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The heart [cor] is a hollow organ principally composed of muscle, the myocardium. It is lined internally by endocardium which is continuous with the intima of the blood-vessels. Externally, it is covered by the epicardium, a serous membrane continuous with the serous lining of the pericardium. The form of the heart, when removed from the body without previous hardening, is that of a fairly regular truncated cone. The base [basis cordis] is poorly circumscribed but corresponds, in a general way, to the area occupied by the roots of the great vessels and the portion of the heart-wall between them. The base of the heart is held in position (Not necessarily fixed, for during systole the base performs a greater excursion than does the apex.) chiefly by the great vessels, which are attached to the pericardium; the remainder of the organ is capable of free movement within the pericardial cavity. The interior of the heart is longitudinally divided, into right and left cavities, by a septum passing from base to apex. Each cavity is subdivided into an atrium [atrium cordis] and a ventricle [ventriculus cordis], the former receiving the ultimate venous trunks and the latter giving rise to the main arteries. Thus the left atrium receives the four pulmonary veins, and the right atrium the superior and inferior vena cava and the coronary sinus; the aorta issues from the left ventricle and the pulmonary artery from the right. The ventricles, which constitute the major portion of the heart, may be recognized by their very thick walls. The atria have thinner walls and are less capacious than the ventricles; projecting from each is a diverticulum or auricle [auricula cordis]. The auricles (which receive their name from their resemblance to dog's ears) partially embrace the roots of the pulmonary artery and aorta.

Orientation of the heart

The apex of the heart [apex cordis] points forward, to the left and downward. The base is directed backward, to the right and upward. The longitudinal axis of the heart forms an angle of about 40° with the horizontal plane and also with the median sagittal plane of the body.

The long axis of the heart is therefore slightly more horizontal than vertical, and slightly more antero-posterior than transverse. The atria are posterior to rather than above the ventricles. To arrive approximately at the longitudinal axis, it is necessary to select the central point of the base. By cutting the vessels short in several hearts, hardened by formalin before removal, a point immediately to the left of the left lower pulmonary vein was selected in determining the data above given. A steel pin was passed through this point to the apex cordis, and the angles controlled by frontal and transverse sections of the thorax. Mention of angular measurements of the axis of the heart could be found only in the text-books of Testut and Luschka; the former gives 40° to the horizontal plane, the latter 60° to the mid-sagittal. Luschka's angle appear to be too large; but further investigation in this direction is desirable.

Size and weight

In the adult, the heart measures about 12.5 cm. (5 in.) from base to apex, 8.7 cm. (31 in.) across where it is broadest, and 6.2 cm. (24 in.) at its thickest portion. In the male, its weight averages about 312 gm. (eleven ounces), and in the female about 255 gm. (nine ounces). It increases both in size and weight up to advanced life, the increase being most marked up to the age of twenty-nine years. The proportion of heart-weight to body-weight is about 1:205 in the adult.

Exterior of the heart

In hearts which have been hardened by injection before removal from the body, the regularity of the heart-cone is disturbed by a well-marked triangular facet, imparted by contact with the diaphragm. This facet is the diaphragmatic surface [facies diaphragmatica], which is directed downward and slightly backward. It ends abruptly along a sharp margin extending from the apex toward the right. This margin is the margo acutus; it separates the diaphragmatic surface from the sternocostal surface. The other margin of the diaphragmatic surface is more rounded and shades gradually into the very wide margo obtusus, which passes almost insensibly into the sternocostal surface. The convex sternocostal surface [facies sternocostalis], directed forward and somewhat upward and to the right, is triangular and bounded below by the margo acutus. To the left, it goes over into the margo obtusus along a line extending from the apex of the heart to the root of the pulmonary artery. The margo obtusus corresponds to the rounded left side of the left ventricle.

The interventricular sulcus is a slightly marked groove indicating the separation of the ventricles upon the exterior of the heart. It lodges coronary blood vessels and a moderate quantity of fat which can be seen through the epicardium.

