282 GENERAL PHYSIOLOGY 



living substance. If, therefore, as has been seen, 1 all organisms 

 without exception respire as long as they live, i.e., if the processes 

 of oxidation are an integral link in the chain of metabolic pro- 

 cesses, it necessarily follows that the presence of oxygen belongs 

 to the general vital conditions of living substance. 



As is well known, the composition of the atmosphere, as regards 

 its essential constituents, is as follows : Nitrogen and argon, 79'02, 

 oxygen 20'95, carbonic acid 0*03 volumes. This composition is essen- 

 tially the same at all times and all places upon the earth's surface. If, 

 therefore, land organisms be considered and upon them have been 

 made the greater number of the investigations regarding the 

 dependence of living things upon oxygen it may be said that 

 they live continually in an atmosphere in which in round numbers 

 21 per cent, of oxygen is present. The striking investigations of 

 W. Mliller and Paul Bert have, however, shown that organisms are 

 not bound exclusively to this percentage and the pressure of one 

 atmosphere, but within certain limits are independent of the 

 partial pressure of oxygen. W. Mtiller ('58) found, for example, 

 that mammals can continue to exist with 14 per cent, of oxygen, 

 they begin to be disturbed at 7 per cent., while at 3 per cent, 

 death by asphyxia takes place ; on the other hand, they thrive 

 in pure oxygen at a pressure of one atmosphere. In like manner 

 a series of experiments published by Paul Bert ('73) shows in 

 animals a far-reaching independence of the partial pressure of 

 oxygen. In atmospheric air animals can still exist with a minimal 

 pressure of about 250 mm. mercury and with a maximal pressure 

 of fifteen atmospheres ; in pure oxygen the minimum of pressure is 

 considerably lower, but a pressure of two atmospheres for plants 

 and of three atmospheres for animals is fatal. In general, it follows 

 from the experiments of Paul Bert that the effects of a too small 

 percentage of oxygen can within certain limits be compensated for 

 by a rise of pressure, and the effects of a too high pressure by a 

 fall of the percentage. The remarkable fact that organisms in 

 pure oxygen with too high partial pressure die, and, as Paul Bert 

 has shown, die of asphyxia, has been made clear by Pflliger ('75, 1), 

 by means of an analogy between living substance and active phos- 

 phorus. As is well known, in atmospheric air phosphorus becomes 

 oxidised actively, gives out light, and evolves fumes of phosphorous 

 acid, while in pure oxygen it is not oxidised at all. So living 

 substance in pure oxygen with a high pressure ceases to oxidise, 

 and hence appears the paradoxical phenomenon of death by 

 asphyxia in pure oxygen. 



The minima and maxima of the percentage and the partial pres- 

 sure of oxygen are very different for different organisms, and thus 

 far are known only in a few cases. These details are of little 



1 Cf. p. 141. 



