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 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