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aperture. Above and anteriorly it is in relation with the ascending aorta, and from the junction of this aspect with the right lateral boundary the right auricle is prolonged anteriorly and to the left. Its right side forms the right margin of the heart, and is in relation with the right phrenic nerve and its accompanying vessels, and with the right pleura and lung, the pericardium intervening. On the left the right atrium is limited by the oblique septum which separates it from the left atrium. The sulcus terminalis is a shallow groove on the surface of the right atrium, which passes from the front of the superior vena cava to the front of the inferior vena cava. It indicates the junction of the primitive sinus venosus with the atrium proper.

The interior of the right atrium is lined with a glistening membrane, the endo

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Vena cava superior

Upper right pulmonary vein

Lower right pulmonary vein

Musculi pectinati

Limbus fossæ ovalis

Fossa ovalis

Valve of vena cava

Vena cava inferior

Anterior cusp of
tricuspid valve

Chorda tendines

Moderator band

Musculi papillares

FIG. 753. THE CAVITIES OF THE RIGHT ATRIUM AND RIGHT VENTRICLE OF THE HEART.
From a formalin fixed heart.

cardium; its walls are smooth, except anteriorly and in the auricle where muscular bundles, the musculi pectinati, form a series of small vertical columns. The musculi pectinati terminate, above, in a crest, the crista terminalis, which corresponds in position with the sulcus terminalis on the external surface.

At the upper and posterior part of the cavity is the opening of the superior vena cava, devoid of a valve. At the lower and posterior part is the orifice of the inferior vena cava, bounded, anteriorly, by the rudimentary valve of the vena cava (O.T. Eustachian); and immediately anterior and to the left of this valve, between it and the atrio-ventricular orifice, is the opening of the coronary sinus, guarded by the unicuspid coronary valve. The atrio-ventricular aperture, guarded by a tricuspid valve, is known as the tricuspid orifice. It is situated in the inferior part of the anterior boundary, and admits three fingers. A number of small fossæ, foramina venarum minimarum, are scattered over the walls, and into some of them the venæ cordis minimæ open. In the septal wall is an oval depression, the fossa ovalis, bounded, above and anteriorly, by a raised margin, the limbus fossæ ovalis, which is continuous, inferiorly, with the valve of the vena cava; the fossa

is the remains of an aperture, the foramen ovale, through which the two atria communicated with one another before birth. Even in the adult a portion of the aperture persists at the upper part of the fossa in about one in five cases. Between the apertures of the superior and inferior vena cava, and posterior to the upper part of the fossa ovalis, a small eminence may be distinguished, which is called the tuberculum intervenosum; it probably directs the blood from the superior vena cava to the tricuspid orifice, during foetal life.

The valvula vena cava inferioris is a thin and sometimes fenestrated fold of endocardium and subendocardial tissue, which extends from the anterior and lower margin of the orifice of the inferior vena cava to the anterior part of the limbus ovalis. It varies very much in size, and is usually of falciform shape; its apex is attached to the limbus fossæ ovalis and its base to the margin of the inferior caval orifice. It is an important structure in the foetus, directing the blood from the inferior vena cava through the foramen ovale into the left atrium. The valvula sinus coronarii is usually a single fold of endocardium which is

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FIG. 754.-THE BASES OF THE VENTRICLES OF THE HEART, showing the auriculo-ventricular, aortic, and pulmonary orifices and their valves.

placed at the right margin of the orifice of the coronary sinus. It is almost in variably incompetent.

Atrium Sinistrum.-The left atrium is in relation posteriorly with the descending thoracic aorta and the oesophagus, but is separated from them by the pericardium and the oblique sinus of the pericardium. Below and anteriorly it is continuous with the left ventricle. Its sterno-costal surface is concave, and lies in close relation to the ascending aorta, the pulmonary artery, and the left coronary artery. Its right side, formed by the interatrial septum, is directed anteriorly and to the right. Its left side forms a very small portion of the left margin of the heart, and from it, at its junction with the antero-superior surface, the long and narrow left auricle is prolonged, forwards, round the left side of the ascending portion of the aorta and the trunk of the pulmonary artery.

The four pulmonary veins enter the upper part of the posterior surface, two on each side.

The interior of the left atrium is lined with endocardium, and its walls are smooth, except in the auricle, where musculi pectinati are present, and on the septum, in a position corresponding with the upper part of the fossa ovalis on the right side, where there are several musculo-fibrous bundles radiating anteriorly and upwards. These septal bundles are separated at their bases by small semilunar depressions, in the largest of which remains of the foramen ovale may be found. Foramina venarum minimarum, and the apertures of venæ cordis minimæ, are scattered irregularly over the inner aspect, whilst in the inferior

part of the anterior boundary is the atrio-ventricular aperture or mitral orifice. The orifice is oval in form; its long axis is placed obliquely antero-posteriorly, and from left to right, and is capable of admitting two fingers. It is guarded by a valve formed of two large cusps, known as the mitral valve.

