Pleura and Mediastinum

The Pleura Structure

The pleura is a twin walled serous lined sac. The two layers are termed visceral and parietal. The visceral pleura invests the lung tissue and the parietal pleura invests the diaphragm, chest wall, apex of the thorax and the mediastinum. The upper limit of the pleura is about 3 cm above the mid-point of the clavicle. From here the line of pleural reflections pass behind the sternoclavicular joints to meet in the midline at the level of the second costal cartilage. At the level of the fourth costal cartilage the lines deviate from each other. On the left the line of reflection deviates laterally descending along the lateral margin of the sternum as far as the sixth costal cartilage. On the right the line continues vertically downwards. At the level of the sixth costal cartilage both lines of reflection pass to the eighth rib in the mid clavicular line and then to the tenth rib in the mid axillary line and finally onto the twelfth rib in the paravertebral line.

Blood Supply

The pleura receives a supply of arterial and venous vessels from the organs which it covers. Nerve Supply

The pulmonary pleura has no sensory supply but the parietal pleura is supplied by fibres from all subjacent tissue.

Lymphatic Drainage

Visceral pleura initially drain to the superficial plexus of the lung and thence to the hilar nodes. The parietal pleura drains into parasternal, diaphragmatic and posterior mediastinal nodes.

The Mediastinum

The mediastinum is the area lying between the pleural sacs. The pericardium makes an artificial division into four regions: The middle mediastinum is the space occupied by the pericardium and its contents; the anterior mediastinum lies between the middle mediastinum and the sternum; the posterior mediastinum is that space between the pericardium and diaphragm and the superior mediastinum lies between pericardium below and thoracic inlet above.

The Diaphragm

A major muscle of respiration, the diaphragm is a musculotendinous septum between thorax and abdomen. It comprises a central tendinous portion and a peripheral muscular portion. The attachments of the diaphragm are central and peripheral. See Figures CA.12, CA.13.

Central Attachment

The fibres of the tendinous section are concentrated into a trilobed central tendon, which blends superiorly with the fibrous pericardium.

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Left vogui— Cfthfral lentfori—| Mario* vtno covo—| flight phrenic nerve--

Figure CA.12 The diaphragm

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Oesophagus

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Figure CA.13 The diaphragmatic orifices

Peripheral Attachment

A complex arrangement consisting of attachments to the crura, medial, median and lateral arcuate ligaments, costal margin and the xiphoid.

Diaphragmatic Foraminae

The inferior vena cava passes through the diaphragm within the central tendon to the right side at the level of T8. This foramen also transmits the right phrenic nerve.

The oesophagus passes through the diaphragm to the left of the midline at the level of T10. This foramen also transmits the vagi.

The aorta passes through the diaphragm behind the median arcuate ligament at T12. The thoracic duct and azygos vein also pass through this foramina.

The left phrenic nerve pierces the left dome of the diaphragm.

Relations

The heart and lungs within their respective sacs lie above. Inferiorly, on the right are the liver and right kidney and on the left the fundus of the stomach spleen and left kidney.

Nerve Supply

The diaphragm is supplied by two phrenic nerves (C3, 4, 5).

The Heart

Structure

The heart is a muscular pump consisting of four chambers delineated by the coronary and interventricular sulci. The heart lies within the pericardial sac effectively suspended by the great vessels. Its shape is said to be that of an irregular cone with a base and an apex, lying obliquely across the middle mediastinum. The base is posterior facing and mainly consists of left atrium. The apex represents the tip of the left ventricle. The anterior surface is formed by the right ventricle (Figure CA. 14).

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