THE BLOOD AND ITS CIRCULATION. 



647 



op, and the blood is diverted from the former to the latter, there is a 

 corresponding change in the carrying capacity of the blood-vessels, 

 resulting in the final disappearance of the vessels connected with the 

 gills. Moreover, while the blood was not returned directly from the 

 gills to the heart, it is returned directly from the lungs, and a second 

 auricle is developed. But the aerial respiration of the frog, with its 

 mixed circulation, is more rapid than the aquatic respiration with the 

 perfect circulation of the tadpole. 



In the reptiles circulation is essentially the same as in the amphibi- 

 ans ; but the ventricle is more or less divided by a partition into two 

 chambers. This membranous partition is perfect only in the croco- 

 dile, where we find a right and left ventricle without communication, 

 and the heart structurally like that of a bird or mammal. But the 

 circulation is still the same as in the lower reptiles, for the pure and 

 impure blood are somewhat mingled by a communication between the 

 two arteries near their point of origin. 



Although birds in their general organization are closely allied to 

 reptiles, their circulation is similar to that of the mammals. In these 



Fig. 5. MuscrLAR Fibers of the Ventricles. 1, superficial fibers, common to both ventricles ; 

 2, fibers of the iett ventricle ; 3, deep fi'jers, passing upward toward the base of the heart ; 

 4, fibers penetrating the left ventricle. 



two highest classes of the animal kingdom, there are always two auri- 

 cles and two ventricles, and the right and left sides of the heart are 

 entirely distinct. Functionally these are two hearts : a systemic heart, 



