HEART, ANATOMY OF. The heart in man is a four chambered muscular bag, which lies in the cavity of the thorax between the two lungs. It is surrounded by another bag, the pericardium, for protective and lubricating purposes (see COELOM AND SEROUS MEMBRANES). Externally the heart is somewhat conical, its base being directed upward, backward, and to the right, its apex downward, forward, and to the left. In transverse section the cone is flattened, so that there is an anterior and a posterior surface and a superior and inferior border. The superior border runs obliquely downward and to the left, the inferior border is horizontal (see fig. i ). The divisions between the four chambers of the heart (namely, the two auricles and two ven tricles) are indicated on the surface by grooves, and when these are followed it will be seen that the right auricle and ventricle lie on the front and right side, while the left auricle and ventricle are behind and on the left.
The right auricle is situated at the base of the heart, and its outline is seen on looking at the organ from in front. Into the posterior part of it open the two venae cavae (see fig. 2) . In front and to the left of the superior vena cava is the right auric ular appendage which overlaps the front of the root of the aorta, while running obliquely from the front of one vena cava to the other is a shallow groove, which indicates the original separation between the true auricle in front and the sinus venosus behind. When the auricle is opened by turning the front wall to the right as a flap the following structures are exposed: I. A series of ridges on the anterior wall and in the appendage, running downward from the last and at right angles to it, like the teeth of a comb ; these are known as musculi pectinati.
2. The orifices of the superior and inferior vena cava.
3. The remains of the Eustachian valve (see fig. 2), which in the foetus directs the blood from the inferior vena cava, through the foramen ovale, into the left auricle.
4. On the posterior or septal wall, between the two auricles, is an oval depression, called the fossa ovalis (see fig. 2), the re mains of the original communication between the two auricles. In about a quarter of all normal hearts there is a small valvular communication between the two auricles in the left margin of this depression.
5. On the left side, opening into the right ventricle, is the right auriculo-ventricular opening.
The right ventricle is a triangular cavity (see fig. 2) the base of which is largely formed by the auriculo-ventricular orifice. To the left of this it is continued up into the root of the pul monary artery. Its anterior wall forms part of the anterior sur face of the heart, while its posterior wall is chiefly formed by the septum ventriculorum, between it and the left ventricle. In transverse section it is crescentic, since the septal wall bulges into its cavity. In its interior the following structures are seen: I. The tricuspid valve (see fig. 2) which guards against reflux of blood into the right auricle. This consists of a short cylindrical curtain of fibrous tissue, which projects into the ventricle from the margin of the auriculo-ventricular aperture, while from its free edge three triangular flaps hang down, the bases of which touch one another.
2. The chordae tendineae are fine fibrous cords, which fasten the cusps to the musculi papillares and ventricular wall, and pre vent the valve being turned inside out when the ventricle contracts.
3. The columnae carneae are fleshy columns, and are of three kinds. The first are attached to the wall of the ventricle in their whole length and are merely sculptured in relief, as it were; the second are attached by both ends and are free in the middle; while the third are known as the musculi papillares and are attached by one end to the ventricular wall, the other end giving attachment to the chordae tendineae. These musculi papillares are grouped into three bundles (fig. 2).
4. The pulmonary valve (see fig. 2) at the opening of the pulmonary artery has three crescentic, pocket-like cusps, which, when the ventricle is filling, completely close the aperture, but during the contraction of the ventricle fit into three small niches known as the sinuses of Valsalva, and so are quite out of the way of the escaping blood. In the middle of the free margin of each is a small knob called the corpus Arantii (see fig. 2), and on each side of this a thin crescent-shaped flap, the lunula (see fig. 2), which is only made of two layers of endocardium, whereas in the rest of the cusp there is a fibrous backing between these two layers.
The left auricle is situated at the back of the base of the heart, behind and to' the left of the right auricle. Behind it are the oesophagus and the thoracic aorta. Internally, it is much lighter in colour than the other cavities, owing to the greater thickness of its endocardium. There are no musculi pectinati except in the auricular appendage. The openings of the four pulmonary veins, usually, are placed two on each side of the posterior wall. On the septal wall is a small depression which corresponds to the anterior part of the fossa ovalis and often forms a valvular • com munication with the right auricle. The auriculo-ventricular orifice is large and oval, and is directed downward and to the left.
Lying in the left auriculo-ventricular furrow, is the coronary sinus, which receives most of the veins returning the blood from the heart substance ; these are the right and left coronary veins at each extremity and the posterior and left cardiac veins from below.
