uld enter the thorax again at P, and make exit below N,
opposite. If a cutting instrument were passed horizontally from before
backward, a little below M, it would first open the abdomen, then pierce
the arching diaphragm, and pass into the thorax, opposite the ninth or
eighth dorsal vertebra.

The outward form or superficies masks in some degree the form of the
interior. The width of the thorax above does not exceed the diameter
between the points I I, of Plate 1, or the points W W, of Plate 2. If we
make percussion directly from before backwards at any place external to
I, Plate 1, we do not render the lung vibrative. The diameters between I
I and N N, Plate 1, are not equal; and these measures will indicate the
form of the thorax in the living body, between the shoulders above and
the loins below.

The position of the heart in the thorax varies somewhat with several
bodies. The size of the heart, even in a state of perfect health, varies
also in subjects of corresponding ages, a condition which is often
mistaken for pathological. For the most part, its form occupies a space
ranging from two or three lines right of the right side of the sternum
to the middle of the shafts of the fifth and sixth ribs of the left
side. In general, the length of the osseous sternum gives the exact
perpendicular range of the heart, together with its great vessels.

The aorta, C, Plates 1 and 2, is behind the upper half of the sternum,
from which it is separated by the pericardium, D, Plate 1, the thin edge
of the lung, and the mediastinal pleurae, U E, Plate 1, &c. If the heart
be injected from the abdominal aorta, the aortal arch will flatten
against the sternum. Pulmonary space would not be opened by a
penetrating instrument passed into the root of the neck in the median
line above the sternum, at L, Plate 1. But the apices of both lungs
would be wounded if the same instrument entered deeply on either side of
this median line at K K. An instrument which would pierce the sternum
opposite the insertion of the second, third, or fourth costal cartilage,
from H downwards, would transfix some part of the arch of the aorta, C,
Plate 1. The same instrument, if pushed horizontally backward through
the second, third, or fourth interspaces of the costal cartilages close
to the sternum, would wound, on the right of the sternal line, the vena
cava superior, G, Plate 1; on the left, the pulmonary artery, B, and
the descending thoracic aorta. In the