The anterior part of this groove, sulcus longitudinalis anterior, beginning posteriorly, runs obliquely over the upper part of the margo obtusus on to the sternocostal surface. Crossing the margo acutus to the right of the apex, it is continuous with the sulcus longitudinalis posterior upon the diaphragmatic surface. The diaphragmatic surface is formed about equally by the right and left ventricles, and the sterno-costal surface mainly by the right. Where the longitudinal sulcus crosses the margo acutus it produces a slight notch, the incisura (apicis) cordis.

The atria are separated externally from the ventricles by the sulcus coronarius. This is a horseshoe-shaped groove well marked below and laterally, and interrupted above by the roots of the pulmonary artery and aorta. It lodges the coronary sinus, smaller coronary vessels and fat.

View of the Base and Diaphragmatic Surface of the Heart

 View of the Base and Diaphragmatic Surface of the Heart

Left pulmonary artery Left superior pulmonary vein Left inferior pulmonary vein Reflexion of pericardium Coronary sinus Aorta. Superior vena cava. Right pulmonary artery Margo obtusus

Atrial portion

The atrial portion of the heart is situated behind, and slightly to the right of and above, the ventricular portion. The separation between the right and left atrium is not indicated behind except in distended hearts; in these it is marked by a slight groove connecting the left sides of the superior and inferior venae cavae. In front, the auricles are separated by the deep notch which lodges the aorta and pulmonary artery. A slight groove on the back of the right atrium which connects the right sides of the superior and inferior venae cavae, is the sulcus terminalis. This represents the right limit of what was, in the embryo, the sinus venosus. It also indicates that the embryonic sinus venosus has become an integral part of the adult right atrium. The superior and inferior cavae have each a nearly vertical direction and join the posterior part of the right atrium above and below, respectively. The coronary sinus runs downward, backward and to the right to join the lower part of the right atrium anterior to the inferior vena cava. The four pulmonary veins run nearly transversely and somewhat forward into the right and left sides of the left atrium.

The interior of the atrial portion of the heart is divided into right and left cavities by the septum atriorum. This septum is a composite structure, having been developed (see morphogenesis of the heart) in two independent parts, each forming an incomplete septum in itself. The two incomplete septa, however, partly overlap one another so that, by the lateral fusion at the time of birth, they together produce the impervious structure of the adult heart. Of these septa, the first to be formed is the membranous septum [pars membranacea septi atriorum]. Later there is formed to the right of this the muscular septum, the margin of which forms, in the adult atrium, the greater part of the limbus fossae ovalis. The margin of the membranous septum is recognizable as a fold of endocardium on the septal wall of the left atrium; it is called the valvula foraminis ovjilis.

Posteriorly into the right atrium [atrium dextrum], above and below, respectively, open the superior and the inferior vena cava. Upon the septal wall, immediately above the inferior cava is the fossa ovalis, a depression of which the floor is formed by the membranous septum. Surrounding the fossa ovalis except below (indeed producing the fossa) is the limbus fossae ovalis which is continuous anteriorly and below with the valvula venae cavae (inferioris Eustachii). Just anterior to the fossa ovalis is the orifice of the coronary sinus guarded by the valvula sinus coronarii (Thebesii). Leading from the front of the atrium forward and slightly downward and to the left is the ostium venosum (right atrioventricular orifice) guarded by the tricuspid valve. Above and behind this is the auricle, the exterior of which is in contact medially with the root of the aorta. To the right of the superior and inferior caval orifices there is a vertical ridge, the crista terminalis, which corresponds to the sulcus terminalis on the exterior.

The portion of the atrium medial to the crista is smooth and is called the sinus venarum; in the embryo it is separated from the atrial cavity proper by the right and left sinus valves. The crista terminalis marks the original line of attachment of the right sinus valve. The valve itself has disappeared, except at the lower part where it persists as the caval and coronary valves. These valves vary in size considerably in different specimens, and are frequently netlike from numerous perforations.

