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. 
Pfliiger 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 