The left ventricle is conical, the base being above, behind and to the right, while the apex corresponds to the apex of the heart and lies opposite the fifth intercostal space, 31in. from the mid line. The following structures are seen inside it : I. The mitral valve guarding the auriculo-ventricular opening has the same arrangement as the tricuspid, already described, save that there are only two cusps, named marginal and aortic, the latter of which is the larger.
2. The chordae tendineae and columnae carneae resemble those of the right ventricle, though there are only two bundles of musculi papillares instead of three. These are very large.
3. The aortic valve has the same structure as the pulmonary, but the cusps are more massive. From the anterior and left posterior sinuses of Valsalva the coronary arteries arise.
The walls of the left ventricle are three times as thick as those of the right, except at the apex, where they are thinner. The septum ventriculorum is concave towards the left ventricle, so that a transverse section of that cavity is nearly circular. The greater part of it has nearly the same thickness as the rest of the left ventricular wall and is muscular, but a small portion of the upper part is membranous and thin; it lies between the aortic and pulmonary orifices.
Structure of the arrangement of the muscular fibres of the heart is very complicated and only imperfectly known. For details one of the larger manuals, such as Cunning ham's Anatomy (1922), or Gray's Anatomy (1926), should be consulted. The general scheme is that there are superficial fibres common to the two auricles and two ventricles and deeper fibres for each cavity. Passing from auricle to ventricle is a muscular bundle (best seen in the heart of young Ruminants) through which the wave of contraction passes from the auricles to the ventricles.
The central fibrous body is a triangular mass of fibro-cartilage, situated between the two auriculo-ventricular and the aortic orifices. The upper part of the septum ventriculorum blends with it. The endocardium is a delicate layer of endothelial cells backed by a very thin layer of fibro-elastic tissue ; it is con tinuous with the endothelium of the great vessels and lines the whole of the cavities of the heart.
The heart is roughly as large as the closed fist and weighs from 8 to ; it continues to increase in size up to about $o years of age, but the increase is more marked in the male than in the female. Each ventricle holds about 4 f. oz. of blood, and each auricle rather less. The nerves of the heart are derived from the vagus, spinal accessory and sympathetic, through the superficial and deep cardiac plexuses.
The fate of the sinus venosus and auricle must now be fol lowed. Into the former, at first, only the two vitelline veins open, but later, as they develop, the ducts of Cuvier and the umbilical veins join in (see VEINS) . As the ducts of Cuvier come from each side the sinus spreads out to meet them and becomes trans versely elongated. The slight constriction, which at first is the only separation between the sinus and the auricle, becomes more marked, and later the opening is into the right part of the auricle. Later the right side of the sinus and the right part of the aperture enlarge until the back part of the right side of the auricle and the right part of the sinus venosus are thrown into one. The left part of the sinus venosus, which does not enlarge at the same rate as the right part, remains as the coronary sinus. It will now be seen why, in the adult heart, all the veins which open into the right auricle open into its posterior part. The permanent division between the two auricles occurs at a later date than that between the ventricles. It is formed by two partitions, the first of which, grows down from the auricular roof and ultimately reaches the endocardial cushions in the auricular canal. The other partition grows down on the right of the former and is never complete ; it grows round and largely overlaps the foramen ovale and its edges form the annulus ovalis, so that, in the later months of foetal life, the foramen ovale is a valvular opening, the floor of which is formed by the first partition and the margins by the second. The closure of the foramen is brought about by adhesion of the two septa.
The pulmonary veins of the two sides at first join one another, dorsal to the left auricle, and open into that cavity by a single median trunk, but, as the auricle grows, this trunk and part of the right and left veins are absorbed into its cavity.
The mitral and tricuspid valves are formed by the shortening of the auricular canal which becomes telescoped into the ventricle, and the cusps are the remnants of this telescoping process.
The columnae carneae and chordae tendineae are the remains of a spongy network which originally filled the cavity of the primary ventricle.
The aortic and pulmonary valves are laid down in the ventral aorta, before it is divided into aorta and pulmonary artery, as four endocardial cushions; anterior, posterior, and two lateral. The septum aorticum cuts the latter two into two, so that each artery has the rudiments of three cusps.
Abnormalities of the heart are very numerous, and can usually be explained by a knowledge of its development. They may cause grave clinical symptoms. A clear and well illustrated review of the most important of them will be found in the chapter on congenital disease of the heart in Clinical Applied Anatomy, by C. R. Box and W. McAdam Eccles (1906).
