This consists primarily of two elastic bags, the lungs, placed in the thorax, filled with air, and communicating by the air-passages with the surrounding atmosphere. In the lungs the pulmonary capillary blood-vessels form a very close network: through their walls the blood gives off to the air in the lungs carbon dioxide, and takes from this air oxygen. The air in the lungs consequently needs renewal from time to time: otherwise it would no longer have oxygen to give to the blood, and would become so loaded with carbon dioxide as to no longer take that waste product from it. This renewal is effected by the working of a system of muscles, bones, and cartilages whose co-operation brings about that alternating expansion and contraction of the chest which we call breathing. When the chest contracts, air deprived of its oxygen and polluted with wastes is expelled from the lungs; and when it expands, fresh air, rich in oxygen, and containing hardly any carbon dioxide, is taken into them.

How is the blood renewed as regards ordinary food matters ? What must occur that energy be set free for use by the body ? What is necessary that the oxidations may continue ? What do the oxidations produce? Which is the most abundant waste substance of the body?

What are the objects of respiration ?

Of what does the respiratory apparatus primarily consist ?

The respiratory organs are, therefore, (1) the lungs; (2) the air-passages; (3) the vessels of the pulmonary circulation, including the pulmonary artery bringing the blood to the lungs, the pulmonary capillaries carrying it through them, and the pulmonary veins conveying it from them; (4) the muscles, bones, and gristles which are concerned in producing the breathing movements.*