2 4 2 



THE SCIENCE OF LIFE. 



and one ventricle. The venous blood from the body is 

 received into the right auricle and the purified blood 



from the lungs into the 

 left. Both communi- 

 cate with the ventricle, 

 which pumps the mixed 

 blood part to the lungs 

 and part around the 

 system. 



The highest form of 

 circulation is seen in 

 the warm-blooded Ver- 

 tebrates, Birds, and 

 Mammals. The heart 



FK;. 141. A. Plan of Circulation in Amphibia 



and Reptiles. B. Plan of Circulation in Birds and has foUT CaVltlCS 3. 



Mammals, a. Right Auricle receiving venous blood 



from tbe system, b. Left Auricle receiving arterial Tight aild left auricle, 



blood from the lungs, c. c' . Ventricles, d. e.f. Sys- , . 1 j i r 



temic Artery, Vein, and Capillaries, g. h. k. Pul- and a ngftt and left 



monary Artery, Vein, and Capillaries. ventricle. The right 



auricle receives the blood from the veins, transmits it to 

 the right ventricle, which sends it to the lungs. The 

 left auricle receives it from the lungs, and sends it to the 

 left ventricle, which propels it over the body. The two 

 auricles contract together, and so also do the ventricles, 

 making certain faint sounds, which may be imitated by 

 the words lubb tup. (Fig. 141.) 



The greatest differences between Vertebrates and other 

 animals are found in the Nervous system, which, as we 

 have seen, has a distinct tube or cavity in this type, al- 

 together unlike the plan of structure elsewhere. 



In living things, like the Protozoa, or Protophytes, 

 which are composed of a simple mass of bioplasm, all 