Ventriculi. The ventricular portion of the heart is conical and somewhat flattened. The base, directed upwards and posteriorly, is partly continuous with the atrial portion and partly free. It is perforated by four orifices, the two atrio-ventricular, the aortic, and the pulmonary. The atrio-ventricular orifices are placed, one on each side, inferiorly and posteriorly; anteriorly and between them is the aortic orifice, whilst the orifice of the pulmonary artery is still more anterior, and slightly to the left of the aortic orifice.

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In the triangle between the atrioventricular and the aortic orifices is embedded a mass of dense fibrous tissue which is the representative of the os cordis of the ox. It is continuous with the upper part of the interventricular septum, and with fibrous rings which surround the apertures at the bases of the ventricles.

The diaphragmatic surfaces and the sterno-costal surfaces of the two ventricles constitute respectively the greater portions of the corresponding surfaces of the heart; the former rest upon the diaphragm, whilst the latter are directed upwards and anteriorly towards the sternum and the costal cartilages of the left side. The apex of the left ventricle forms.

I to VII, Costal cartilages.

A, Aortic orifice.

Ao, Aorta.

C, Clavicle.

LA, Left atrium.

LV, Left ventricle.

FIG. 755. THE RELATIONS OF THE HEART AND THE the apex of the heart..
ATRIO VENTRICULAR, AORTIC, AND PULMONARY
ORIFICES TO THE ANTERIOR THORACIC WALL.

The inferior margin of the ventricular region, which is thin, forms the inferior margin of the heart; and the left margin, which is thick and rounded, forms the greater part of the left margin of the heart. The ventricular portion of the heart is divided into right and left chambers by the ventricular septum, which is placed obliquely, with one surface directed anteriorly and to the right, and the other posteriorly and to the left; it bulges into the right ventricle, and its lower margin lies to the right of the apex of the heart, which is, therefore, formed entirely by the left ventricle. The margins of the septum are indicated on the two surfaces of the ventricular part of the heart by anterior and inferior interventricular sulci.

M, Mitral orifice.
P, Pulmonary orifice.
RA, Right atrium.
RV, Right ventricle.
SVC, Superior vena cava.
T, Tricuspid orifice.

Ventriculus Dexter.-The right ventricle is triangular in form.

directed upwards and to the right, and, in the greater part of its extent, it is continuous with the right atrium, with which it communicates by the atrioventricular orifice; but its left and anterior angle is free from the atrium, and gives origin to the pulmonary artery. Its inferior wall rests upon the diaphragm. The sterno-costal wall lies posterior to the lower part of the left half of the sternum and the cartilages of the fourth, fifth, and sixth ribs of the left side. The left or septal wall, which is directed posteriorly and to the left, bulges into its interior, and on this account the transverse section of the cavity has a semilunar outline. The cavity itself is a bent tube consisting of an inferior portion or body into which the atrio-ventricular orifice opens, and of an antero-superior part, the infundibulum or conus arteriosus, which terminates in the pulmonary artery.

the angle between the two limbs is a thick ledge of muscle, the supraventricular crest.

The right atrio-ventricular orifice is guarded by a tricuspid valve. The three cusps are an anterior, which intervenes between the atrioventricular orifice and the infundibulum, a medial or septal, and an inferior. Each cusp consists of a fold of endocardium, strengthened by a little intermediate fibrous tissue. The bases of the cusps are generally continuous with one another at the atrioventricular orifice, where they are attached to a fibrous ring, but they may be separated by small intermediate cusps which fill the angles between the main segments. The apices of the cusps project into the ventricle. The margins, which are thinner than the central portions, are notched and irregular. The atrial surfaces are smooth. The ventricular surfaces are roughened, and, like the margins and apices, they give attachment to fine tendinous cords, the chordæ tendineæ. The opposite extremities of the chorda tendineæ are attached to muscular bundles, the musculi papillares, which project from the wall into the cavity of the ventricle.

The pulmonary orifice, which lies anterior and to the left of the tricuspid orifice, is guarded by a pulmonary valve composed of three semilunar segments, two of which are placed anteriorly and one posteriorly. The convexity or outer border of each semilunar segment is attached to the wall of the pulmonary artery. The inner border is free, and it presents at its centre a small nodule, the nodulus valvulæ semilunaris. On each side of the nodule there is a small, thin marginal segment, of semilunar form, the lunule. Each segment of the valve is formed by a layer of endocardium on its ventricular surface, an endothelial layer of the inner coat of the artery on its arterial surface, and an intermediate stratum of fibrous tissue. Both the attached and the free margins of the cusps are strengthened by fibrous bands, and strands of condensed fibrous tissue radiate from the outer border of each cusp to the nodule, but they do not enter the lunulæ. When the valve closes the noduli are closely apposed, the lunulæ of the adjacent segments of the valve are pressed together, and both noduli and lunulæ project vertically upwards into the interior of the artery.

