The heart, great blood-vessels, and the lungs are placed in the air-tight cavity of the thorax, and are subjected to the pumping action of the respiratory movements. The inspiratory muscles elevate the ribs, at the same time that the diaphragm, by its contraction, pushes the contents of the abdomen downward. The cavity of the chest, so enlarged, causes the pressure around the heart and the great blood-vessels within the chest to be less than that on the blood-vessels outside the chest; hence, during each inspiration the venous blood is sucked back into the right side of the heart. The tissues deprive the blood of its oxygen, so that which flows back to the heart in the veins is blue. The right heart then sends this blue blood to the lungs, that it may get rid of its carbonic acid, which is not only not needed, but is actually injurious to the body, and to receive a fresh supply of oxygen, which has been carried into the lungs in breathing. The _blood_ is the great medium of exchange between all parts of the body. It is, at the same time, the nourisher and the scavenger of all the tissues. After the food has been liquefied and converted into new substances in the digestive system it is poured into the blood. From the blood all the tissues draw material to renew their own worn-out parts and other material which they store up as latent force, which, when it unites with the oxygen of the blood, becomes active force, such as heat and motion. [Illustration: Fig. 10.—The diaphragm (after Kitchen).] The blood holds in suspension a vast number of minute cells or corpuscles; the red corpuscles give its color to the blood, and are the oxygen carriers, while the white are the phagocytes or the protective agents of the body against disease. The blood constitutes about one-thirteenth of the body weight. Of this, one-fourth is distributed to the heart, lungs, and great blood-vessels, one-fourth to the liver, one-fourth to the skeletal muscles, and the remainder to other organs. In order that the blood may be a satisfactory medium of exchange between all the tissues of the body two things are necessary—first, there must be through all parts of the body a flow of blood of a certain rapidity and general constancy; and, second, this flow must be susceptible of general and local modifications. The =lungs= are the essential organs of respiration or ventilators of the body. They are two in number, separated from each other by the heart, are placed in a semi-distended state in the air-tight thorax, which we have seen they, together with the heart and great blood-vessels, completely fill. The lungs ultimately consist of air-cells, surrounded by dense plexuses of capillaries and nerves. The air-cells communicate with the exterior through the bronchial tubes, trachea, larynx, throat, and nose.