The conversion of a portion of a single valve into two separate valves, which meet at an acute angle, is brought about by an attachment between the sinus valve and an embryonic structure called the sinus-septum. This septum is a ridge dividing the right horn of the sinus venosus from the transverse portion of the sinus (the coronary of the adult); it probably con tributes somewhat to the formation of both the coronary and caval valves. The left sinus valve usually disappears by blending with the septum atriorum on which it unites with the limbus fossae ovalis; it occasionally remains partially separate in the adult.

The interior of the right auricle and of the portion of the atrium lateral to the crista terminalis is thrown into ridges (musculi pectinati) by prominent bands of the atrial myocardium. The musculi pectinati end abruptly by joining the crista. The orifice of the superior cava has no valve and is directed downward and somewhat forward; below it, on the posterior wall of the atrium, there has been described a tubercle or ridge, the tuberculum intervenostun (Loweri).

Apart from the posterior circumference of the superior cava itself and the limbus fossae ovalis, the human heart appears to contain nothing in this region that could be described as a tubercle. With regard to the segregation of the streams entering the fetal right atrium from the superior and inferior cavae, respectively, in which the tubercle of Lower has been supposed to participate, it is to be noted that the fossa ovalis is just above (almost within) the inferior caval orifice. Also that the caval opening and the fossal ovalis (containing the fetal foramen ovale) are, in hearts well hardened before removal, situated in a distinct diverticulum to the left of the remainder of the atrium. Between this diverticulum and the atrium proper, the caval valve and the limbus fossae ovalis form a prominent flange, better marked in the fetus than the adult. Opening into the right atrium, particularly upon the septal and right lateral walls, are numerous foromina venarum minimarum (Thebesii).

The left atrium [a. sinistrum] is to the left and somewhat posterior to the right. It is behind the root of the aorta and its auricle is to the left of the pulmonary root. Opening into it posteriorly on the right and left sides, respectively, are the right and left upper and lower pulmonary veins. The valvula foraminis ovalis forms a more or less distinct crescentic ridge on the septal wall. This may not be attached to the limbus fossas ovalis, in which case there is a communication between the two atria. Absence of lateral adhesion between the two septa atriorum does not necessarily lead to admixture of arterial and venous blood during life. The left ostium venosum (atrio-ventricular orifice) guarded by the mitral valve leads from the anterior part of the atrium forward and slightly downward and to the left. The interior of the left atrium is smooth except in the auricle, in which musculi pectinati are well marked.

Atrio-ventricular valves

The atrio-ventricular valves are attached around the venous ostia of the ventricles in such a way as to open freely into the ventricles, but to prevent regurgitation of the blood into the atria during ventricular systole. Each valve is continuous along its line of attachment, but its free edge is notched so as to produce an irregular margin; some of the notches are so deep as to partially divide the valve into cusps. The right atrio-ventricular valve is commonly divided by three deep notches into three cusps; this valve is therefore called the tricuspid [valvula tricuspidalis]. The left is similarly divided into two cusps and is called the bicuspid [v. bicuspidalis] or mitral. The depth of the notches, however, is very variable and there may be an increase or (more rarely) a diminution in the number of cusps; the addition of small subsidiary cusps is quite common. Each valve cusp is tied down to the papillary muscles [mm. papillares] of the ventricle by chordae tendinese. The latter are fibrous cords, generally branched, of varying thickness. The thinnest cords are attached to the free margin of the cusp; those of intermediate thickness to the ventricular surface a few millimeters from the margin, and the thickest to the ventricular surface near the attached margin. The valves are smooth and glistening on the atrial aspect, but rough and fasciculated, from the attachment of the chordae, on the ventricular. The cusps of the mitral valve are called anterior and posterior; those of the tricuspid, anterior, posterior and medial. Each cusp receives chorda from more than one papillary muscle and each papillary muscle sends chordse to more than one cusp. The chordae tendineae of the mitral valve are thicker than those of the tricuspid.

