Middle Mediastinum PDF

Dr.Ahmed Abdulhussain  The middle mediastinum is centrally located in the thoracic cavity. It contains the pericardium, heart, origins of the great vessels, various nerves, and smaller vessels.  It is a fibro-serous sac surrounding the heart and the roots of...

Dr.Ahmed Abdulhussain  The middle mediastinum is centrally located in the thoracic cavity. It contains the pericardium, heart, origins of the great vessels, various nerves, and smaller vessels.  It is a fibro-serous sac surrounding the heart and the roots of the great vessels. It consists of two components, the fibrous pericardium and the serous pericardium. The fibrous pericardium is a tough connective tissue outer layer that defines the boundaries of the middle mediastinum. The serous pericardium is thin and consists of two parts:  Parietal layer lines the inner surface of of surface the Fibrous pericardium. Visceral layer (epicardium) of serous pericardium adheres to the heart and forms its outer covering. The two layers of serous pericardium are continuous at the roots of the great vessels. The narrow space created between the two layers of serous pericardium, containing a small amount of fluid, is the pericardial cavity. This potential space allows for the relatively uninhibited movement of the heart.  It is the strong fibrous part of the sac. It is firmly attached below to the central tendon of the diaphragm. It fuses with the outer coats of the great blood vessels passing through it namely, the aorta, the pulmonary trunk, the superior and inferior venae cavae, and the pulmonary veins. The fibrous pericardium is attached in front to the sternum by the sternopericardial ligaments.  It lines the fibrous pericardium and coats the heart. It is divided into parietal and visceral layers. The parietal layer lines the fibrous pericardium and is reflected around the roots of the great vessels to become continuous with the visceral layer of serous pericardium that closely covers the heart.  The visceral layer is closely applied to the heart and is often called the epicardium. The slitlike space between the parietal and visceral layers is referred to as the pericardial cavity. Normally, the cavity contains a small amount of tissue fluid (about 50 mL), the pericardial fluid, which acts as a lubricant to facilitate movements of the heart.  On the posterior surface of the heart, the reflection of the serous pericardium around the large veins forms a recess called the oblique sinus. Also on the posterior surface of the heart is the transverse sinus, which is a short passage that lies between the reflection of serous pericardium around the aorta and pulmonary trunk and the reflection around the large veins.  The pericardium is supplied by branches from the internal thoracic, pericardiacophrenic, musculophrenic, and inferior phrenic arteries, and the thoracic aorta. Veins from the pericardium enter the azygos system of veins and the internal thoracic and inferior phrenic veins. Nerves supplying the pericardium arise from the vagus nerve, the sympathetic trunks, and the phrenic nerves.  The heart is a hollow muscular organ that is somewhat pyramid shaped and lies within the pericardium in the mediastinum. It is connected at its base to the great blood vessels but otherwise lies free within the pericardium.  The heart has three surfaces: i. Sternocostal (anterior) ii. Diaphragmatic (inferior) iii. Base (posterior). It also has an apex, which is directed downward, forward, and to the left.  The anterior surface faces anteriorly and consists mostly of the right ventricle with some of the right atrium on the right and some of the left ventricle on the left. The right border is formed by the right atrium; the left border, by the left ventricle and part of the left auricle. The right ventricle is separated from the left ventricle by the anterior interventricular groove.  It consists of the left ventricle and anda a small portion of the right ventricle separated by the posterior interventricular groove. This surface faces inferiorly, rests on the diaphragm, is separated from the base of the heart by the coronary sinus, and extends from the base to the apex of the heart.  The base of the heart is quadrilateral and directed posteriorly. It consists of: the left atrium; a small portion of the right atrium; and the proximal parts of the great veins (superior and inferior venae cavae and the pulmonary veins). It lies opposite the vertebrae T5-T8. The esophagus lies immediately posterior to the base.  It is formed by the infero-lateral part of the left ventricle and is positioned deep to the left fifth intercostal space, 8-9 cm from the mid-sternal line.  