mediastinum in the midline and the right and left pleural cavities on either side have already been identified.
Mediastinum
The mediastinum is the bulky midline septum be-tween the pleural cavities and their contents—the lungs. It is thick and extends from the root of the neck to the diaphragm and from the sternum to the vertebral column [see Figs. 2.2, 2.5]. The me-diastinum is elongated in inspiration and may be displaced to one side or the other if the pressures in the two pleural cavities are unequal. For descrip-tive purposes, the mediastinum is subdivided by an imaginary horizontal plane, drawn from the ster-nal angle in front to the intervertebral disc between the fourth and fifth thoracic vertebrae behind. The superior mediastinum lies above this plane.
Below the plane, the inferior mediastinum is further subdivided into: (1) the middle mediasti-num which contains the heart in the pericardial sac, and a phrenic nerve on each side; (2) the an-terior mediastinum between the pericardium and the sternum; and (3) the posterior mediasti-num between the pericardium and the diaphragm anteriorly and the vertebral column posteriorly [Fig. 4.1].
The superior mediastinum has a smaller antero-posterior depth than the inferior mediastinum.
The posterior mediastinum extends furthest
in-CHAPTER 4
The cavity of the thorax
feriorly because it passes into the angle between the diaphragm and the vertebral column to reach down to the level of the twelfth thoracic vertebra [Fig. 4.1].
Pleura
On each side of the mediastinum is a lung covered with a double layer of pleura. The inner layer of the pleura is the pulmonary or visceral pleura, and the outer layer is the parietal pleura [see Fig. 2.5]. The narrow space between the two layers is the pleural cavity. The smooth, glistening lay-er of visclay-eral pleura invests the lung closely, except on a small part of its medial surface [see Figs. 4.11, 4.12]. The parietal pleura lies in close contact with the structures adjacent to the lungs. These include the inner aspect of the thoracic cage, the upper surface of the diaphragm, the mediastinum, and structures in the root of the lung. Different parts of the parietal pleura are given different names, depending on the structure on which they lie [Fig. 4.2]. The part of the parietal pleura that lines the inner surfaces of the thoracic wall is known as the costal pleura. Inferiorly, the part which cov-ers the superior surface of the diaphragm is the diaphragmatic pleura. Superiorly, the dome of the pleura extends through the superior ap-erture of the thorax into the root of the neck. The mediastinal pleura covers the mediastinum lat-erally. Anteriorly and posteriorly, the mediastinal pleura is continuous with the costal pleura at the anterior and posterior margins. The mediastinal and costal pleura continue with the dome of the pleura at the margin of the first rib. The costal pleura continues with the diaphragmatic pleura at the margins of the diaphragm. The parietal pleura
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The cavity of the thorax
of the embryonic lung growing laterally into the hollow pleural cavity [Fig. 4.3]. The invaginated medial wall of the primitive pleural cavity now invests the lung closely and forms the pulmonary pleura covering the lung. As the lung expands out-wards, the pleural cavity is reduced to a mere slit between the outer wall of the cavity (parietal pleu-ra) and the pleura covering the lung (pulmonary pleura). The original point of outgrowth from the mediastinum remains as the relatively narrow root of the lung around which the pulmonary and par-ietal parts of the pleura are continuous with each other. The lung moves freely within the pleural cavity during respiration. (A similar arrangement permits free movement of the heart within the pericardial cavity and parts of the gut tube within the peritoneal cavity.) The pleural cavity contains a small quantity of thin serous pleural fluid which has a thick, fibrous layer (endothoracic fascia)
ex-ternal to it.
The visceral pleura invests the lung closely, ex-cept on a small part of its medial surface where the structures in the root of the lung—the bron-chi and the pulmonary vessels and nerves—enter or leave the lung. In this region, the pulmonary pleura which surrounds the root of the lung is con-tinuous with the parietal pleura which covers the surface of the mediastinum (mediastinal pleura).
The pleura which surrounds the root of the lung extends inferiorly as a narrow fold—the pulmo-nary ligament. This fold attaches the pleura on the medial surface of the lung to the adjacent me-diastinal pleura below the root of the lung [see Figs.
4.11, 4.12].
The arrangement of the parietal and visceral pleura to the lungs and to each other is the result
Oesophagus Trachea
Upper limit of superior mediastinum
Arch of aorta Lower boundary of superior mediastinum
Anterior mediastinum
T.12 T.4
T.1
HEART
LIVER
Fig. 4.1 A diagram of a median section through the thorax to show the general disposition of the structures in the mediastinum. The heart in the pericardium forms the middle mediastinum. The anterior and posterior mediastina lie between it and the sternum and vertebral column, respectively. The posterior mediastinum also extends downwards behind the diaphragm.
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Introduction
ertheless, the cavity should be carefully explored and the root of the lung and pulmonary ligament should be identified.
Dissection 4.1 describes the identification of lung root, removal of lungs, and exploration of the pleural cavity.
