm-blooded animals the heart's action ceases in a very few minutes after the destruction of the nervous centres, or after the circulation of the blood in its own vessels is arrested by haemorrhage, by ligature, by the stoppage of respiration, or by excision of the heart itself; since, like the other organs in these animals, it requires a constant supply of freshly arterialized blood to preserve its vitality. But in the cold-blooded animals it will continue to beat for many hours after it has been emptied of blood, and even after it has been cut out of the chest and separated from all its nervous and vascular connections. The pulsations of the separated heart have been seen to go on in the eel for six hours, in the torpedo for nine hours, and in the salmon for twenty-four hours.

In the turtle, the brain and medulla oblongata having been destroyed, the heart, left in situ but drained of blood, continued to pulsate for more than eight hours; and the heart of the same animal, cut out of the body, drained of blood, divested of pericardium, and exposed upon an earthen plate, continued its action for four hours. This shows that the heart, as a muscular organ, is endowed with an unusual degree of irritability. Other muscles contract only occasionally, on the application of a special stimulus; but with the heart the contractions are incessantly repeated, with only momentary intervals of relaxation, from the first periods of embryonic existence to the latest moment of life. This irritability is so great that the contact of any foreign substance, even that of the atmospheric air or the plate upon which the separated organ rests, is sufficient to produce a contraction, which is repeated at intervals as soon as the muscular irritability has again accumulated by a short interval of relaxation. During life, it is believed, the immediate stimulus to each cardiac contraction is the filling of its cavities by the blood which flows into them.

When this distention is complete, the ventricles respond by a contraction, empty themselves, and then remain quiescent until again filled to their full capacity, when the motion is repeated. But for this to go on, as it does, indefinitely, the inherent irritability of the heart must be very great, as compared with other muscles; and in fact, as mentioned above, its motions may continue to be excited for a considerable period by the contact of the external atmosphere or other foreign bodies. - In the inferior animals the heart varies in size, form, and construction, according to the general external configuration of the body, and particularly according to the arrangement of the organs of respiration, and the activity with which this function is performed. In the warm-blooded animals, namely, mammalia and birds, whose respiration is very active and performed by lungs, the heart is a double organ with four cavities, as in man; consisting of a right auricle and right ventricle destined for the pulmonary circulation, and a left auricle and left ventricle for the general circulation.

Since in these animals, in order to provide for the necessary activity of respiration, all the venous blood must constantly pass through the lungs before reaching the arterial system, the two sets of cavities in the heart are completely distinct from each other, the venous blood being carried exclusively to the lungs for aeration, and the pure arterialized blood alone being disseminated through the arterial system. But in the reptiles which are air-breathing animals but of sluggish respiration, the two ventricles are imperfectly separated from each other, the septum between them being more or less perforated, or, as in the crocodiles, the two ventricles communicating with the same artery.

The Heart, Great Vessels, and Lungs   Front View.

Fig. 1. - The Heart, Great Vessels, and Lungs - Front View.

R. V, right ventricle; L. V„ left ventricle; R. A., right auricle; L.A., left auricle; Ao., aorta; P.A., pulmonary artery; P. V., pulmonary veins; R.L., right lung; L.L., left lung; V.S., vena cava superior; V.I., vena cava inferior.

Muscular Fibres of the Heart.

FIG. 2. - Muscular Fibres of the Heart.

Horizontal Section of the Human Heart.

Fig. 3. - Horizontal Section of the Human Heart, at the level of the Ventricular Orifices.

A. Orifice of pulmonary artery. B. Orifice of aorta. C, C, C. Tricuspid valves. D., D. Mitral valves.

In the batrachia, the heart consists of but three cavities, two auricles, and one ventricle. Thus the venous blood from the right auricle and the aerated blood from the left auricle are mingled in the cavity of the single ventricle, and this mixed blood is sent partly to the lungs and partly to the general circulation. Thus the blood sent to the organs of the general circulation is never so highly aerated as in the mammals, and the blood sent to the lungs is never completely venous. This is no doubt one reason why respiration can be so long suspended in these animals without producing death. In fishes there is but a single auricle and a single ventricle, destined to receive the venous blood coming from the body, and to propel it into an arterial trunk, by which it is conveyed to the gills. After passing through these organs the arterialized blood is again collected in a single trunk corresponding to the aorta, and thence distributed throughout the body. In the fishes, accordingly, the entire heart represents the right side of that organ, as it exists in the mammalia; its contractions being sufficient to insure the passage of the blood through the organs of respiration, and afterward also through the whole arterial system.