The cavity of the right ventricle is lined with endocardium; the walls are smooth in the conus arteriosus, but are rendered rugose and sponge-like in the body by the inward projection of numerous muscular bundles, the trabeculæ carneæ. The fleshy trabeculæ are of two kinds: the simpler are merely columns raised in relief on the wall of the ventricle; the other class are rounded bundles, free in the middle, but attached at each end to the wall of the ventricle. One special bundle of the second group, called the moderator band, is attached by one extremity to the septum, and by the other to the sterno-costal wall, at the base of the anterior papillary muscle; it tends to prevent over-distension of the cavity. In addition to the trabecula carneæ conical muscular eminences, the musculi papillares, project into the cavity of the ventricle. The bases of the papillary muscles are continuous with the wall of the ventricle, and their apices terminate in numerous chordæ tendineæ which are attached to the apices, the borders, and ventricular surfaces of the cusps of the tricuspid valve.

The musculi papillares of the right ventricle are (1) a large anterior muscle, from which the chorda pass to the anterior and inferior segments of the valve; (2) a smaller and more irregular inferior muscle, sometimes represented by two or more segments, from which chorda pass to the inferior and medial cusps; and (3) a group of muscular bundles, varying in size and number, which spring from the septum and are united by chorda to the anterior and medial cusps.

The walls of the right ventricle, the septal excepted, are much thinner than those of the left, but the trabeculae carnea are coarser and less numerous in the right than in the left ventricle.

Ventriculus Sinister. The left ventricle is a conical chamber, and its cavity is oval in transverse section. The base is directed upwards and posteriorly, and in the greater part of its extent it is continuous with the corresponding atrium, with which it communicates through the mitral orifice; but anteriorly and to the

right of its communication with the atrium it is continued into the ascending

aorta.

The mitral orifice is oval; its long axis runs obliquely from above downwards and to the right, and it is guarded by a valve consisting of two cusps, which is known as the bicuspid or mitral valve. The two cusps of the valve are triangular and of unequal size. The smaller of the two is placed to the left and inferiorly; and the larger, placed to the right and anteriorly, between the mitral and aortic orifices, is known as the aortic cusp. The bases of the cusps are either continuous with one another, at their attachments to the fibrous ring around the mitral orifice, or they are separated by small intermediate cusps of irregular form and size. The apices of the cusps project into the cavity of the ventricle. The atrial surfaces are smooth; the ventricular surfaces are roughened by the attachments of the chorda tendineæ, which are connected also with the irregular and notched margins and with the apices. The structure is the same as that of the cusps of the tricuspid valve, but the ventricular surface of the anterior cusp is relatively smooth; therefore the blood flow into the aorta is not impeded.

The aortic orifice is circular; it lies immediately anterior and to the right of the mitral orifice, from which it is separated by the anterior cusp of the mitral valve, and it is guarded by the aortic valve, formed of three semilunar segments, one of which is placed anteriorly and the other two posteriorly. The structure and attachments of the cusps of the aortic valve are similar to those of the cusps of the pulmonary valve (see p. 877).

The cavity of the left ventricle is separable, like that of the right, into two portions, the body and the aortic vestibule; the latter is a small section placed immediately below the aortic orifice; its walls are non-contractile, consisting of fibrous and fibro - cartilaginous tissue. The wall of the cavity is lined by endocardium. The inferior wall and the apex are rendered sponge-like by numerous fine trabeculæ carnea, whilst the upper part of the sterno-costal wall and the septum are relatively smooth.

There are two papillary muscles of much larger size than those met with in the right ventricle-an anterior and an inferior; each is connected by chordæ tendineæ with both cusps of the mitral valve.

The walls of the left ventricle are three times as thick as those of the right ventricle, and they are thickest in the region of the widest portion of the cavity, which is situated about a fourth of its length from the base. The muscular portion of the wall is thinnest at the apex, but the thinnest portion of the boundary lies at the upper part of the septum, and it consists entirely of fibrous tissue; this part is occasionally deficient, and an aperture is left through which the cavities of the two ventricles communicate.

The ventricular septum is a musculo-membranous partition. It is placed obliquely, so that one surface looks anteriorly and to the right, and bulges into the right ventricle, whilst the other looks posteriorly and to the left and is concave towards the left ventricle. Its sterno costal and inferior margins correspond respectively with the anterior and the inferior portions of the interventricular sulcus, and it extends from the right of the apex to the interval between the pulmonary, the aortic, and the atrio-ventricular orifices. The main part of its extent is muscular, and is developed from the wall of the ventricular part of the heart; but its upper and posterior portion, the pars membranacea, which is developed from the septum of the truncus arteriosus, is entirely fibrous, and constitutes the thinnest portion of the ventricular walls. The pars membranacea lies between the aortic vestibule of the left ventricle, on the left, and the upper part of the right ventricle and the lower and left part of the right atrium, on the right.

STRUCTURE OF THE HEART.

The walls of the heart consist mainly of peculiar striped muscle, the myocardium, which is enclosed between the visceral layer of the pericardium, or epicardium, externally, and the endocardium internally. The muscular fibres differ from those of ordinary voluntary striped

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