The thoracic cavity lies within the walls of the thorax and is separated from the abdominal cavity by the diaphragm. The cavity contains the right and left lungs, each surrounded by a serous membrane called the pleura. Between the lungs is a central partition, the mediastinum, which includes the heart and great vessels, the trachea and the oesophagus. Superiorly, numerous mediastinal structures enter or leave the root of the neck through the superior thoracic aperture (p. 322). Inferiorly, important structures including the aorta, inferior vena cava and oesophagus pass between the mediastinum and the abdomen through openings in the diaphragm (p. 203).
The pleura surrounds the lungs and lines the walls of the thoracic cavity and is subdivided into visceral and parietal parts. The visceral layer covers the surface of the lung and is continuous with the parietal layer around the mediastinal attachment of the lung at the lung root. The parietal layer covers the lateral aspect of the mediastinum, the upper surface of the diaphragm and the inner aspect of the chest wall (Fig. 2.17). Although the parietal and visceral layers are normally in contact, a space, the pleural cavity (Fig. 2.18), exists between them and contains a thin film of serous fluid. The fluid ensures close apposition of the two pleural surfaces and reduces friction during respiratory movements. Injury or disease may produce an accumulation of air (pneumothorax) or fluid (pleural effusion) within the pleural cavity, causing the lung to collapse.
The parietal pleura is named according to the surfaces it covers. Thus, the mediastinal pleura conforms to the contours of the structures forming the lateral surface of the mediastinum and is innervated by sensory branches of the phrenic nerve. Inferiorly, the diaphragmatic pleura clothes the upper surface of the diaphragm. The central portion receives sensory branches from each phrenic nerve, while the periphery is innervated by lower intercostal nerves. The pleura covering the inner surface of the thoracic wall is called the costal pleura and is innervated segmentally by the intercostal nerves (Fig. 2.17).
The periphery of the diaphragm slopes steeply downwards towards its attachment to the thoracic wall, creating a narrow gutter, the costodiaphragmatic recess. Within this recess, which is particularly deep laterally and posteriorly, the costal and diaphragmatic parts of the pari- etal pleura lie in mutual contact.
The parietal pleura extending into the root of the neck is called the cervical pleura and is innervated by the first intercostal nerve. It is applied to the undersurface of a firm fascial layer, the suprapleural membrane, which prevents upward movement of the apex of the lung and pleura during ventilation (Fig. 7.15).
Surface markings of the parietal pleura Because the parietal pleura is reflected from the thoracic wall onto both the mediastinum and the diaphragm, a line of pleural reflection can be mapped out on the body surface. Traced from its upper limit, approximately 2.5 cm above the medial third of the clavicle, this line descends behind the sternoclavicular joint.
This cervical pleura may be accidentally perforated when a central venous line is being placed in the subclavian vein. The pleural reflection line approaches the midline at the level of the manubriosternal joint. On the right, the pleural reflection descends vertically to the level of the sixth costal cartilage, while on the left, the heart displaces the pleura laterally (Fig. 2.17) so that from the fourth to the sixth costal cartilages, the line of reflection lies just lateral to the edge of the sternum. This displacement exposes part of the pericardium underlying the medial ends of the fourth and fifth intercostal spaces. Traced laterally from the sixth costal cartilage, the surface marking is the same on each side, crossing the eighth rib in the midclavicular line and the tenth rib in the midaxillary line.
Posteriorly, the parietal pleura continues horizontally, crosses the twelfth rib 5 cm from the midline and continues medially for a further 2.5 cm. Thus, a small area of parietal pleura lies below the level of the twelfth rib.
The visceral pleura (Fig. 2.18) is continuous with the mediastinal parietal pleura around the root of the lung. Structures entering or leaving the hilum of the lung occupy the upper part of this pleural sleeve, the lower part consisting of an empty fold of pleura, the pulmonary ligament (Fig. 2.25). The visceral pleura firmly adheres to the surface of the lung and extends into the depths of the fissures. Unlike the parietal layer, the visceral pleura does not have a somatic innervation.
Surface markings of the visceral pleura Since the visceral pleura covers the surface of the lung, its surface markings coincide with those of the lung (p. 41).