ribs. The respiratory murmur
can be heard below the level of these ribs posteriorly, for the lung
descends behind the arching diaphragm as far as the eleventh rib.

When fluid is effused into the pleural cavity, the ribs are not moved by
the intercostal muscles opposite the place occupied by the fluid, for
this has separated the lung from the ribs. The fluid has compressed the
lung; and in the same ratio as the lung is prevented from expanding, the
ribs become less moveable. The presence of fluid in the pleural sac is
discoverable by dulness on percussion, and, as might be expected, by the
absence of the respiratory murmur at that locality which the fluid
occupies. Fluid, when effused into the pleural sac, will of course
gravitate; and its position will vary according to the position of the
patient. The sitting or standing posture will therefore suit best for
the examination of the thorax in reference to the presence of fluid.

Though the lungs are closely applied to the costal sides at all times in
the healthy state of these organs, still they slide freely within the
thorax during the respiratory motions--forwards and backwards--over the
serous pericardium, E, and upwards and downwards along the pleura
costalis. The length of the adhesions which supervene upon pleuritis
gives evidence of the extent of these motions. When the lung becomes in
part solidified and impervious to the inspired air, the motions of the
thoracic parietes opposite to the part are impeded. Between a solidified
lung and one which happens to be compressed by effused fluid it requires
no small experience to distinguish a difference, either by percussion or
the use of the stethoscope. It is great experience alone that can
diagnose hydro-pericardium from hypertrophy of the substance of the
heart by either of these means.

The thoracic viscera gravitate according to the position of the body.
The heart in its pericardial envelope sways to either side of the
sternal median line according as the body lies on this or that side. The
two lungs must, therefore, be alternately affected as to their capacity
according as the heart occupies space on either side of the thorax. In
expiration, the heart, E, is more uncovered by the shelving edges of the
lungs than in inspiration. In pneumothorax of either of the pleural sacs
the air compresses the lung, pushes the heart from its normal position,
and the space which the air occupies in the pleura yields a clear hollo