372. The blood which has finished its circulation through the system, returned by the great systemic veins (fig. CXL. 12, 13), to the right side of the heart (fig. CXL. 3), is driven by the right ventricle (fig. CXL. 5), into the pulmonary artery (fig. CXL. 7); by the branches of which (fig. CXL. 8, 9) it is distributed to the air vesicles of the lungs: consequently the right heart of man bears precisely the same relation to the lungs, that the single heart of the fish bears to the branchiæ; the former is a pulmonic, as the latter is a branchial heart; one half of the double heart of the more highly organized creature is employed to circulate the venous blood of the system through the lungs, as the whole of the single heart of the less highly organized animal, is employed to propel the blood through the branchiæ (368). From the capillary branches of the pulmonary artery in the Rete Mirabile (fig. CXL. 9), arises another set of vessels termed the pulmonary veins (fig. CXL. 10), which receive the blood from the venous vessels spread out on the air vesicles: for the pulmonary artery is functionally a vein, since it contains venous blood, though it is nominally an artery because it carries blood from the heart (269); and in like manner the pulmonary veins are functionally arteries since they contain arterial blood, though they are nominally veins because they carry blood to the heart (272). The branches of the pulmonary arteries are larger in size and greater in number than those of the pulmonary veins, the reverse of what is observed in any other part of the body; because the pulmonary artery contains the blood which is to be acted upon by the air, while the pulmonary veins merely receive the blood which has been acted upon by the air, and the former ramifies more minutely than the latter, in order that the air may act on a larger surface of blood. 373. In the Rete Mirabile the junction of the air-vessel with the blood-vessel is accomplished. The combination of these two sets of vessels constitutes the lung; for the lung is composed of air-vessels and blood-vessels united, and sustained by cellular tissue, and inclosed in the thin but firm membrane called the pleura (104 and 105). 374. Such is the arrangement of that part of the respiratory apparatus which contains the fluids that are to act on each other. The object of the remaining portion of it is to produce the movements which are necessary to bring the fluids into contact. This is accomplished by the mechanism and action of the thorax and diaphragm (figs. CXLI. and CXXXIV. 10).