Breathing, in normal conditions, is regulated by the concentration of carbon dioxide in the blood; the “respiratory centre” of the brain, which directs the movements of the lungs, controls matters with a highly delicate degree of regulation so as to maintain that concentration at a fixed level. When, through hard exertion or through the influence of carbon monoxide poisoning, the organism becomes seriously affected by want of oxygen, that deficiency itself begins to influence the respiratory centre and to stimulate breathing. During the course of poisoning by carbon monoxide, then, the lung-ventilation increases; the increase accelerates the rate of saturation of the blood by the gas, which in turn reacts in intensifying the oxygen-shortage. A consequence of the hyperpnœa which thus progressively develops is the washing out of much of the carbon dioxide in the blood.