THE SEROUS MEMBRANES. 
667 
rax above and below from the spine to the sternum, and before 
and behind from the first rib to the diaphragm ; but when I at¬ 
tempt to pass my hand beyond the spine or the sternum to the 
other side of the chest, I meet with an insuperable obstacle : the 
pleura is leaving the spine above and is reflected or turned down¬ 
wards, and it is also leaving the sternum below, and is reflected 
upwards>it is leaving the diaphragm, and I can trace it forwards; 
and it is quitting the first rib, and I can trace it backward. I 
follow it, and I most conveniently follow it from below upwards. 
I can trace it all along the sternum, reflected upwards, and I can 
trace a portion of it both anteriorly and posteriorly as high as the 
spine, but centrally, and introducing my hand between the lungs, 
and directing it towards the spine, my progress is opposed. There 
is a voluminous mass, round which, however, I can plainly feel 
the pleura turning. On examination, I find this to be the origin 
of the bronchial tubes, or the bifurcation of the trachea on either 
side, together with the vessels proceeding from the heart to the 
lungs, and from the lungs to the heart. These, to a certain ex¬ 
tent, are covered by this reflection of the pleura, and closely em¬ 
braced by it; and then, examining more closely both the outer and 
inner surface of the lung, I find that the pleura dilates, and forms 
a kind of bag, and spreads over and contains the whole of that 
lung. 
Two Pleurae and the Lungs between them .—On the other side 
I find precisely the same structure, except that the reflection 
from the sternum always takes place a little on the left of theme- 
sian line. Then I have two distinct pleurae, each lining the ribs 
and a part of the diaphragm, and constituting the wall of partition 
along the thorax, and including the lung in a fold or prolongation 
of itself; and I have two distinct cavities, the right being rather 
the larger, because that lung is of somew hat greater size than the 
left. 1 have also this singular fact, that the lungs are, in one 
sense, as much out of the pleuritic cavity as are the bony parietes 
of the thorax, and are covered, like the walls of the thorax, 
by the outer surface of the membrane. The sacs are empty, 
and the lungs are curiously enveloped in folds or dilatations 
of one of the sides, and in contact with the outside of the 
bag and not the inside. There result, however, from this singular 
arrangement some important circumstances which could not other¬ 
wise have been accomplished : the polished and glistening sur¬ 
faces of the pleurse are opposed to each other, and the utmost 
f reedom of motion is thus permitted ; a duplicature of membrane 
is interposed between the lungs and their bony wall, and a closed 
cavity is formed in which there may always exist a moisture or 
dew, to obviate injurious friction, to prevent concussion by its in- 
