Z ; THE PLEURAL MEMBRANES. 927 
chamber from the other. The mediastinum is not median in position. Owing to 
the marked projection of the heart to the left side, and to the position of the 
descending thoracic aorta on the left side of the mesial plane, the left pleural 
chamber, although it is deeper than the right, is greatly reduced in width. The 
two pleural cavities, the refore, are very far from bei ‘ing symmetrical in form. 
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 a space termed the mediastinal 
or interpleural space, within which the parts which build up the mediastinum are 
placed. 
THE TWO PLEURAL MEMBRANES. 
The pleura or pleural membrane of each side not only lines the corresponding 
pleural cavity, but at the pulmonary root it 1s prolonged on to the lung so as to 
give it a complete investment. It is customary, therefore, to recognise a visceral 
or investing part (pleura visceralis) and a parietal or lining part (pleura parietalis). 
The inner surface of the membrane (7.e. that surface which is turned towards the 
interior of the cavity) is coated with squamous epithelium, and presents a smooth, 
ghstening, 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 visceral 
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 surface of he membrane becomes roughened by inflammatory exudation, and 
the so-called “ friction sounds” become 
evident when the ear is applied to the Visceral 
chest. De Ne 
Visceral Pleura.— The visceral 
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 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 dif- 
ferent lobes of the lungs from each other. 
The visceral pleura becomes continuous 
with the mediastinal part of the parietal 
pleura over the root of the lung, and 
also through the ligamentum latum 
pulnonis. 
Parietal Pleura.—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 is the costal pleura, the diaphragmatic 
pleura, the mediastinal pleura, and the cervical pleura; but it must be borne in 
mind that these terms are merely used for convenience in description, and the 
portions of the membrane so designated are all directly continuous with each 
other. 
The cervical pleura rises up into the root of the neck, through the superior 
aperture of the thorax, and forms a dome-shaped roof for the pleural cavity. Its 
summit or highest point reaches the level of the lower 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 the anterior extremity of the first rib, and 
from a half to one and a half inches above the clavicle. The cervical dome of 
pleura is supported on the outer side by the scalenus anticus and scalenus medius 
muscles, whilst the subclavian artery arches over it, and lies in a groove on its inner 
and anterior aspect a short distance below its summit. At a lower level the 
innominate and subclavian veins also lie upon its inner and anterior aspects. 
Parietal 
~ / pleura 
PERICARDIUM 
ROOT OF ROOT OF 
LUNG LUNG 
Fic. 627.—DIAGRAM SHOWING ARRANGEMENT OF 
PLEURAL SACs, as seen in transverse section. 
