386 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



goes to the tissues above and below. That which goes 

 to the tissues comes back to the right auricle as dark, 

 and that which goes to the lungs to the left auricle as 

 bright blood. The auricles con- 

 tract and empty their contents 

 into the common ventricle as 

 mixed blood, which is again 

 thrown into the aorta, to be 

 again divided between the tis- 

 sues and the lungs. 



There is a double reason why 

 these animals are cold-blooded : 

 (i) We have already seen that 

 the lungs in them are a coarse 

 sponge, and therefore the sur- 

 face of exposure of the blood 

 to the air is small ; (2) and now 

 we see that not the whole but 

 only a part of the blood is oxi- 

 dized in each round of the cir- 

 culation. Mixed blood goes to 

 the tissues. The metabolic process — i. e., waste and 

 supply — is less active, and therefore the heat is less. 



We have given in Fig. 265 a schematic diagram illus- 

 trating the general principle. This is sufficient for 

 physiology but not for morphology, and especially not 

 for evolution. The actual course is far more complex, 

 and it differs also a little in different reptiles. We can 

 not take all the cases. We take the typical case of the 

 lizard. 



The lizard (Fig. 266) has three aortic arches on each 

 side — six in all. What conceivable use can there be for 

 six aortic aches? The blood from two of these — the 

 lower one on each side — goes to the lungs to be aerated, 

 while that of the other four, after sending a branch to 



Fig. 265. — Diagram showing 

 the structure of the heart 

 and the general course of 

 the circulation in reptiles 

 and amphibians. 



