646 



THE POPULAR SCIENCE MONTHLY. 



Amphibians and reptiles exist under conditions incompatible with 

 a high temperature of the body. In the adult state they are air- 

 breathers, and, if their circulation were complete, they would be 

 " warm-blooded." But the temperature is subdued by imperfect cir- 

 culation, which results from the arrangement of the heart-chambers. 

 There is but one ventricle for the two auricles, hence the j)ure blood 

 from the breathing organs and the impure blood from the body are 

 mingled, so that, besides the venous and arterial, they have a mixed 

 blood. The blood which goes to the lungs is never wholly impure, 

 and that which goes to the body is never entirely pure. However, by 

 a complex and beautiful action of the parts and valves of the heart 

 too complex to be here described the mingling of venous and arterial 

 blood is not complete. 



The change which the amphibians undergo in outward form, from 

 the tadpole or larval state to the frog-like condition, is accompanied 



Fig. 3. 



Fi3. 4. 



/ 



Pig. 8. Diagram of the Circulation in a Reptile. (The part contniiiitie; pure blood ie black ; 

 that coiitainiiiir impiiro blood is white ; and that containinp^ mixed bUxd is cross-shaded. i a, 

 iit,'hr auricle, receivinti: impure blood from the body ; ', left auricle, receiviufr pure blood from 

 the luiitrs ; v. ventricle, contiiininu mixed blood, which is cari'ied by the pulmonary artery {p) 

 to the lunjjp, and by the aorta (0) to the body. 



Fig. 4. Diaokam op the CiuruLATioN op a Bird or Mammal. (The venous system is blnck ; 

 the arterial system is white.) a, right auricle ; ?;, rich t ventricle; /?, pulmonary artery, carry- 

 ing venous blood to the lungs; p v, pulmonary veins, carrying arterial blood from the lungs ; 

 a', left auricle ; v\ left ventricle ; 6, aorta, carrying arterial blood to the body; c, vena cava, 

 carrying venous blood to the lungs. 



by a remarkable inward change in the circulation. In the larval 

 stage, with respiration by gills, the heart and circulation resemble 

 that of the fishes a single auricle and ventricle and complete purifi- 

 cation of the blood. But, as the gills disappear and the lungs devel- 



