194 



APPLIED ANATOMY. 



THE PLEUR/E. 



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 dis- 

 tended. 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, 



' 



FIG. 213. The posterior regions of the chest. 



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. 



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. (i in.) 

 above the level of the upper surface of the clavicle. 