Ventricular portion

The ventricles form the greater portion of the heart. In the adult, the relation of the ventricles to one another is as follows. The left [ventriculus sinister] has the form of a narrow cone, the apex of which is the apex of the heart. The right ventricle [ventriculus dexter] is crescentic in section and appears to be partially wrapped around the right or lower wall of the left ventricle which forms the septum ventriculorum. The left ventricle forms the margo obtusus of the heart, about half the diaphragmatic surface, and a shght part of the sterno-costal surface. The right ventricle forms about half the diaphragmatic surface and the major part of the sterno-costal surface; it takes no share in the formation of the apex of the heart.

The interventricular septum [septum ventriculorum] is thick and muscular except for a small area near the root of the aorta which is membranous [septum membranaceum ventriculorum]. The latter can be seen from the left ventricle in the angle between the attached edges of the right and posterior aortic valves. The membranous septum is partly concealed from the right heart by the medial cusp of the tricuspid valve which is attached to it near its upper part. The portion of the membranous septum above the medial tricuspid cusp is therefore atrio-ventricular, i. e., between the right atrium and left ventricle.

The membranous septum is the extreme lower part of the independent septum (s. aorticum) which divides the aortic root from the pulmonary artery and conus arteriosus (and partially subdivides, also, the right ventricle by separating the conus arteriosus from the remainder of the ventricle). The relation of the part of the aortic septum between the conus arteriosus and aortic root to the septum ventriculorum is beautifully shown by His.

The greater part of the interior of the ventricles is thrown into ridges by myocardial bundles of large size. These fasciculi [trabeculae cordis] either stand out in relief only, or, by being undermined, form bands covered except at either end by endothelium. A careful examination of the endocardium of fresh hearts will reveal a plexiform network of Purkinje fibers. These fibers, belonging to the atrio-ventricular conducting system, become very obvious when the endocardium has been exposed to the air long enough to become partially dry.

The wall of the right ventricle [ventriculus dexter] is much thicker than that of the atria, but less so than that of the left ventricle. The upper and anterior part of the right ventricle is in relation posteriorly with the root of the aorta. This portion of the ventricle is called the conus arteriosus and is separated from the remainder of the right ventricle by a muscular spur which extends from the back of the conus to the right venous ostium. The spur is the crista supraventricularis; its relation to the partition between the conus and aorta, and to the septum membranaceum, shows that it is the free edge of the embryonic aortic septum (see morphogenesis of the heart).

Two papillary muscles in the right ventricle are constant in position, the large anterior papillary muscle, and the small papillary muscle of the conus (Luschka). The anterior papillary is situated on the sterno-costal wall, near the junction of this with the septal wall. The papillary of the conus is placed just below the septal end of the crista supraventricularis. The posterior papillary muscles form an irregular group springing from the diaphragmatic wall. Some chordae tendinese stretch directly from the septal wall (with or without small muscular elevations at their bases) to the medial cusp of the tricuspid valve. The chordae tendinese from the anterior papillary go to the anterior and posterior cusps; those from the conus papillary to the medial and anterior, and those from the posterior papillary muscles to the medial and posterior cusps of the tricuspid valve, respectively.

There is frequently a band of myocardium extending from the septal wall of the right ventricle to the anterior papillary muscle near its middle. This is the moderator band, which contains a part of the right limb of the atrioventricular bundle. If the moderator band joins the anterior papillary near its base, as it frequently does, it is difficult to distinguish it from the ordinary trabeculae in this situation.

The term moderator band was originally applied to this bridge or band of muscle under the impression that it prevented overdistention of the ventricle. Subsequent discovery of the conducting system of the heart makes it plain that there is always a band conducting the right limb of the atrioventricular bundle from the septum to the anterior papillary muscle. Whether the band is isolated from the other trabecules, and therefore readily recognizable, appears to depend somewhat upon the relation of the base of the papillary muscle to the septum ventriculorum.

The wall of the left ventricle [veiitriculus sinister] is very thick except at the extreme apex, and at the membranous septum. In the left ventricle are two large papillary muscles, generally known as anterior and posterior; both send chordae tendineae to each cusp of the mitral valve. On the septal wall of the ventricle the left limb of the atrioventricular bundle can usually be seen as a broad, flattened band beneath the endocardium. The band appears just below the septum membranaceum and divides into strands which go to the two papillary muscles. The strands in many places bridge across part of the ventricle to reach the papillary muscles covered only by tubes of endocardium.

