IX.] THE COMMON FROG. 131 



to a bishop's mitre), and consists of two flaps. The 

 aperture leading from the left ventricle to the aorta 

 is guarded by three crescentric flaps — called the 

 ''semilunar'' valves of the aorta. 



In man the whole of the blood is sent to the lunsfs 

 for purification during each circuit of this most im- 

 portant fluid, and every organ is supplied with oxy- 

 genated blood. 



If in any animals the process of purification is in- 

 complete, it is manifestly desirable that these organs 

 of the body, the functions of which are the most im- 

 portant, should be supplied with that part of the blood 

 which is pure. This consideration eminently applies 

 to the brain, the director and controller of the entire 

 body. 



Now all birds and beasts without exception, share 

 with man this perfect aeration of the entire blood, the 

 whole of the blood in the classes Mammalia and Avcs 

 being purified in the lungs before being distributed 

 to the body. 



The conditions by which the frog, at the various, 

 stages of its existence, oxygenates its blood and 

 directs the purified stream, in the most desirable 

 manner, are curious and instructive. 



It is generally known that the lower air-breathing 

 Vertebrates (Reptiles and Batrachrans) have the heart 

 less completely divided than in the higher classes, so 

 that the oxygenated (or arterial) blood and the un- 

 oxygenated (or venous) blood become mixed in the 

 single or imperfectly divided ventricle. 



It might well be supposed, and in fact has gene- 

 rally been so, that in animals with a heart so imper- 



K 2 