Dome of the pleura
The dome of the pleura is that part of the parietal pleura which bulges into the root of the neck above the inner margin of the first rib [Fig. 4.4]. The apex of the dome lies at the level of the neck of the first rib. Because of the obliquity of the first rib, the apex is 4.5 cm above the anterior end of the first rib and 1.2 cm above the medial third of the clavi-cle. The right and left domes are separated by the midline structures of the neck.
The subclavian vessels arch across the anterior surface of the dome, the artery near its highest point, with the vein antero-inferior to the artery.
The internal thoracic artery descends on the front of the dome from the subclavian artery to the back of the first costal cartilage. Between these vessels and the dome is a layer of dense fascia called the suprapleural membrane. The suprapleural membrane extends from the transverse process of the seventh cervical vertebra to the inner margin of the first rib. The membrane may contain some muscle fibres—the scalenus minimus—which lubricates the pleural surfaces and allows them to
slide freely on each other.
If the pleural cavity is opened in the body of an individual who has healthy lungs, the lung is free to be lifted away from the parietal pleura, except at the root and pulmonary ligament. In the aged, it is common to find the pulmonary and parietal pleura adherent at various points because of the spread of inflammatory or cancerous processes from the lung. Such adhesion makes it difficult to confirm the extent of the pleural cavity by dissection.
Nev-Rib 1
Costal pleura
Rib 10 Mediastinal pleura
Diaphragmatic pleura Dome of pleura
Fig. 4.2 Schematic diagram showing parts of the parietal pleura.
Fig. 4.3 Schematic diagram to show how the lungs expand into the pleural cavity, carrying part of the pleura in front of it as a covering. By expanding within the cavity, the lungs reduce the cavity to a mere slit between the outer (parietal) and the inner (visceral) layers of the pleura. Right side: adult position. Left side:
during development.
Trachea
Principal bronchus Parietal pleura
Principal bronchus Parietal pleura
Visceral pleura
Pleural cavity
Visceral pleura
Lung Pleural cavity
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The cavity of the thorax
uous with the corresponding inferior margin of the pleura at the level of the xiphisternal joint.
Trace the inferior margin of each pleural cavity with a finger in the costodiaphragmatic recess. This margin passes postero-inferiorly deep to the seventh costal cartilage and crosses the tenth rib in the mid-axillary line, about 5 cm superior to the costal mar-gin [Fig. 4.5B]. It then passes anterior to the eleventh and twelfth ribs to reach the side of the vertebral column inferior to the medial part of the twelfth rib, at the level of the twelfth thoracic spine. It is impor-tant to remember how far inferiorly the pleural cav-ity reaches; otherwise it may be damaged in clinical procedures done on the upper abdomen.
The rounded posterior margin of the pleura ex-tends along the vertebral column, from the dome to the end of the inferior margin [Fig. 4.5C].
tighten it and help to maintain the pleural dome despite changes in intrapleural pressure.
Margins of the parietal pleura
The anterior margins of the right and left parietal pleura start at a point posterior to the sternoclav-icular joint, and converge towards the midline as they descend from the domes. They come close to each other at the sternal angle and below this re-main in apposition till the level of the fourth cos-tal cartilage [Fig. 4.5A]. At the fourth coscos-tal carti-lage, the left pleural margin deviates to the left and descends posterior to the fifth and sixth costal car-tilages, close to the sternum. (The heart and peri-cardium project to the left here.) The right pleural margin continues its vertical descent, close to the midline. Each anterior margin becomes
contin-DISSECTION 4.1 Identification of the pleura and removal of the lungs Objectives
I. To identify the layers of the pleura, the root of the lung, and the pulmonary ligament. II. To remove the lungs.
Instructions
1. Pull the lung laterally from the mediastinum and find the root of the lung and the pulmonary ligament ex-tending laterally from it.
2. Cut through the root and pulmonary ligament from above downwards close to the lung.
3. Remove the lung on each side and store in a plastic bag to prevent from drying.
4. Examine the extent of the pleural cavity.
Oesophagus
Trachea
Phrenic nerve Subclavian artery Thoracic duct
Internal thoracic artery Vagus nerve
Subclavian vein Sympathetic trunk
Fig. 4.4 The root of the neck to show the structures adjacent to the dome of the pleura (blue).
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Introduction
Fig. 4.5 (A) Position of the lungs and their fissures (solid blue lines), the margins of the pleural sacs (broken blue lines), and the diaphragm, as seen from the anterior aspect of the trunk. The same structures seen (B) from the right lateral aspect and (C) from the posterior aspect.
Oblique fissure
Diaphragm
Outline of parietal pleura Horizontal fissure
Oblique fissure
Cardiac notch of left lung (A)
Dome of diaphragm
Outline of parietal pleura at costodia-phragmatic recess Horizontal fissure Oblique
fissure (B)
Diaphragm
Outline of parietal pleura at costodiaphragmatic recess
Oblique fissure (C)
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The cavity of the thorax