Thoracic cavity

Question 1

Describe the thoracic cavity.

Answer 1

• The cavity of the thorax is completely filled laterally by the lungs, each lying in its pleural cavity.
• The space between the pleural cavities occupying the centre of the thoracic cavity is the mediastinum.
• It contains the heart and great blood vessels, oesophagus, trachea and its bifurcation, thymus, thoracic duct, lymph nodes, phrenic and vagus nerves.
• The loose connective tissue between these structures connects freely with that of the neck.
• Mediastinitis may complicate infections in the neck.

Question 2

Outline the divisions of the mediastinum.

Answer 2

• There is a plane of division to which the whole topography of the mediastinum can be related, namely a plane passing horizontally through the sternal angle (of Louis), i.e. the manubriosternal joint.
• From the second costal cartilages, this plane passes backwards to the lower border of T4 vertebra.
• Above, between it and the thoracic inlet, lies the superior mediastinum.
• Below the plane, the inferior mediastinum is divided into three compartments by the fibrous pericardium - a part in front, the anterior mediastinum; a part behind, the posterior mediastinum; and the middle mediastinum in between containing the pericardium and heart together with the adjoining parts of the great vessels and the lung roots.

Question 3

Which structures pass through the plane of division of the mediastinum?

Answer 3

• The anterior and posterior mediastina are in direct continuity with the superior mediastinum; their separation from it is purely descriptive, not anatomical. * The plane passes through the bifurcation of the trachea, the concavity of the arch of the aorta, and just above the bifurcation of the pulmonary trunk.
• On the plane
• The azygos vein enters the superior vena cava.
• The thoracic duct reaches the left side of the oesophagus in its passage upwards from the abdomen.
• Also lying in the plane are the ligamentum arteriosum, with the left recurrent laryngeal nerve recurving below it.
• Tracheobronchial lymph nodes.
• The superficial and deep parts of the cardiac plexus.

Question 4

Describe the boundaries of the superior mediastinum.

Answer 4

• The superior mediastinum is wedge shaped.
• The anterior boundary is the manubrium.
• The posterior boundary is much longer, due to the obliquity of the thoracic inlet.
• It consists of the bodies of the first four thoracic vertebrae; this wall is concave towards the mediastinum.

Question 5

Outline the thoracic inlet.

Answer 5

• At the thoracic inlet, often called clinically the thoracic outlet, the oesophagus lies against the body of T1 vertebra.
• The trachea lies on the oesophagus and may touch the jugular notch of the manubrium.
• The midline of the inlet is thus wholly occupied by these two tubes.
• At the inlet the apices of the lungs lie laterally, separated by the trachea and oesophagus and by vessels and nerves passing between the neck and the superior mediastinum.
• Below the inlet, the trachea slopes back and the manubrium slopes forward; the brachiocephalic trunk, the left brachiocephalic vein and the thymus occupy the space thus provided.
• The concavity of the arch of the aorta lies in the plane of the sternal angle, and the arch of the aorta lies wholly in the superior mediastinum, behind the manubrium.
• It arches over the beginning of the left bronchus and the bifurcation of the pulmonary trunk.
• The brachiocephalic trunk begins as a midline branch from the arch and diverges to the right as it ascends in front of the trachea. The two other branches of the arch, the left common carotid and left subclavian arteries, pass upwards on the left side of the trachea.
• These great arteries keep the left vagus nerve and apex of the left lung away from contact with the trachea.
• On the right side there is no structure to separate the trachea from the right vagus and apex of the right lung.
• The veins entering the superior mediastinum are the right and left brachiocephalic veins, each formed by the confluence of the internal jugular with the subclavian vein. They lie in front of the arteries and are asymmetrical.
• The right brachiocephalic vein passes vertically down- wards; the left vein runs across the superior mediastinum, above the arch of the aorta, to join the right.
• The confluence of the brachiocephalic veins produces the superior vena cava, which passes vertically downwards behind the right edge of the sternum, anterior to the right pulmonary hilum.
• The right phrenic nerve descends in contact with the lateral aspect of the right brachiocephalic vein and superior vena cava.