172 SCIENCE PROGRESS 



increased with the tension, rapidly at first, and then more and 

 more slowly. In all probability the caterpillars kept in air were 

 absorbing oxygen almost at their maximum rate. In the snail, 

 however, the affinity of the tissues for oxygen was so small 

 that from the 5 per cent, mixture only half as much oxygen was 

 absorbed as in the case of the caterpillar, and the maximum 

 absorption rate was not attained in an atmosphere containing 

 50 per cent, of oxygen. In mammals it would appear that the 

 affinity of the tissues for oxygen is very much greater than in 

 these lowly organisms, for the maximum absorption rate is 

 attained at very much smaller tensions. More than a century 

 ago Seguin and Lavoisier showed that the oxygen absorption 

 was, within certain limits, independent of the oxygen content 

 of the atmosphere breathed. This question has since been 

 repeatedly examined by other observers, and the general 

 consensus of opinion upholds the original doctrine. Lowy, for 

 instance, has found that the oxygen absorption is constant 

 whether the oxygen in the atmosphere breathed be increased to 

 more than double the normal, or diminished to less than half 

 the normal. Of course it does not follow that the tension of 

 oxygen in the blood reaching the tissues varies to the same 

 degree as that in the air respired, but there can be no doubt 

 that it can vary within wide limits without affecting the gaseous 

 metabolism of the tissues. In the case of excised mammalian 

 kidneys, the writer found that at a temperature of 20 the oxygen 

 absorption diminished if the tension of oxygen in the einals 

 used for perfusion fell below half atmospheric pressure, but 

 otherwise it was unaffected. 



Pflt'iger endeavoured to account for this constancy in the 

 oxygen-absorptive capacity of an animal by supposing that 

 through the metabolism of the cells in any given moment only 

 a fixed number of oxygen-binding affinities are liberated, and 

 hence that any further increase in the available oxygen above 

 that required by them is without influence. On this view the 

 tissues must have more oxygen at their command than they can 

 use up, or be always saturated to their full capacity with 

 intramolecular oxygen. Thunberg explains his results by 

 supposing that the restitution of the intramolecular oxygen 

 absorbed in the metabolism is a somewhat slow process, and 

 that the greater the oxygen tension the quicker the restitution, 

 and the greater the supply of intramolecular oxygen available 



