The Thoracic Trachea lies in the posterior portion of the superior mediastinum, separated from the vertebrae by the oesophagus. In front it is related to the aortic arch at the level of the fourth dorsal vertebra, the great vessels, left innominate vein, and remains of the thymus. Immediately above the bifurcation the deep cardiac nerve plexus lies in front and laterally. On the right side it is in relation to the vagus and pleura, and on the left to the subclavian artery and recurrent laryngeal nerve. The bifurcation of the trachea lies opposite the interval between the third and fourth dorsal spines, about the level of the sternal angle in front. The two bronchi proceed down and outwards towards the hilus of the corresponding lung, the left bronchus generally being twice as long as the right, which is the wider. The vena azygos major arches over the right bronchus, and the aorta over the left, while the interval between the two is occupied by a group of bronchial lymphatic glands. On the left side the pulmonary artery crosses above all the collateral branches of the bronchus, while on the right side the first collateral bronchus lies above the artery, and the others below it.

Foreign bodies, small enough to pass the glottis, sometimes drop into the trachea, lodging probably about the bifurcation, and giving rise to dyspnoea and cough. When small enough to enter the bronchus, it is generally the right one which is affected, owing to its larger size, and the position of the septum, which lies rather to the left of the middle line.

The pleurae are closed sacs which line the chest-walls (parietal) and the surfaces of the lungs (visceral). They consist of elastic and connective tissue, lined with flat endothelium, the parietal and visceral layers being in contact, save for the interposition of a little clear serous fluid at all points, except at the incisura of the left lung, and at the lower and anterior portion on both sides (pleural sinuses).

While the visceral pleura is intimately adherent to the lung, the parietal pleura is generally comparatively free. The cervical portion of the parietal pleura is connected to the spine and first rib by fibrous bands which support it: the costal portion, which is the strongest, is separated from the chest-wall by a layer of connective tissue, the endothoracic fascia ; the diaphragmatic portion is fairly adherent, but does not extend to the Tjottom of the groove between the thoracic wall and diaphragm. The mediastinal portion is loose above the pericardium, and extends direct from sternum to spine. Over the pericardium, however, it is adherent, and it is invaginated over the root of the lung to join the visceral layer. This dipping in of the parietal pleura to meet the visceral continues from the root of the lung to the base, and the two layers, which thus come in contact, form a vertical fold, the ligamentum latum pulmonis. The lines of reflection of the pleura from the chest-wall are of considerable importance. Anteriorly, on the right side, the line begins opposite the interval between the two heads of the sterno-mastoid, runs down and inwards behind the sterno-clavicular articulation, to meet its fellow at the manubrio-gladiolar junction, generally a little to the left of the middle line, whence it descends almost vertically to the sixth intercostal space, and then curves outwards, becoming continuous with the costo-diaphragmatic reflection. On the left side it behaves similarly, save that at the level of the fourth costal cartilage it deviates outwards behind the inner extremities of the fifth and sixth intercostal spaces, which thus are in contact for a short distance with the pericardium. Below, starting from the sternal end of the sixth intercostal space, the reflection on the right side runs down and outwards, crosses the junction of the seventh rib and its cartilage, crosses the seventh space in the mammary line, and so runs down to the tenth rib behind the midaxillary line. Then it runs inwards to the vertebral end of the twelfth rib. On the left side this reflection commences behind the sixth costal cartilage, but otherwise resembles the right. Generally speaking, a line from the lower end of the gladiolus over the cartilage of the ribs to the lower border of the last rib indicates this reflection with sufficient accuracy. Posteriorly, the pleurae extend to the costo-vertebral junctions. The lowest part of the pleural sac is posterior, behind the twelfth rib, but operations on the pleura at this part are rather dangerous, as, owing to the rapid arching of the diaphragm, a trocar introduced would almost certainly penetrate both layers of pleura and diaphragm, and enter the abdomen-probably the stomach, if done on the left side. It is well to remember, however, that the lower level of the pleura reaches the twelfth rib, even when that rib is rudimentary, and occasionally it descends to the transverse process of the first lumbar vertebra, thus rendering it liable to wounding in operations on the kidney in the lumbar region. The pleura extends further down in the child than in the adult, and slightly further on the left side than on the right. Normally, except at the pleural sinuses, the pleural surfaces are in contact with one another, save for the interposition of a thin layer of serous fluid. As a result of the smooth, polished surfaces which the pleurae present to one another, and the intervening thin layer of fluid, the force of cohesion between the two layers is very great. This force is not merely sufficient to counterbalance the contractile elasticity of the lung substance, but tends to extrude any accumulation of fluid which may exist between the two surfaces. Thus, it is quite safe to remove two or three ribs, and expose the visceral pleura; not only will the lung not collapse, but if there be a recent accumulation of fluid between the layers this will be expelled, the lung being pulled out to its original size by the cohesive force acting on the pleurae. In some cases, due possibly to forced expiratory efforts, the lung has even protruded from an opening in the chest (hernia of lung).

