Nitrous oxide gas, administered from an inhaler, rapidly enters the circulation; is absorbed by the plasma; converts the arterial into venous blood, in the course of about sixty seconds; and thus produces partial asphyxia. It does so apparently by diminishing the amount of oxygen in combination with the red corpuscles, without itself uniting with the haemoglobin, like CO and NO; in this respect it is an "indifferent" gas, like N and H, simply taking the place of the Oxygen, if this be completely excluded at the same time, and exerting of itself no poisonous action upon the corpuscles. It must, therefore, be given pure, i.e. without any admixture of air. The effect of the incipient asphyxia, and the use to which it may be turned, will be described in the next section.

2. Specific Action And Uses

Nitrous oxide gas not only renders the blood venous, but simultaneously enters the nervous centres, upon which it acts first as a stimulant, and speedily as an anaesthetic. Thus the gas produces a series of phenomena which can he resolved into the parallel effects of venosity of the blood or asphyxia, and a specific influence on the nerve cells of the convolutions. After a few seconds' excitement, the subject for anaesthesia by nitrious oxide begins to breathe laboriously; the mind becomes rapidly obsured; and by the end of about sixty seconds consciousness is lost, the face may be livid and bloated in appearance, respiration becomes stertorous, mnscular twitchings occur, the pulse fails at the wrist, and the whole appearance is alarming to a novice. If the inhalation be now interrupted, perfect recovery of consciousness and of natural breathing occurs in thirty to sixty seconds, with disappearance of all the urgent symptoms. It is clear that asphyxia is carried into the second stage - that of respiratory excitement, hut not beyond, neither the movements of the chest nor the action of the heart being arrested. But even if these untoward results should occur, resuscitation is easy by means of artificial respiration; it is said even after five minutes in the case of rabbits. No attempt to carry the asphyxia beyond the second stage is permissible in man.

Nitrous oxide gas is extensively used to produce anaesthesia during operations lasting but one minute or less, and especially by dental surgeons during the extraction of teeth, destruction of the nerve, etc. It must always be given pure, by the arrangement above described in the hands of a skilled anaesthetist. The moment for operating is best indicated by stertorous breathing and twitching of the muscles. Persons with diseased vessels, such as the subjects of chronic Bright's disease, ought not to take this anaesthetic, which produces (like all asphyxiating agents) a great and sudden rise of the arterial pressure, likely to cause rupture within the brain.