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marked projection of the heart to the left side, and to the position of the thoracic aorta on the left side of the median plane, the left pleural chamber, although it is deeper than the right, is greatly reduced in width. The two pleural cavities, therefore, are very far from being symmetrical in form, and consequently the mediastinal septum tends to extend so the left of the median plane of the body.

Each pleural cavity is completely lined by a separate serous membrane termed the pleura. The portion of this membrane which clothes the mediastinum or intervening partition forms the lateral boundary of an area termed the mediastinal or interpleural space, within which the parts which build up the mediastinal septum are placed.

PLEURE.

The pleura of each side not only lines the corresponding pleural cavity, but at the pulmonary root, it is prolonged on to the lung so as to give it a complete investment. It is customary, therefore, to recognise a pulmonary or investing part (pleura pulmonalis) and a parietal or lining part (pleura parietalis). The inner surface of the pleura (i.e. that surface which is turned towards the interior of the cavity) is coated with squamous endothelium, and presents a smooth, glistening, and polished appearance; further, it is moistened by a small amount of serous fluid. In consequence of this the surface of the lung covered by pulmonary pleura can glide on the wall of the cavity, lined as it is by parietal pleura, with the least possible degree of friction. In the pathological condition known as pleurisy the inner surface of the pleura becomes roughened by inflammatory exudation, and the so-called 'friction sounds" are heard when the ear is applied to the chest.

Costal part of parietal pleura

Pleural cavity Visceral pleura

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Pleura Pulmonalis.-The pulmonary pleura is very thin, and is so firmly bound down to the surface of the lung that it cannot be detached without laceration of the

SACS AS SEEN IN TRANSVERSE SECTION.

FIG. 862.—DIAGRAM SHOWING ARRANGEMENT OF PLEURAL pulmonary substance, and then only in small pieces. It dips into the fissures of the lungs, lines them down to the very bottom, and thus completely separates the different lobes of the lungs from each other. The pulmonary pleura becomes continuous with the mediastinal pleura at the root of the lung, and also through the ligamentum pulmonale.

Pleura Parietalis.-Different names are applied to the parietal pleura as it lines the different parts of the wall of the cavity in which the lung lies. Thus there are the costal pleura, the diaphragmatic pleura, the mediastinal pleura, and the cupula pleurae; but it must be borne in mind that these terms are merely used for convenience in description, and that the portions of the pleura so designated are all directly continuous with one another.

The cupula pleuræ or the cervical pleura rises into the root of the neck, through the superior aperture of the thorax, and forms a dome-shaped roof for the pleural cavity. Its highest point or summit reaches the level of the inferior border of the neck of the first rib; but owing to the great obliquity of the first costal arch, this point is placed from one to two inches above to the ventral or anterior extremity of the first rib, and from a half to one and a half inches above the clavicle. The cupula pleuræ is supported on the lateral side by the scalenus anterior and scalenus medius muscles, whilst the subclavian artery, arching laterally, lies in a groove on its medial and ventral aspects a short distance below its summit.

At a lower level the innominate and subclavian veins also lie upon its medial and ventral aspects.

The cupula pleuræ is strengthened and held in place by an aponeurotic expansion, first described by Sibson, which is spread over it, and is attached to the internal concave margin of the first rib. This fascia is derived from a small muscular slip which takes origin from the transverse process of the seventh cervical vertebra.

Right vagus nerve Trachea

Esophagus

Left subclavian artery

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Right

Pleura Costalis. The cos- innominate vein Innominate artery

tal pleura is the strongest and thickest part of the parietal pleura. It lines the internal surfaces of the costal arches and of the intervening intercostal muscles. Ventrally it reaches the sternum, whilst dorsally it passes from the ribs over the sides of the bodies of the vertebræ. It is easily detached from the parts which it covers, except as it passes from the heads of the ribs on to the vertebral column. There it is somewhat tightly bound down.

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Left common carotid artery Left innominate vein

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Pleura Diaphragmatica.The diaphragmatic pleura covers that portion of

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face of the dia- FIG. 863.-DISSECTION OF A SUBJECT HARDENED BY FORMALIN INJECTION, to show the

relations of the two pleural sacs, as viewed from the front. The anterior and diaphragmatic lines of pleural reflection are exhibited by black dotted lines, whilst the outlines of the lungs and their fissures are indicated by the blue lines.

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lies to the lateral side of the base of the pericardium, but it does not dip down to the bottom of the narrow interval between the thoracic wall and the diaphragm. In other words, a strip of the thoracic surface of the diaphragm adjoining its costal attachment is left uncovered.

Pleura Mediastinalis.-The mediastinal pleura extends from the dorsal surface of the ventral thoracic wall to the vertebral column, and clothes the side of the mediastinal septum, which intervenes between the two pleural cavities. It is continuous with the costal pleura of its own side, both ventrally and dorsally, along two lines which are respectively termed the sternal and vertebral lines of

pleural reflection; whilst inferiorly it becomes continuous with the diaphragmatic pleura, of its own side, at the base of the pericardium.

Above the root of the lung the mediastinal pleura passes directly from the sternum to the vertebral column. In that region the left mediastinal pleura is applied to the arch of the aorta and the phrenic and vagus nerves; to the left innominate vein, the left superior intercostal vein and the left common carotid and left subclavian arteries; to the oesophagus and the thoracic duct. The right mediastinal pleura, on the other hand, is applied, above the root of the lung, to the superior part of the vena cava superior and the right innominate vein; to the innominate artery; to the vena azygos, as it hooks forwards above the bronchus; to the vagus and phrenic nerves; and to the right side of the trachea.