Pneumothorax, or air in the pleural cavity, is generally produced by a valve-shaped wound, as in some cases of compound fracture of the ribs, the respiratory action forcing air in between the layers of pleura, and so separating them. In severe cases the pneumothorax may be so great as to threaten collapse of the lung, and produce urgent dyspnoea. In such cases the wound should be laid freely open, thus destroying the valve ; the chest-wall compressed so as to express the air, and restore the condition of cohesion between the layers of pleura, when the lung will expand to its normal size. Sometimes it is sufficient to introduce a trocar and cannula, leaving the latter in situ. It is noteworthy that in some cases of wound of the chest involving the pleura an actual bulging forwards, or hernia of the lung, has occurred, and elaborate theories have been advanced to account for such phenomena. Where, from any cause, cohesion is destroyed, collapse of the lung may occur readily. Thus, wounds of the pleura affecting the pleural sinuses are particularly prone to result in collapse, air at these points getting most readily between the two surfaces.

Emphysema of the subcutaneous tissues frequently results from Such valve-shaped wounds as mentioned, the air being pumped by the respiratory movements into the tissues. It may also arise from valved superficial wounds which do not communicate with the pleura or lung, the chest movement supplying the pumping action. Pleural effusions sometimes result in soldering of the two surfaces together. On the other hand, they may force the two surfaces apart, and produce grave changes in the serous membranes. This is well seen in empyema, or accumulation of pus in the pleural cavity, which sometimes is of great size, causing bulging of the chest-wall on the affected side, collapse, more or less complete, of the lung, and thickening of the pleurae and roughness of their surfaces. The thickening of the pleurae tends to prevent re-expansion of the lung, while the roughness of the surface destroys the force of cohesion, thus rendering it difficult to keep the lung expanded, even if it tends to do so. Empyemas, however, are frequently localized, the two layers of pleura becoming soldered round the periphery by fibrous exudation. A similar soldering frequently prevents the formation of a pneumothorax in cases of perforation of the visceral pleura from tubercular disease of the lung. An untreated empyema may burst through the chest-wall or into the lung or pericardium, or sometimes may extend into the abdomen through the internal arcuate ligament.

Empyema is generally treated by evacuation through the sixth or seventh intercostal space in front of the posterior axillary fold. Incisions made lower down may wound the diaphragm, or even penetrate the abdomen, owing to obliteration of the lower portion of the pleural cavity from disease; and even if this should not occur, the diaphragm frequently ascends after the empyema has been evacuated, rendering the communication very oblique. Where the empyema is localized, its site is generally determined by percussion, and confirmed by an exploring needle. The operation may consist of an incision along the intercostal space, keeping near the upper border of the lower rib, or a portion of rib may be excised. As already stated, however, changes frequently occur, after empyema has been present for some time, in the pleura, and disease may be present in the lung, so that even after evacuation of the pus the lung may not tend to expand.

In such cases healing is very slow, the chest not collapsing readily, and a large cavity remaining in the chest, from which there is a continual discharge. In these circumstances a thoracoplasty is performed-a large flap being raised from the chest-wall, several ribs removed subperiosteally, the abscess cavity cleared out thoroughly, and the two refreshed surfaces brought into contact by collapsing the chest-wall (Estlander).