Oxygen treatment rests on the fact that by raising the pressure of the oxygen from 113 mm., as it is generally in ordinary air, to 675 mm., which is reached in presence of pure oxygen, the quantity of oxygen absorbed in the blood rises from 0·3 to 1·8 per 100 c.c. Further, the saturation of the hæmoglobin, the colouring matter of the blood, undergoes an increase of 2·4 per cent. This increase of oxygen in the blood can save life in cases where through poisoning a deficiency of oxygen has resulted. The introduction of oxygen is done by special apparatus which acts essentially on the principle that during inhalation oxygen is pressed into the lungs which are below normal physiological pressure, while exhalation is effected by a deflating arrangement when the poisoned individual no longer breathes of his own accord. When natural breathing begins, the introduction of oxygen without special apparatus generally suffices. [Illustration: FIG. 30.—Dräger’s Oxygen Box I Oxygen cylinder; A Valve on cylinder; B Manometer; C Key for opening and closing the flow of oxygen; F Economiser; H Facepiece.] Dräger’s _oxygen apparatus_ (fig. 30) consists of a small oxygen cylinder provided with a closing valve, a small manometer, a so-called ‘automatic’ reducing valve with an arrangement for opening and closing the oxygen supply, a bag to act as a receiver or economiser, a breathing mask, and a metal tube connecting the breathing mask with the other parts of the apparatus. The oxygen cylinder, when filled, contains about 180 litres of oxygen, and the manometer allows the manipulator to control at any time whatever oxygen it still contains. The automatic arrangement not only reduces the pressure but at the same time controls the supply of oxygen. This dose is fixed at three litres of oxygen per minute, so that the apparatus with the same oxygen cylinder will last for sixty minutes. The oxygen is not inhaled pure, but is mixed with atmospheric air according to need, and in order to make this possible the breathing mask is provided with a small hole through which atmospheric air finds entrance. [Illustration: FIG. 31.—Oxygen Inhaling Apparatus] [Illustration: FIG. 32.—Showing apparatus in use (_Siebe, Gorman & Co._)] As the oxygen flows continuously from the cylinder waste during exhalation is prevented by the economiser, in which, during exhalation, the inflowing oxygen accumulates, to be absorbed again in inhalation. A small relief valve in the screw head of the bag prevents the entrance into it of exhaled air. [Illustration: FIG. 33.—Dräger’s Pulmotor (_R. Jacobson_)] Another oxygen inhaling apparatus for resuscitating purposes, that of Siebe, Gorman & Co., is illustrated in figs. 31 and 32. Dräger also constructs an apparatus called the ‘Pulmotor’ which simultaneously accomplishes the introduction of oxygen and artificial respiration.