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Opposite the root of the lung, as well as in the region below it, the mediastinal pleura clothes the corresponding aspect of the pericardium (pleura pericardiaca), and is somewhat firmly attached to it. As the phrenic nerve passes downwards upon the pericardium it likewise is covered over by the pleura. In the region corresponding to the superior part of the lateral aspect of the pericardium the mediastinal pleura is prolonged laterally, so as to form an investment for the root of the lung, and becomes continuous around the hilum of the lung with the pulmonary pleura. Below the root of the lung the two layers of pleura which invest it come into apposition with each other, and are prolonged downwards as a distinct fold, termed the ligamentum pulmonale. This fold stretches between the pericardium and the inferior part of the mediastinal surface of the lung, and ends inferiorly in a free border.

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Dorsal to the root of the lung and the ligamentum pulmonale the mediastinal pleura on the right side passes over the oesophagus to the vertebral column, whilst on the left side it passes dorsally over the thoracic aorta, and to a small extent over the lower end of the oesophagus, in the region immediately adjoining the diaphragm and ventral to the thoracic aorta.

FIG. 864.-LATERAL VIEW OF THE RIGHT PLEURAL SAC IN

A SUBJECT HARDENED BY FORMALIN INJECTION. The blue lines indicate the outline of the right lung, and also the position of its fissures.

Lines of Pleural Reflection. These are three in number-viz., the sternal, the vertebral, and the diaphragmatic. The pleural cavities are not symmetrical. The left is longer and narrower than the right, and it thus happens that the lines of pleural reflec tion do not accurately correspond on the two sides of the body. Further, although the

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vertebral line of reflection is fairly constant, the other two reflection-lines are subject to marked variations in different subjects. Consequently the following description must be regarded as merely giving the average condition.

The vertebral line of pleural reflection is that along which the costal pleura is continued ventrally from the vertebral column to become the mediastinal pleura. On the right side, above the root of the lung, the pleura passes from the bodies of the vertebræ on to the right side of the trachea; whilst lower down, and dorsal to the pericardium, it passes from the vertebral bodies on to the œsophagus. On the left side, and above Left the arch of the aorta, the pleura along this line of reflection is carried from the vertebral column on to the oesophagus and thoracic duct; below that level it passes on to the thoracic aorta. In the superior part of the chest the right and left lines of reflection are placed well apart from each other, and about equidistant from the median plane. As they are traced downwards they approach more closely to each other and deviate to the left, so that whilst the reflection on the right side takes place from the ventral aspect of the vertebral bodies, on the left side it takes place from the left aspect of the vertebral column. This is

due to the position of the thoracic aorta.

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The sternal line of

- pleural reflection is that along which the costal pleura leaves the ventral thoracic wall to become the mediastinal pleura. The lines differ somewhat on the two sides, and in both cases show a tendency to deviate to the left (Fig. 863, p. 1085). In the vicinity of the manubrium sterni the two pleural sacs are separated from each other by an angular interval. The lines of reflection at the superior thoracic aperture or inlet correspond to the sterno-clavicular joints. From those points the lines, as they are traced downwards, converge behind the manubrium, until at last they meet at its inferior border. There the two sacs come into contact with each other, and the lines of reflection coincide. Thence they proceed downwards, on the back of the body of the sternum, with a slight deviation to the left of the median plane, until a point immediately above the level of the sternal attachments of the fourth costal cartilages The line of reflection of the right is reached, and there the two sacs part company. pleura is continued downwards in a straight line to the xiphoid process, where the

FIG.

865.-LEFT PLEURAL SAC IN A SUBJECT HARDENED BY FORMALIN INJECTION, opened into by the removal of the costal part of the parietal pleura. The lung has also been removed so as to display the mediastinal pleura.

sternal reflection-line passes into the right diaphragmatic reflection-line. Opposite the sternal attachment of the fourth costal cartilage the reflection-line of the left pleura deviates laterally, and is continued downwards at a variable distance from the right pleura. A small triangular area of pericardium is thus left uncovered by pleura, and therefore in direct contact with the ventral chest-wall. Leaving the sternum, the reflection-line of the left pleura passes downwards, parallel and close to the left margin of the sternum, dorsal to the fourth intercostal space, the fifth costal cartilage and the fifth intercostal space, to the sixth costal cartilage. There it turns laterally and downwards, and passes into the diaphragmatic flection-line of the left side.

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From the back of the sternum the right pleura is reflected, in the superior part of the chest, on to the remains of the thymus, the right innominate vein and the vena cava superior, and, at a lower level, directly on to the ventral aspect of the pericardium. The left pleura is reflected from the back of the manubrium sterni on to the left innominate vein and the aortic arch, and, at a lower level, directly on to the ventral side of the pericardium.

The diaphragmatic line of reflection is that along which the pleura leaves the thoracic wall and is reflected on to the thoracic surface of the diaphragm. This reflection takes place along a curved line, which, except as it approaches the ver tebral column, is placed a short dis tance above the in

SPLEE

LEFT KIDNEY

RIGHT KIDNEY

FIG. 866.-DISSECTION OF THE PLEURAL SACS FROM BEHIND.
The blue lines indicate the outlines and the fissures of the lungs.

ferior border of the thoracic wall. It differs somewhat on the two sides of the body.

On the left side the diaphragmatic line of reflection proceeds downwards along the ascending part of the sixth costal cartilage, crosses the ventral end of the sixth intercostal space and the descending part of the cartilage of the seventh rib (Fig. 865). Still continuing to pass downwards, it crosses the eighth costal arch at the junction between its cartilaginous and bony portions. This is a fairly constant relation on both sides of the body, and it should be noted that a vertical line-the mamillary line,-drawn downwards from the nipple of the breast, intersects the line of pleural reflection, close to the point where it presents this relation to the eighth costal arch. Beyond that point

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