356 MANUAL OF PHYSIOLOGY. 



to be capable of sustaining life without producing any marked 

 change in respiration. 



Dyspnoea is produced by an atmosphere containing only 7.5 per 

 cent, of O. This dyspnoea rapidly increases as the percentage of 

 O is further decreased, and when it gets as low as 3 per cent, 

 suffocation speedily ensues. 



The output of CO 2 can be accomplished if the lungs be venti- 

 lated by any harmless or indifferent gas, and since the manufacture 

 of the CO 2 does not take place in the lungs, its elimination can 

 go on independently of the presence of O in them. The 79 per 

 cent, of N contained in the atmosphere has a passive duty to per- 

 form in diluting the O and facilitating the escape of the CO 2 from 

 the lungs. 



Indifferent gases are those which produce no unpleasant effect 

 of themselves, but which, in the absence of O, are incapable of sus- 

 taining life, such as nitrogen, hydrogen, and CH 4 . 



Irrespirable gases are such as, owing to the irritating effect on the 

 air passages, cannot be respired in quantity, as they cause instant 

 closure of the glottis. In small quantities they irritate and pro- 

 duce cough, and if persisted in, inflammation of the air passages; 

 among these are chlorine, ammonia, ozone, nitrous, sulphurous, 

 hydrochloric, and hydrofluoric acids. 



Poisonous gases are those which can be breathed without much 

 inconvenience, but when brought into union with the blood cause 

 death. Of these there are many varieties. (1.) Those which per- 

 manently usurp the place of oxygen with the haemoglobin, viz. : 

 carbon monoxide (CO), hydrocyanic acid (HCN). (2.) Narcotic : 

 (a) carbonic dioxide (CO 2 ), of which 10 per cent, is rapidly fatal, 

 1.0 per cent, poisonous, and over 0.1 per cent, injurious. (/5) nitro- 

 gen monoxide (N 2 O). Both of these gases lead to a peculiar 

 asphyxia without convulsion, (y) chloroform, ether, etc. 



(3.) Sulphuretted hydrogen (H 2 S), which reduces the oxyhsemo- 

 globin, and produces sulphur and water. 



(4.) Phosphoretted hydrogen (PH 3 ), arseniuretted hydrogen 

 (AsH 3 ), and cyanogen gas (C 2 N 2 ) also have specially poisonous 

 effects. 