These bridging strands connecting the papillary muscles with the septum ventriculorum, which were formerly called "false chordae tendinese," are exactly comparable to the moderator band of the right ventricle which occasionally consists of atrioventricular bundle and endocardium only.

Semilunar valves

The semilunar valves [valvulse semilunares] guard the arterial ostia of the ventricles. The aortic ostium is directed upward and slightly forward and to the right; the pulmonary backward and slightly upward and to the left. Each valve, of which there are three to each ostium, is a pocket-like fold of endocardium strengthened by fibrous tissue. The free edge of each valve is directed away from the ventricle, so that excess of pressure within the great vessels brings the three valves of either ostium into mutual apposition. In the middle of the free edge of each valve there is a small fibro-cartilaginous nodule; radiating from this toward the entire fundus, and along the extreme free edge of the valve, are fibrous thickenings. On either side of the nodule, between the thicker margin and fundus, the valve is thin over a crescentic area called the lunula.

The aortic valves are called the right, left, and posterior; the pulmonary valves, the right, left, and anterior.* The aortic semilunar valves are stronger than the pulmonary; opposite them there are three dilatations in the aortic wall, the aortic sinuses [sinus aortae] or sinuses of Valsalva. From the right and left sinuses, the right and left coronary arteries, respectively, arise.

After ventricular systole, the increased pressure in the great vessels distends the valves with blood. The noduli meet in the center and the lunulas, coming into mutual contact, produce a tri-radiate line of contact between the valves.

* The BNA names of the aortic and pulmonary valves are not based upon their relative positions in the body. From transverse sections through the thorax (see any good atlas) it may be seen that one aortic valve is anterior, one pulmonary valve posterior, and the other aortic and pulmonary valves are right and left. If the removed heart is held so that the ventricles are on the right and left of the septum, respectively, the valves take the positions indicated by the BNA. The names given by the BNA to the valves, although conventional (Uke many other terms of orientation applied to parts of the heart), are convenient, particularly from a developmental standpoint.

Architecture of the myocardium

In the adult heart the myocardium of the atria is separate from that of the ventricles. There is, between the atria and ventricles, a fibrous partition, the upper and lower surfaces ol which give attachment to the muscle fibers of these cavities, respectively.

The fibrous partition is thickest in the triangle formed by the meeting of the aortic, and right and left atrioventricular ostia. This interval is filled by a mass of fibrous tissue, which in the angles between the aortic and the left atrioventricular ostium forms two thickened triangular masses, the trigona fibrosa. The fibrous mass is continued to the pulmonary ostium as the tendon of the conus. Below the point of junction of the trigona and the tendon of the conus these structures blend with the septum membranaceum ventriculorum. The septum membranaceum, tendon of the conus, and part of the trigona are derived from the aortic septum. The trigona give off laterally, on either side, atrioventricular rings which encircle the venous ostia and give attachment to the atrioventricular valves. There are also weak rings surrounding the pulmonary and aortic orifices; the aortic and left atrioventricular rings being partly confluent. The rings surrounding the arterial and venous ostia axe the annuli fibrosi.

The atrial musculature is attached to the trigona and atrioventricular rings only. The superficial fibers are attached to both rings and either encircle both atria in one loop, or enter the septum and form a figure 8. The deeper fibers are attached to one ring and encircle one atrium only; some fibers encircle only the auricle.

The ventricular musculature is very complex and consists of numerous superimposed layers distinguished from one another by the direction taken by the muscle fibers. In a general way, the fibers of the deepest layer have a direction crossing those upon the surface of the same area at a right angle. The intervening layers of fibers pass through all stages of obliquity.

Recent work upon the origin and distribution of the ventricular fibers has resulted in the recognition of a certain uniformity of behavior, thus: -

1. AH fibers arise from, and are inserted into, the fibrous partition at the base. The attachment may be directly to the trigona or annuli, or indirectly to them by means of the chordae tendineae and atrioventricular valves.