BIBLIOGRAPHY.-For further details of the embryology of the Bibliography.-For further details of the embryology of the heart see Oscar Hertwig's Entwickelungslehre der Wirbeltiere (1902) ; G. Born, "Entwicklung des Saugetierherzens," Archiv f. mik. Anat. Bd. 33 (1889) ; W. His, Anatomie menschlicher Embryonen (1881 1885) ; Quain's Anatomy, vol. i. (19o8) ; C. S. Minot, Human Embry (1892) ; and A. Keith, Human Embryology and Morphology (19o5)• In the Acrania (e.g., lancelet) there is no heart, though the vessels are specially contractile in the ventral part of the pharynx.
In the Cyclostomata (lamprey and hag), and Fishes, the heart has the arrangement which has been noticed in the human embryo. There is a smooth, thin-walled sinus venosus, a thin reticulate walled auricle, produced laterally into two appendages, a thick walled ventricle, and a conus arteriosus containing valves. In addition to these the beginning of the ventral aorta is often thick ened and expanded to form a bulbus arteriosus, which is non contractile, and, strictly speaking, should rather be described with the arteries than with the heart. Between the auricle and ventricle is the auriculo-ventricular valve, which primarily consists of two cusps, comparable to the two endocardial cushions of the human embryo, though in some forms they may be subdivided. In the interior of the ventricle is a network of muscular trabeculae. The conus arteriosus in the Elasmobranchs (sharks and rays) and Ganoids (sturgeon) is large and provided with several rows of semilunar valves, but in the Cyclostomes (lamprey) and Teleosts (bony fishes) the conus is reduced and only the anterior (cephalic) row of valves retained. With the reduction of the conus the bulbus arteriosus is enlarged. So far the heart is a single tubular organ expanded into various cavities and having the characteristic 0-shaped form seen in the human embryo ; it contains only venous blood which is forced through the gills to be oxidized on its way to the tissues. In the Dipnoi (mud fish), in which rudimentary lungs, as well as gills, are developed, the auricle is divided into two, and the sinus venosus opens into the right auricle. The conus arteriosus too begins to be divided into two chambers, and in Protopterus this division is complete. This division of the heart is one instance in which the sequence of mammalian embryonic changes is not an epitome of evolu tionary development--i.e., mammalian ontogeny does not repeat the processes of phylogeny, because in the human embryo the ventricular septum appears before the auricular.
In the Amphibia there are also two auricles and one ventricle, though in the Urodela (tailed amphibians) the auricular septum is often fenestrated. The sinus venosus is still a separate cham ber, and the conus arteriosus, which may contain many or few valves, is usually divided into two by a spiral fold. Structurally the amphibian heart closely resembles the dipnoan, though the increased size of the left auricle is an advance. In the Anura (frogs and toads) the whole ventricle is filled with a spongy net work which prevents the arterial and venous blood from the two auricles mixing to any great extent. (For the anatomy and physiology of the frog's heart, see The Frog, by Milnes Marshall.) In the Reptiles the ventricular septum begins to appear ; this in the lizards is quite incomplete, but in the crocodiles, which are usually regarded as the highest order of living reptiles, the parti tion has nearly reached the top of the ventricle, and the con dition resembles that of the human embryo before the pars membranacea septi is formed. The conus arteriosus becomes in cluded in the ventricular cavity, but the sinus venosus still remains distinct, and its opening into the right ventricle is guarded by two valves which closely resemble two venous valves in the auricle of the human embryo.
In the Birds the auricular and ventricular septa are complete; the right ventricle is thin-walled and crescentic in section, as in Man, and the musculi papillares are developed. The left auriculo ventricular valve has three membranous cusps with chordae ten dineae attached to them, but the right auriculo-ventricular valve has a large fleshy cusp without chordae tendineae. The sinus venosus is largely included in the right auricle, but remains of the two venous valves are seen on each side of the orifice of the inferior vena cava.
In the Mammals the structure of the heart corresponds closely with the description of that of Man already given. In the Ornithorynchus, among the Monotremes, the right auriculo-ven tricular valve has two fleshy and two membranous cusps, thus showing a resemblance to that of the bird. In the Echidna, the other member of the order, however, both auriculo-ventricular valves are membranous. In the Edentates the remains of the venous valves at the opening of the inferior vena cava are better marked than in other orders. In the Ungulates the moderator band in the right ventricle is especially well developed, and the central fibrous body at the base of the heart is often ossified, forming the os cordis so well known in the heart of the ox.
The position of the heart in the lower mammals is not so oblique as it is in Man.
BIBLIOGRAPHY.-For further details, see C. Rose, Beitr. z. vergl. Bibliography.-For further details, see C. Rose, Beitr. z. vergl. Anat. des Herzens der Wirbelthiere Morph. Jahrb., Bd. xvi. (189o) ; R. Wiedersheim, Anatomie der Wirbelthiere (Jena, iqo2) (for literature). (F. G. P.; X.)