136 CHANGES OF AIR AND BLOOD IN RESPIRATION. 



surface and the reduced atmospheric pressure diminishes the capacity of the 

 blood for retaining gases. 



Magendie and Bernard, in experimenting on the minimum proportion of 

 oxygen in the air which is capable of sustaining life, found that a rabbit, 

 confined under a bell-glass, with an arrangement for removing the carbon 

 dioxide and water exhaled, as fast as they were produced, died of asphyxia 

 when the quantity of oxygen became reduced to between three and five per 

 cent. 



A few experiments are on record in which the human subject and the 

 lower animals have been made to respire for a time pure oxygen. Allen and 

 Pepys confined animals for twenty-four hours in an atmosphere of pure oxy- 

 gen without any notable results ; but these experiments do not show that it 

 would be possible to respire unmixed oxygen indefinitely without incon- 

 venience. As it exists in the air, oxygen is undoubtedly in the best condi- 

 tion for the permanent maintenance of the respiratory function. The blood 

 seems to have a certain capacity for the absorption of oxygen, which is not 

 materially increased when the pure gas is respired. 



The only other gas which has the power of maintaining respiration, even 

 for a time, is nitrogen monoxide. This is appropriated by the blood-cor- 

 puscles with great avidity, and for a time it produces an exaggeration of the 

 vital processes, with delirium etc., which has given it the common name of 

 the laughing gas ; but this condition is followed by anaesthesia, and finally 

 by asphyxia, probably because the gas has so strong an affinity for the blood- 

 corpuscles as to remain to a certain extent fixed, interfering with the inter- 

 change of gases which is essential to life. Notwithstanding this, experiment- 

 ers have confined with impunity rabbits and other animals in an atmosphere 

 of nitrogen monoxide for a number of hours. In all cases they became 

 asphyxiated, but in some instances they were restored on being brought again 

 into the ordinary atmosphere. 



Other gases which may be introduced into the lungs either produce as- 

 phyxia, negatively, from the fact that they are incapable of carrying on respi- 

 ration, like hydrogen or nitrogen, or positively, by a poisonous effect on the 

 system. The most important of the gases which act as poisons are carbon 

 monoxide, hydrogen monosulphide and arsenious hydride. Carbon mo- 

 noxide unites with the coloring matter of the red corpuscles, forming carbon- 

 monoxide-haemaglobine. This union is so stable, that it paralyzes the cor- 

 puscles as oxygen-carriers and produces death by asphyxia. It is probable 

 that carbon dioxide is not in itself poisonous. Regnault and Reiset exposed 

 animals (dogs and rabbits) for many hours, to an atmosphere containing 

 twenty-three per cent, of carbon dioxide artificially introduced, with between 

 thirty and forty per cent, of oxygen, without any ill effects. 



Consumption of Oxygen. The determination of the quantity of oxygen 

 which is removed from the air by the process of respiration is important ; and 

 on this point, there is an accumulated mass of observations which are com- 

 paratively unimportant from the fact that they were made before the means 

 of analysis of the gases were as accurate as they now are. In the observations 



