The pleurae form closed sacs which line the thorax (parietal pleura) and cover the surface of the lungs (visceral pleura). As the lungs expand and contract, the pleurae are only completely in contact with the lungs when the latter are fully distended. In ordinary breathing the lungs are not completely expanded, hence the edges of the pleurae fall together and so prevent the formation of a cavity. This collapsing of the pleurae takes place mainly along its anterior and lower edges. The apex of the pleura is prevented from collapsing by its attachment to the first rib, and also, as pointed out by Sibson, by the attachment to it of an expansion of the deep cervical fascia and some fibres of the scalenus anticus muscle. Posteriorly the chest wall is unyielding. Anteriorly when the lungs are collapsed they fill out the pleurae as low down as the fourth costal cartilage; below that, in front of the heart, a space or sinus is left unoccupied by lung. It is called the costomediastinal sinus. Likewise between the diaphragm and chest-walls is another space, in which the parietal or costal and visceral layers of the pleura are in contact, called the costo-phrenic sinus or complemental space of Gerhardt. From these facts it follows that the outlines of the pleurae and lungs are identical posteriorly, superiorly, and anteriorly, as low as the fourth costal cartilage. Here they diverge, the pleurae descending lower than the lungs.

Fig. 213.   The posterior regions of the chest.

Fig. 213. - The posterior regions of the chest.

The top of the pleura is about on a plane with the upper surface of the first rib. This makes its posterior portion at the head of the first rib 5 cm. higher than its anterior portion at the anterior end of the first rib. The upper border of the clavicle is level with a point midway between the anterior and posterior ends of the first rib. This, therefore, shows the pleura to extend 2.5 cm. (1 in.) above the level of the upper surface of the clavicle.

The top of the pleura does not project into the neck in the form of a cone, but resembles a drum-head, being stretched in the form of a plane almost or quite level with the top of the first rib. Its upper surface is strengthened by fibres from the deep fascias of the neck and, according to Sibson, by some fibres from the scalene muscle.

The pleura then slopes forward behind the sternoclavicular joint to meet the pleura of the opposite side at the level of the second costal cartilage, a little to the left of the median line. They then descend until opposite or a little below the fourth costal cartilage, when they each diverge toward the side, reaching the upper border of the seventh costal cartilage near its sternal junction. They then slope down and out, reaching the lower border of the seventh rib in the mammary line, the ninth rib in the axillary line, and the twelfth rib posteriorly (Joessel and Waldeyer, page 51). The scapular line intersects the lower edge of the pleura at about the eleventh rib.

Fig. 214.   Anterior surface relations of the lungs and pleurae.

Fig. 214. - Anterior surface relations of the lungs and pleurae.

In operations involving the lumbar region, if the incision is carried high up posteriorly the pleura may be opened along the lower border of the posterior portion of the twelfth rib. It soon recedes from the costal margin and in the axillary line is about 6 cm. (2 2/5 in.) above it.

A heavy body, as a bullet, gravitates to the lowest portion of the pleural cavity, hence it can be removed through an incision in the eleventh interspace posteriorly.

(Paracentesis and empyema will be alluded to after the lungs have been described, see p. 200).