the
sternum, the line of median cleavage is maintained and sketched out in
membrane. This membranous middle is formed by the adjacent sides of the
opposite pleural or enveloping bags in which the lungs are enclosed. The
heart, A, Plate 1, is developed between these two pleural sacs, F F, and
separates them from each other to a distance corresponding to its own
size. The adjacent sides of the two pleural sacs are central to the
thorax, and form that space which is called mediastinum; the heart is
located in this mediastinum, U E, Plate 1. The extent of the thoracic
region ranges perpendicularly from the root of the neck, Q, Plate 1, to
the roof of the abdomen--viz., the diaphragm, P, transversely from the
ribs of one side, I N, Plate 1, to those of the other, and
antero-posteriorly from the sternum, H M, to the vertebral column. All
this space is pulmonary, except the cardiac or median space, which, in
addition to the heart, A, Plate 1, and great bloodvessels, G C B,
contains the oesophagus, bronchi, &c. The ribs are the true enclosures
of thoracic space, and, generally, in mammalian forms, they fail or
degenerate at that region of the trunk which is not pulmonary or
respiratory. In human anatomy, a teleological reason is given for
this--namely, that of the ribs being mechanically subservient to the
function of respiration alone. But the transcendental anatomists
interpret this fact otherwise, and refer it to the operation of a higher
law of formation.

The capacity of the thorax is influenced by the capacity of the abdomen
and its contents. In order to admit of full inspiration and pulmonary
expansion, the abdominal viscera recede in the same ratio as the lungs
dilate. The diaphragm, P P, Plate 1, or transverse musculo-membranous
partition which divides the pulmonary and alimentary cavities, is, by
virtue of its situation, as mechanically subservient to the abdomen as
to the thorax. And under general notice, it will appear that even the
abdominal muscles are as directly related to the respiratory act as
those of the thorax. The connexion between functions is as intimate and
indissoluble as the connexion between organs in the same body. There can
be no more striking proof of the divinity of design than by such
revelations as anatomical science everywhere manifests in facts such as
this--viz., that each organ serves in most cases a double, and in many a
triple purpose, in the animal economy.

The apex of the lung proje