s difference is merely histological. In man and the
mammalia the costal arches hold relation with the pulmonary organs, and
these costae fail at that region where the ventral organs are located.
In birds, and many reptiles, the costal arches enclose the common
thoracico-abdominal region, as if it were a common pulmonary region. In
fishes the costal arches enclose the thoracico-abdominal region, just as
if it were a common abdominal region. I merely mention these general
facts to show that costal enclosure does not actually serve to isolate
the thorax from the abdomen in the lower classes of animals; and on
turning to the human form, I find that this line of separation between
the two compartments is so very indefinite, that, as pathologists, we
are very liable to err in our diagnosis between the diseased and the
healthy organs of either region, as they lie in relation with the
moveable diaphragm or septum in the living body. The contents of the
whole trunk of the body from the top of the sternum to the perineum are
influenced by the respiratory motions; and it is most true that the
diaphragmatic line, H F H*, is alternately occupied by those organs
situated immediately above and below it during the performance of these
motions, even in health.

The organs of the thoracic region hold a certain relation to each other
and to the thoracic walls. The organs of the abdomen hold likewise a
certain relation to each other and to the abdominal parietes. The organs
of both the thorax and the abdomen have a certain relation to each
other, as they lie above and below the diaphragm. In dead nature these
relations are fixed and readily ascertainable, but in living, moving
nature, the organs influence this relative position, not only of each
other, but also of that which they bear to the cavities in which they
are contained. This change of place among the organs occurs in the
normal or healthy state of the living body, and, doubtless, raises some
difficulty in the way of our ascertaining, with mathematical precision,
the actual state of the parts which we question, by the physical signs
of percussion and auscultation. In disease this change of place among
these organs is increased, and the difficulty of making a correct
diagnosis is increased also in the same ratio. For when an emphysematous
lung shall fully occupy the right thoracic side from B to L, then G, the
liver, will protrude considerably into the abdomen beneath the right
aste