The heart by is divide divided by is heart vertical septa into four chambers: the right and left atria and the right and left ventricles. The right atrium lies anterior to the left atrium, and the right ventricle lies anterior to the left ventricle. The walls of the heart are composed of cardiac muscle, the myocardium; covered externally with serous pericardium, the epicardium; and lined internally with a layer of endothelium, the endocardium.  It consists of a main cavity and a small outpouching, the auricle. On the outside of the heart at the junction between the right atrium and the right auricle is a vertical groove, the sulcus terminalis, which on the inside forms a ridge, the crista terminalis.  The part of the atrium in front of the ridge is roughened or trabeculated by bundles of muscle fibers, the musculi pectinati, which run from the crista terminalis to the auricle. The superior vena cava opens into the upper part of the right atrium; it has no valve.  The inferior vena cava opens into the lower part of the right atrium; it is guarded by a rudimentary, nonfunctioning valve. The coronary sinus opens into the right atrium between the inferior vena cava and the atrioventricular orifice. It is guarded by a rudimentary, nonfunctioning valve. The right atrioventricular orifice lies anterior to the inferior vena caval opening and is guarded by the tricuspid valve  A depression is clearly visible in the septum just above the orifice of the inferior vena cava. This is the fossa ovalis (oval fossa), with its prominent margin, the limbus fossa ovalis (border of oval fossa). The fossa ovalis marks the location of the embryonic foramen ovale, which is an important part of fetal circulation.  It communicates with the right atrium through the atrioventricular orifice and with the pulmonary trunk through the pulmonary orifice. As the cavity approaches the pulmonary orifice it becomes funnel shaped, at which point it is referred to as the infundibulum. The walls of the right ventricle are much thicker than those of the right atrium and show several internal projecting ridges formed of muscle bundles.  The projecting ridges give the ventricular wall a sponge- like appearance and are known as trabeculae carneae, which are of three types: 1. Papillary muscles: project inward, being attached by their bases to the ventricular wall; their apices are connected by fibrous chords (the chordae tendineae) to the cusps of the tricuspid valve 2. Moderator band: crosses the ventricular cavity from the septal to the anterior wall. It conveys the right branch of the atrioventricular bundle, which is part of the conducting system of the heart. 3. simply composed of prominent ridges.  It guards the atrioventricular orifice and consists of three cusps formed by a fold of endocardium with some connective tissue enclosed: anterior, septal, and inferior (posterior) cusps. The chordae tendineae connect the cusps to the papillary muscles.  It guards the pulmonary orifice and consists of three semilunar cusps formed by folds of endocardium with some connective tissue enclosed. No chordae or papillary muscles are associated with these valve cusps. The three semilunar cusps are arranged with one posterior (left cusp) and two anterior (anterior and right cusps).  It consists of a main cavity and a left auricle. It is situated behind the right atrium and forms the greater part of the base or the posterior surface of the heart. The interior of the left atrium is smooth, ‫تمتلك‬ but the left auricle possesses muscular ridges as in the right auricle. The four pulmonary veins, two from each lung, open through the posterior wall and have no valves. The left atrioventricular orifice is guarded by the mitral valve.  It communicates with the left atrium through the atrioventricular orifice and aorta through the with the aota aortic orifice. The walls of the left ventricle are three times thicker than those of the right ventricle. (The left intraventricular blood pressure is six times higher than that inside the right ventricle).  There are well-developed trabeculae carneae, two large papillary muscles, but no modrator band. The part of the ventricle below the aortic orifice is called the aortic vestibule. The interventricular septum is described as having two parts: i. a muscular part ii. a membranous part.  It guards the atrioventricular orifice. It consists of two cusps, one anterior and one posterior, which have a structure similar to that of the cusps of the tricuspid valve. The attachment of the chordae tendineae to the cusps and the papillary muscles is similar to that of the tricuspid valve.  It guards the aortic orifice and is precisely similar in structure to the pulmonary valve. One cusp is situated on the anterior wall (right cusp) and two are located on the posterior wall (left and posterior cusps). Behind each cusp the aortic wall bulges to form an aortic sinus. The anterior aortic sinus gives origin to the right coronary the andartery, and artery, the left coronary left posterior sinus gives origin to the left coronary artery.