360 MANUAL OF PHYSIOLOGY. 



Dyspnoea is produced by an atmosphere containing only 7.5 

 per cent, of O. This dyspnoea rapidly increases as the percent- 

 age 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 manufac- 

 ture 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 

 perform 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 

 sustaining 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 in- 

 stant closure of the glottis. In small quantities they irritate and 

 produce cough, and if persisted in, inflammation of the air pas- 

 sages ; among these are chlorine, ammonia, ozone, nitrous, sul- 

 phurous, 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) nitrogen monoxide (N 2 O). Both of these gases lead to a 

 peculiar asphyxia without convulsion, (j) chloroform, ether, etc. 



(3.) Sulphureted hydrogen (H 2 S), which reduces the oxy- 

 hsemoglobin, and produces sulphur and water. 



(4.) Phosphoreted hydrogen (PH 3 ), arseniureted hydrogen 

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

 effects. 



VENTILATION. 



In the open air the effects of respiration on the atmosphere 

 cannot be appreciated, but in enclosed spaces, such as houses, 



