18. But, whatever may be _carried off_ from the blood, during its passage through the lungs, something is certainly _added_ to it, for the blood in the pulmonary artery is of a dark red, but when it has undergone the action of the air in the lungs, and returns by the pulmonary vein, it is then of a bright scarlet, which colour it retains through all the arteries of the body, but loses it on its return through the veins. This scarlet colour is communicated to blood in all cases when exposed to the air; and Dr. Priestley has observed that it is acted upon by the air even through a bladder; much more then must it be so through those very thin membranes which form the coats of the fine pulmonary vessels. What this subtile matter is which the blood receives, shall be afterwards inquired into; at present it is sufficient to take notice that it is absolutely necessary, for the purposes of life, that the blood should pass through the lungs: for, as Dr. Huxham observes, “we see neither nutrition, nor the motion of the muscles, performed by any blood that hath not passed through the lungs; this is observable from the coronary arteries[49] to the ultimate ramifications of the aorta.” As the previous circulation of the blood through the lungs therefore is absolutely necessary to the growth and life of the body, and as the blood certainly receives _something_ from the air, we must account this a proof, and no inconsiderable one, that the air contains a _vital spirit_, which it imparts to the blood in the lungs. But, before we proceed farther on this subject, it is proper to take some notice of [49] The name of the vessels by which the heart itself is supplied with blood. These come from the aorta by the circuitous way of the lungs.