RESPIRATION 



215 



^■the incoming air. Nevertheless it seems hardly probable that the 

 ^Rirrangement is for heat regeneration. The blood passes to the 

 ^Kfland with, presumably, the main physiological object of supply- 

 ^Bng oxygen, and venous blood in returning is already spent as 

 ^■regards its supply of oxygen. Nevertheless I think we can now 

 ^^suggest an explanation. It was discovered by Barcroft and King^ 

 ' that at low temperatures the influence of CO2 in expelling oxygen 

 from haemoglobin is much greater, relatively speaking, than at the 

 temperature of warm-blooded animals. The difference is so great 

 as to suggest that the dissociation of oxyhaemoglobin in the tis- 

 sues of cold-blooded animals is practically dependent, not on fall 



END 



Figure 64. 

 (X 1000). Transverse section through anterior end of rete mirabile of 

 Gobius niger, showing the peculiar endothelium (END) of the arterial 

 capillaries (A) as compared with the venous capillaries. (V) (Woodland). 



of oxygen pressure, but on rise of CO2 pressure. It seems probable, 

 therefore, that the function of the rete mirabile is to enable venous 

 blood to communicate part of its COg to the arterial blood. The 

 effect of this will be to raise the CO2 pressure of the blood sup- 

 plied to the gland, and so raise the oxygen pressure. There may 

 be active secretion of CO2 into the arterial capillaries; and this 



" Barcroft and King, Journ. of Physiol., XXXIX, p. 374, 1909. 



