The gas is markedly more poisonous when inhaled than when taken in any other way. It hinders exhalation of the carbonic acid normally existing in the blood, and is itself absorbed in small quantity, thus inducing dyspnoea; a proportion of 10 per cent in the air is irrespirable and fatal. The undiluted gas first excites irritation and sometimes spasmodic contraction of the glottis with consequent asphyxia (Wareing); in any case, and independently of such spasm, it soon arrests respiration. It has been thought that the gas is itself inert, and induces death only by preventing the due interchange of oxygen and carbonic acid in the lungs (Bichat, Regnault, etc.), but recent observations suggest that it is actively poisonous, since young mammals die by cardiac arrest after two or three minutes in an atmosphere charged with it, while they live fifteen to twenty minutes in nitrogen or hydrogen (Paul Bert,

Rabuteau), and the heart continues to beat in the latter case after respiration has ceased. The experiments of Collard de Martigny, Orfila, van Hasselt, and others, point to the same conclusion.

Circulatory System

The effect of respired carbonic acid in preventing oxygenation of blood is quickly shown by the appearance of more or less cyanosis, with slow, labored pulse, and ultimate arrest of heart-action. It does not, however, intimately combine with, and fix itself upon, the haemoglobin, since this remains red and unreduced in an atmosphere even highly charged with carbonic acid, provided that a normal amount of oxygen is present also; while in animals dying deprived of oxygen, the blood is found black, haemoglobin being completely reduced. The effects of the internal administration of the gas, or even its careful injection into the larger venous trunks, differ from those produced by its inhalation, and are such as slight stimulation of the heart-action, quickening of respiration, and increase of the peripheral circulation, with a slight prickling of skin and brief sense of exhilaration; this is often experienced from sparkling beer, wine, and even waters. Husemann remarks that experiments with "direct injection" of carbonic acid into the blood (Nysten, Demarquay) have not led to great results, on account of the smallness of the amount that can be injected without death occurring from the entrance of air into veins. Even small quantities thus injected cause muscular weakness, a symptom which only appears late in the inhalation of dilute carbonic acid gas.