ioo RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



part of the explanation of the divergence between the observed facts 

 and the "theory". 1 



It might be thought that when the oxygen pressure, at a certain 

 temperature, is the limiting factor for the respiratory exchange, because 

 it is or approximates zero at those points, at least, in the tissues which 

 are at the greatest distance from the source of the oxygen supply (the 

 capillary wall in the higher animals, the outer surface of the body in 

 the lowest forms), no increase whatever would be possible at increasing 

 temperature. This is not the case however. When the temperature 

 rises the (intercapillary) spaces in which no oxygen is available will 

 increase in size, and the average distance which an oxygen molecule 

 has to travel before it enters into chemical combination will become 

 shortened and an increase in oxygen consumption must therefore take 

 place. Henze [1910] has examined the influence of a rise in temperature 

 upon the oxygen absorption of Actinia^ in which animal he has shown 

 that the oxygen pressure is a limiting factor for the respiration. By 

 increasing the temperature from about 14 to about 22 he observed a 

 considerably increased oxygen absorption which could be further in- 

 creased by raising the oxygen pressure. 



It is in the opinion of the writer very significant that Vernon has 

 found the largest temperature increments of the gas exchange (above, p. 

 89) in the hyaline pelagic animals (Salpa, Cestus, Beroe\ the respiratory 

 exchange of which is not affected by variations in the oxygen pressure, 

 and which must therefore be supposed to have a positive oxygen pres- 

 sure in their tissues. 2 In the case of these animals, if in any, we might 

 expect the temperature-metabolism curve to follow the rule of van't 

 HofT. Unfortunately the determinations of Vernon, which were not 

 made under standard conditions, are too discordant to decide the issue. 



1 Piitter [1914] considers that different processes may be limiting factors at different 

 temperatures, that a van't Hoff curve with one definite constant corresponds to each process, 

 and that therefore the actual temperature-metabolism curve is made up of several distinct 

 parts, of which each is a perfect van't Hoff curve. In an example (constructed) he lets 

 the permeability of the cells for oxygen be the limiting factor from o to 5 and assumes that 

 this permeability follows van't Hoff's law with a constant = 8'o. From 5 upwards the 

 velocity of the oxidative process is taken to be the limiting factor with a van't Hoff con- 

 stant = 2*0, but from 15 upwards the reaction is modified by some noxious factor, also of 

 course acting according to the rule but with a constant = 16*0. The combined result of 

 all this is a curve which does not resemble any metabolism curve known to the writer, but 

 it is not unlike the curve for the temperature influence upon the rate of development of 

 certain embryos. Putter's reasoning is possible because he deals with processes which have 

 been very inadequately studied. 



2 Their oxygen consumption per unit body weight is very low at all temperatures be- 

 cause their dry organic substance amounts only to about \ per cent, of the weight. 



