1. Again remove the cut portion of the sternum along with the attached costal cartilages and ribs. Preserve the removed portion of the sternum and costal cartilages for later study.

2. Examine the mediastinum, which is the space between the two pleural sacs. The mediastinum is subdivided into a superior and an inferior mediastinum. The part that lies above a plane passing from the sternal angle to the junction between the vertebrae T4 and T5 is the superior mediastinum and the part below the plane is the inferior mediastinum. The inferior mediastinum is further subdivided into an anterior mediastinum situated in front of the heart and a posterior mediastinum lying behind the heart and the posterior surface of the diaphragm, while the heart and the roots of the great vessels lie in the middle mediastinum.

3. Identify the bilobed thymus gland if present, occupying the superior mediastinum and the upper part of the anterior mediastinum. Observe that it overlaps not only the great vessels, but also the upper portion of the pericardium. The gland receives its blood supply from the internal thoracic artery.

4. Remove the thymus. The fibrous pericardium covering the heart as well as the large vessels lying cranial to the pericardium can now be viewed. Displace the remainder of the manubrium upwards and forwards along with the first rib in order to clean the left brachiocephalic vein which lies under cover of the upper half of the manubrium. This vein commences behind the left sternoclavicular joint and passes obliquely in front of the branches of the arch of the aorta to join the right brachiocephalic vein at the lower border of the right first costal cartilage to form the superior vena cava. Next clean the right brachiocephalic vein which commences behind the right sternoclavicular joint.

5. Clean the vessels arising from the arch of the aorta. These are from right to left: brachiocephalic trunk, left common carotid artery and left subclavian artery. Note a small artery, the thyroidea ima which if present arises from the brachiocephalic trunk and ascends into the neck to supply the thyroid gland. Identify the trachea and oesophagus which lie behind the arch of the aorta.

6. Replace the cut portion of the sternum and ribs, and note that the arch of the aorta lies behind the lower half of the manubrium, i.e. entirely in the superior mediastinum.

7. Trace the phrenic nerves cranially. Note that the right nerve lies lateral to the right brachiocephalic vein and superior vena cava, while the left nerve descends between the left common carotid and left subclavian arteries to cross the arch of the aorta. In the interval between these two arteries, find the left vagus nerve which lies posterior to the left phrenic nerve. Trace the left vagus nerve as it descends in front of the arch of the aorta where it gives off the left recurrent laryngeal nerve. Secure this nerve as it hooks under the aortic arch and the ligamentum arteriosum. The ligamentum arteriosum is a fibrous band passing from the left pulmonary artery to the aortic arch. It is a remnant of the ductus arteriosus. Clean the superficial part of the cardiac plexus lying in the concavity of the aortic arch and superficial to the ligamentum arteriosum. Sympathetic branches from the superior cervical, middle cervical, cervicothoracic and 2nd, 3rd, 4th and 5th thoracic ganglia as well as branches from the right and left vagus nerves contribute to the cardiac plexus. The cardiac plexus consists of a superficial and a deep part.

8. Find the left superior intercostal vein passing between the left phrenic and left vagus nerves to drain into the left brachiocephalic vein. It drains the second and third intercostal spaces.

9. Now identify the right vagus nerve as it passes behind the root of the right lung and trace it upwards and note how it enters the thoracic inlet by passing superficial to the right subclavian artery.

10. Next examine the attachments of the fibrous pericardium to the central tendon of the diaphragm and to the walls of the great vessels. The sternopericardial ligaments attaching it to the back of the sternum have already been cut. Expose the heart by a cruciform incision through the front of the fibrous pericardium. Note that the inner surface of the fibrous pericardium is smooth as it is lined by the parietal layer of the serous pericardium. The serous layer of pericardium which is intimately adherent to the heart is the visceral layer. Observe that this layer is reflected along the great vessels where it becomes continuous with the parietal layer.

1. Identify the right and left auricles of the atria, the right and left ventricles, the ascending aorta and the pulmonary trunk. Note that the coronary sulcus separates the atria from the ventricles while the anterior and posterior interventricular sulci lie between the ventricles. Trace the superior and inferior venae cavae towards the right atrium.

2. Note the orientation of the heart. Observe that the base of the heart is situated above and posteriorly, while the apex is directed anteroinferiorly and to the left. Verify that the major part of the base of the heart is formed by the left atrium which lies opposite the middle four thoracic vertebrae. Note that the right atrium lies mostly in front of the left atrium while the diaphragmatic surface of the heart lies against the diaphragm, and the right and left pulmonary surface against the right and left lungs respectively.

3. Pass your index finger behind the ascending aorta and pulmonary trunk and in front of the superior vena cava to explore the transverse sinus of the pericardium. Note that your finger is above the left atrium as it lies within the transverse sinus. Lift the apex of the heart and pass your finger along the posterior surface of the heart and note the oblique sinus of the pericardium, which lies between the inferior vena cava and right pulmonary veins on the right and the left pulmonary veins on the left. Note that the oblique and transverse sinuses are separated from each other by reflections of the visceral pericardium along the superior border of the left atrium.