2. The more superficial fibers tend to encircle the entire heart, passing over the longitudinal sulci. If they enter the septum they do so by passing into the vortex or whorl at the apex of the left ventricle. These fibers have always a definite direction upon the surface, i. e., from right to left upon the sterno-costal surface and from left to right on the diaphragmatic.

3. The deeper fibers all enter the septum in a direction oblique or perpendicular to its longitudinal axis. In addition, they completely encircle one or both ventricles forming, in the latter case, double loops.

During systole, as a result of this arrangement: (1) The papillary muscles and the longitudinal and antero-posterior axes of the ventricles are simultaneously shortened. (2) There is a movement of torsion or "wringing" which reduces the ventricular cavities to their minimum dimensions.

Conducting system fo the heart

Although the ordinary myocardium of the atria is distinct from that of the ventricles there is, at one place, a connection between them. This connection is by means of a small band of muscle which differs histologically from ordinary heart muscle. It is known as the atrioventricular bundle, and serves to transmit the atrial rhythm of contraction to the ventricles.

The atrioventricular bundle begins in the septal wall of the atrium a short distance in front of the coronary orifices. It has an expanded free end, the atrioventricular node, from which branches pass to be quickly lost in the atrial myocardium. The bundle passes forward covered by endocardium and by one or two millimeters of myocardium, and passes beneath the medial cusp of the tricuspid valve. In passing from the atrium to the ventricle, the bundle skirts the lower margin of the septum membranaceum. Immediately in front of the septum membranaceum it divides into a left and right limb, of which the former pierces the muscular interventricular septum. The right limb now passes beneath the crista supra ventricularis and above the papillary muscle of the conus, giving off branches to the latter and to other small papillaries on the septum. Bending somewhat toward the apex, it enters the moderator band which conducts it to the large anterior papillary muscle. From here it passes along one of the trabeculae connected with the sterno-costal wall of the ventricle, or in the wall itself, to reach the posterior papillary muscle or muscles. The right limb is compact and rounded and in the intact heart is usually invisible except, sometimes near the root of the moderator band or in the band itself.

The left limb of the bundle appears in the left ventricle a little below the septum membranaceum. It is a wide band immediately beneath the endocardium, which cannot usually be stripped off without injuring the bundle. It passes along the septal wall toward the apex and divides into two parts, which again subdivide to be distributed to the anterior and the posterior papillary muscles. The branches for the papillary muscles may reach them through thick trabecula; or they may form thin strands which, covered only by endocardium, bridge from septum to papillary muscle.

In addition to the comparatively distinct branches to the papillary muscles of both ventricles, the bundle gives off finer fibers which form a sub-endocardial plexus. This plexus, visible to the naked eye is made up of fibers having a structure similar to those of the ventricular portion of the bundle. The fibers were described by Purkinje as long ago as 1845,* but it was not until 1906, thirteen years after the discovery of the bundle by W. His, Jr., that Tawaraf recognized their significance.

There is another node of muscle having characters similar to that of the conducting system, although not connected with it except by myocardium of the ordinary character. This is the sinus-node which is situated at the upper end of the crista terminalis of the right atrium. Unanimity is still lacking with regard to the physiological significance of this structure.

* Arch. f. Anat., Physiol, u. wissenschafthche Medizin.

t Das Reitungssystem des Saiigertierherzens, Fischer, Jena, 1906.

Vessels and Nerves of the Heart

The arteries. The two coronary arteries arise from the right and left sinuses of the aorta. The right coronary artery [a. coronaria dextre] passes forward between the pulmonary artery and the right atrium, and then follows the right coronary sulcus to the diaphragmatic surface of the heart, to anastomose with the left coronary artery. The posterior descending branch [ramus descendens posterior] arises at the posterior longitudinal sulcus. It easses in the furrow between the ventricles toward the apex, near which it anastomoses with ranches derived from the left coronary artery. In this course the right coronary artery supplies branches to the right atrium and roots of the pulmonary artery and aorta, as well as one that descends near the margo acutus (right marginal), and a second (preventricular) to the anterior wall of the right ventricle. It supplies both ventricles and the septum.

