RESPIRATION 



215 



the incoming air. Nevertheless it seems hardly probable that the 

 arrangement is for heat regeneration. The blood passes to the 

 gland with, presumably, the main physiological object of supply- 

 ing 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 9 

 that at low temperatures the influence of CO 2 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 



S.G. 



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 CO 2 pressure. It seems probable, 

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

 blood to communicate part of its CO 2 to the arterial blood. The 

 effect of this will be to raise the CO 2 pressure of the blood sup- 

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

 be active secretion of CO 2 into the arterial capillaries; and this 



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