4. Clean the superior vena cava and observe that its upper half is extrapericardial and the lower half intrapericardial. Note that the vena azygos joins it posteriorly at the lower end of the extrapericardial portion.

5. Now remove the heart. First divide the inferior vena cava and pulmonary veins close to the heart. Lift the heart upwards and review the reflections of the serous pericardium and the boundaries of the oblique sinus. Note the caval opening in the central tendon of the diaphragm where the inferior vena cava passes through. This corresponds to the level of the eighth thoracic vertebra. Cut the reflections of the visceral pericardium along the superior border of the left atrium. Next divide the ascending aorta, the pulmonary trunk and the superior vena cava. Lift out the freed heart. The heart

1. Now that the heart is freed, identify and trace the left coronary artery as it runs between the pulmonary trunk and the left auricle to enter the coronary sulcus, where it divides into the anterior interventricular and circumflex branches. Trace the anterior interventricular branch along the anterior interventricular sulcus towards the apex of the heart. Follow the circumflex branch which runs in the coronary sulcus. Secure the small left marginal branch arising from the circumflex artery.

2. Next identify the origin of the right coronary artery from the right side of the ascending aorta and trace it first between the pulmonary trunk and right auricle and subsequently along the coronary sulcus where it gives off its right marginal branch. Trace the right coronary artery further along the posterior surface where it divides into a large posterior interventricular branch and a smaller terminal branch which anastomoses with the terminal part of the circumflex branch of the left coronary artery. Trace the posterior interventricular branch which runs in the posterior interventricular sulcus towards the apex along the diaphragmatic surface of the heart.

3. Identify:

(a)the great cardiac vein which first lies in the anterior interventricular sulcus along with the anterior interventricular branch of the left coronary artery and then accompanies the circumflex arterial branch;

(b) the middle cardiac vein running along the posterior interventricular sulcus, and accompanying the posterior interventricular branch of the right coronary artery;

(c) the left and right marginal veins along the corresponding margins of the heart; and

(d) the small cardiac vein occupying the right extremity of the coronary sulcus and receiving the right marginal vein. Trace these veins towards the coronary sinus. The major part of the coronary sinus lies between the left atrium and left ventricle. Follow the coronary sinus to its entry into the right atrium.

4. Now open the four chambers of the heart in the following manner: The right atrium Pass the blade of a long knife through the superior vena caval opening down into the right atrium and out through the inferior vena caval opening. Cut laterally to open the right atrium. Then use a pair of scissors to make a second cut through the anterior wall of the right atrium at right angles to the first cut. Extend this into the apex of the right auricle. Remove any clots from the atrial cavity and thoroughly wash out the atrium so that its internal features are clearly displayed.

The right ventricle

Pass a long-bladed knife through the right atrioventricular orifice between the anterior and septal cusps until it emerges through the apex of the right ventricle. When the knife is in this position, open the ventricle by cutting antero-laterally to the left. Next pass the knife from the lower end of the ventricular incision upwards through the pulmonary valve and out of the cut end of the pulmonary trunk. Now cut anteriorly, thus increasing the exposure of the interior of the right ventricle and at the same time opening the pulmonary trunk. Remove any clots and wash out the ventricle.

The left atrium

Make a vertical cut through the wall of the atrium so as to join the two right pulmonary openings. Do the same with the two left pulmonary veins. Make a transverse cut to join these two small vertical incisions and extend this cut as far as the apex of the left auricle and so open the left atrium posteriorly. Wash out the interior of this chamber. The left ventricle First pass the knife through the left atrioventricular orifice until it emerges through the apex of the left ventricle. When the knife is in this position, open the left ventricle by cutting laterally to the left. Next pass the knife from the lower end of this incision upwards between the anterior cusp of the mitral valve and the interventricular septum through the aortic valve and out through the cut end of the aorta. Now cut posteriorly to expose the interior of the left ventricle and the ascending aorta. Clean and wash out any clots that may still be present.

5. Observe the following features in the right atrium:

(a)crista terminalis, a ridge running between the anterior margins of the superior and inferior venae cavae. (The sinuatrial node lies near the upper end of the crista terminalis.) Note the muscular ridges, the musculi pectinati, running from the crista terminalis into the auricular part of the atrium. Observe that the atrial wall behind the crista terminalis is smooth. This posterior portion of the atrium is the sinus venae cavae;

(b) the interatrial septum separating the two atria. In the septum, note the fossa ovalis (a depression) and limbus fossae ovalis forming a raised margin above and in front of the fossa;

(c) opening of the superior vena cava;

(d) opening of the inferior vena cava;

(e) right atrioventricular orifice; and

(f) opening of the coronary sinus situated between the last two orifices. (Situated above the opening of the coronary sinus and lying in the atrial septum is the atrioventricular node).