The left coronary artery [a. coronarius sinistra] passes for a short distance forward, between the pulmonary artery and the left auricle, and then divides into two principal branches, one of which runs in the anterior longitudinal sulcus to the apex of the heart, the anterior descending branch [r. descendens anterior], around which it sends branches to anastomose with the right coronary; whilst the other, the circumflex [ramus circumflexus], winds to the diaphragmatic surface in the coronary groove, to anastomose with the corresponding twigs of the right artery. In this course it gives off a branch which follows the margo obtusus (left marginal) as well as smaller branches to the left atrium, both ventricles, and the commencement of the aorta and pulmonary vessels.

The cardiac or coronary veins accompany the coronary arteries and return the blood from the walls of the heart.

The great cardiac vein [v. cordis magna], runs in the anterior longitudinal sulcus, passing round the left side of the heart in the coronary sulcus to terminate in the commencement of the coronary sinus. Its mouth is usually guarded by two valves, and it receives in its course the posterior vein of the left ventricle, with other smaller veins from the left atrium and ventricle, all of which are guarded by valves.

The middle cardiac vein (v. cordis media], sometimes the larger of the two chief veins, communicates with the foregoing at its commencement above the heart's apex. It ascends in the posterior longitudinal groove, receiving blood from the ventricular walls, and joins the coronary sinus through an orifice guarded by a single valve, close to its termination.

The posterior vein of the left ventricle [v. post, ventriculi sinistri], lies upon the posterior surface of the ventricle and, receiving branches from it, passes upward to terminate directly in the coronary sinus.

The anterior cardiac veins [vv. cordis anteriores] consist of several small branches from the front of the right ventricle, which vary in number and either open separately into the right atrium or join the lesser cardiac vein.

The small cardiac vein [v. cordis parva] is a small vessel which receives branches from both the right atrium and ventricle, and winds around the right side of the heart, in the coronary sulcus, to terminate in the coronary sinus.

The coronary sinus [sinus coronarius] may be regarded as a much-dilated terminal portion of the great cardiac vein. It is about 2.5 cm. (1 in.) in length, is covered by muscular fibers from the atrium, and lies in the coronary sulcus below the base of the heart. Its cardiac orifice, with the coronary (Thebesian) valve, has already been described. Besides the tributary veins already named, a small oblique vein [v. obliqua atrii sinistri] of the left atrium may sometimes be traced, on the back of the left atrium, from the ligament of the left vena cava (Marshall) to the sinus. This little vein, which is not always pervious or easy of demonstration, never possesses a valve at its orifice, and, Uke the coronary sinus, formed a part of the left superior vena cava of early fetal life.

The smallest cardiac veins [vv. cordis minima;] drain blood from septum and lateral walls of the atria, particularly the right; also from the conus arteriosus. They open directly into the right atrium.

Although anastomoses occur between the two coronary arteries, these are by no means extensive, and are not sufficient to allow of the establishment of a satisfactory collateral circulation in the case of the blocking of one coronary artery. Consequently, such interference with the cardiac circulation produces rapid pathological changes in the heart musculature, provided it is sudden in occurrence. If the obliteration of the artery take place gradually, however, some relief may be afforded by the establishment of a collateral circulation through the venae minimae, which open out from both the atrial and ventricular cavities and communicate with the finer branches of the cardiac veins, and also with the general capillary network in the heart's walls.

The lymphatic vessels of the heart pass chiefly through the anterior mediastinal lymph- nodes into the broncho-mediastinal trunk.

The cardiac nerves, derived from the vagus and the cervical sympathetic, descend into the superior mediastinum, passing in front of and behind the arch of the aorta; they unite in the formation of the superficial and deep cardiac plexuses. The superficial plexus lies above the right pulmonary artery as the latter passes beneath the aortic arch. The deep plexus lies between the trachea and the arch of the aorta, above the bifurcation of the pulmonary trunk. For the connections of the plexuses see section on Nervous System.

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