lungs or the heart. When any organ, by its process of growth, or by
its own functional act, forces a space for itself, it immediately
inhabits that space entirely at the expense of neighbouring organs. When
the heart dilates, the pulmonary space contracts; and when the thoracic
space increases, general space diminishes in the same ratio.

The mechanism of the functions of respiration and circulation consists,
during the life of the animal, in a constant oscillatory nisus to
produce a vacuum which it never establishes. These vital forces of the
respiratory and circulatory organs, so characteristic of the higher
classes of animals, are opposed to the general forces of surrounding
nature. The former vainly strive to make exception to the irrevocable
law, that "nature abhors a vacuum." This act of opposition between both
forces constitutes the respiratory act, and thus the respiratory
thoracic being (like a vibrating pendulum) manifests respiratory motion,
not as an effort of volition originating solely with itself, but
according to the measure of the force of either law; as entity is
relationary, so is functionality likewise. The being is functional by
relationship; and just as a pendulum is functional, by reason of the
counteraction of two opposing forces,--viz., the force of motion and the
force of gravity,--so is a thoracic cavity (considering it as a
mechanical apparatus) functional by two opposing forces--the vital force
and the surrounding physical force. The inspiration of thoracic space is
the expiration of general space, and reciprocally.

The thoracic space is a symmetrical enclosure originally, which
aftercoming necessities modify and distort in some degree. The spaces
occupied by the opposite lungs in the adult body do not exactly
correspond as to capacity, O O, Plate 1. Neither is the cardiac space, A
E G D, Plate 1, which is traversed by the common median line,
symmetrical. The asymmetry of the lungs is mainly owing to the form and
position of the heart; for this organ inclines towards the left thoracic
side. The left lung is less in capacity than the right, by so much space
as the heart occupies in the left pulmonary side. The general form of
the thorax is that of a cone, I I N N, Plate 1, bicleft through its
perpendicular axis, H M. The line of bicleavage is exactly median, and
passes through the centre of the sternum in front, and the centres of
the dorsal vertebral behind. Between the dorsal vertebral and