6. Next observe the interior of the right ventricle and note:

(a)trabeculae carneae which are muscular ridges in the wall of the ventricle. This portion of the ventricle is the inflow tract;

(b) right atrioventricular orifice guarded by the tricuspid valve, composed of anterior, posterior and septal cusps. Identify these cusps which are named according to their positions. The right atrioventricular orifice admits three fingers;

(c) three papillary muscles which are conical muscular projections from the ventricular wall. Note the positions of these papillary muscles (anterior, posterior and septal);

(d) chordae tendineae which are fibrous cords arising from the papillary muscles to be attached to the tricuspid valve cusps a little away from their free margins;

(e) opening of pulmonary trunk guarded by the pulmonary valve with three semilunar cusps. Note that the pulmonary trunk arises from the smooth upper part of the right ventricle is known as the conus arteriosus (the outflow tract); and

(f) note that a band of muscle passed (now torn) from the septum to the anterior wall of the ventricle. This was the septomarginal trabecula which conveyed the right branch of the atrioventricular bundle.

7. N ow examine the interior of the left atrium and note:

(a)openings of the four pulmonary veins into the smooth portion of the atrium;

(b) left atrioventricular orifice; and

(c) left auricle forming the anterior part of the atrium and showing the presence of musculi pectinati.

8. Finally observe the following features in the interior of the left ventricle:

(a)trabeculae carneae as in the right ventricle (this region is the inflow tract);

(b) left atrioventricular orifice guarded by the mitral valve which has an anterior and a posterior cusp. Note that the anterior cusp of the mitral valve intervenes between the mitral and aortic orifices. It is smooth on both surfaces as blood passes to and from the ventricle on either side of this valve cusp;

(c) anterior and posterior papillary muscles;

(d) chordae tendineae arising from the papillary muscles and attached to the anterior and posterior cusps of the mitral valve;

(e) aortic vestibule which is the smooth upper and anterior part of the left ventricle leading into the aorta (this region is the outflow tract);

(f) aortic orifice guarded by the aortic valve which is composed of three semilunar cusps like the pulmonary valve; Observe that the left ventricular wall is about three times thicker than the right. Why?

9. In the cut ascending aorta identify three dilatations (aortic sinuses) at the root of the aorta, just above the valve cusps. Observe that the right coronary artery arises from the right aortic sinus and the left coronary artery from the left aortic sinus.

10. Note that the interventricular septum is placed obliquely so that its right surface which is convex looks forwards and to the right. Feel the upper membranous and the lower muscular parts of the septum.

1. Observe that the pulmonary trunk lies first to the right, then in front and finally to the left of the aorta. Note that the right pulmonary artery passes behind the aorta.

Summary

In order to understand the orientation of the heart, it must be appreciated that the heart has a base placed posteriorly, a diaphragmatic surface in relation to the diaphragm and an anterior surface. The axis of the heart, i.e. from base to apex, runs obliquely downwards, forwards and to the left. Moreover, the heart is rotated in such a way that the right atrium lies almost in front of the left atrium so that the interatrial septum separating the two chambers forms the posterior wall of the right atrium. Similarly, the right ventricle tends to lie in front of the left. As a result, both the interatrial and interventricular septa are inclined at about 45º to the sagittal plane. Thus, the slope of the septa is such that their anterior surfaces are directed forwards and to the right. Moreover, the ventricles lie anterior to the atria so that the blood from the atria flows more or less in a horizontal direction into the ventricles and not vertically downwards as is usually imagined. The right and left coronary arteries which supply the heart arise from the ascending aorta opposite the right and left aortic cusps in relation to the corresponding aortic sinuses. The left coronary artery is preponderant in that it supplies more of the heart than the right. However, the right artery usually supplies such important structures as the sinuatrial and atrioventricular nodes, the former being known as the pace maker. Sometimes these nodes are supplied by the left coronary artery. The pace of the heart is reduced if the sinuatrial node is nonfunctional and the ventricular rhythm is further reduced if the AV node is also damaged. Although these nodes possess intrinsic rhythmic activity, this can be modified by the nerves supplying the nodes. These nerves are derived from the superficial and deep parts of the cardiac plexus, the branches of which run along the right coronary artery to supply the SA and AV nodes.

The branches of the two coronary arteries anastomose both in the coronary sulcus and in the interventricular sulci. These anastomoses are insufficient to maintain a collateral circulation if one of the main arteries or one of its larger branches is blocked suddenly. However, the collateral circulation can sometimes maintain the viability of the heart if the blockage had been gradual. When one of the main branches of the left or right coronary artery is blocked suddenly by a blood clot, i.e. coronary thrombosis, it results in death (infarction) of the cardiac muscle supplied by the vessel. Pain resulting from ischaemia (reduced blood supply) or infarction is felt retrosternally and sometimes referred along the medial side of the left upper limb.