In the upper half of the engraving, H represents the heart; _a_, the commencement of the _aorta_; _v c s_, the termination of the _vena cava superior_. On each side of the heart, are the lungs; _l l_, being the left lobe, and _r l_, the right lobe. They are composed of a network of air-vessels, blood-vessels, and nerves. W, represents the _trachea_, or _windpipe_, through which, the air we breathe is conducted to the lungs. It branches out into myriads of minute vessels, which are thus filled with air every time we breathe. From the heart, run the _pulmonary arteries_, marked _p a_. These enter the lungs and spread out along-side of the branches of the air-vessels, so that every air-vessel has a small artery running side by side with it. When the two _vena cavas_ empty the blood into the heart, the latter contracts, and sends this blood, through these pulmonary arteries, into the lungs. As the air and blood meander, side by side, through the lungs, the superabundant carbon and hydrogen of the blood combine with the oxygen of the air, forming carbonic acid gas, and water, which are thrown out of the lungs at every expiration. This is the process by which the chyle is converted into arterial blood, and the venous blood purified of its excess of carbon and hydrogen. When the blood is thus prepared, in the lungs, for its duties, it is received by the small _pulmonary veins_, which gradually unite, and bring the blood back to the heart, through the large _pulmonary veins_, marked _p v_, _p v_. On receiving this purified blood from the lungs, the heart contracts, and sends it out again, through the _aorta_, to all parts of the body. It then makes another circuit through every part, ministering to the wants of all, and is afterwards again brought back by the veins to receive the fresh chyle from the stomach, and to be purified by the lungs. The throbbing of the heart is caused by its alternate expansion and contraction, as it receives and expels the blood. With one throb, the blood is sent from the right ventricle into the lungs, and from the left ventricle into the aorta.