124 



riilNCIPLES OF ANIMAL lilOlAK}Y 



the circulation is in part related to this feature of heart structure. A dia- 

 gram of the circulatory system in the dogfish, an animal with a two- 

 chambered heart, is shown in Fig. 88. This diagram indicates that the 

 blood of animals with gills and a two-chambered heart passes from the 

 heart through the gills, and then out through the dorsal aorta to the organs 

 of the body, where it passes through capillaries (not shown) and returns 



r* HeaJ •- 



C^crsat aor 



• fa To./ 



Fig. 88. — Diagram of the circulatory system in the dog-fish. The capillaries and 

 finer arteries and veins are omitted. {Modified from Parker and HasweU's Textbook of 

 Zoology.) 



to the heart by means of the veins. The blood thus passes over a single 

 course from the heart back to the heart again. 



In animals with lungs and a heart of more than two chambers the cir- 

 culatory system is more complicated. The heart of amphibians and 

 reptiles, except crocodilians, has three chambers in place of two as in the 



heart of fishes (Fig. 88), and the heart of 



mammals, birds, and crocodilians has four 



^y"^ chambers. The four-chambered heart is com- 



^^ posed of two halves, right and left. Each 



half is made up of two chambers, a thin 



walled auricle and a thick walled muscular 



ventricle. There is no passage between the 



two halves of the heart b^ J^ere is a broad 



•^ ^^  ^ passage guarded by val\ps .connecting each 



\ ^kfl^ auricle with the ventricle of the same side. 



j^Q DiAgram of a four- '^^^ relations of the parts of a four-chambered 



chambe'rCti-'ij^irt. LA, left heart may be understood from Fig. 89, which 



also suggests the path of the blood. 



The blood of such an animal, after having 

 made a circuit through the systemic circulation 

 (through the- body) is returned to the right 

 auricle, whence upon contraction of the auricle it passes through a valve to 

 the right ventricle. Upon contraction of the right ventricle the blood is 

 sent through the pulmonary arteries to the lungs. Having completed 

 the circulation of the; lungs (puhnoncyy circulation) the blood returns to the 

 left auricle through the pulmonary veins and from this auricle passes 

 through the opening to the left ventricle. Upon contraction of the left 



auricle; RA, right auricle; L^< 

 left ventricle; RV, right v^tri- 

 cle;Ll, vessel from luag's; L'2, 

 vessel to lungs; <S'l, vessel to 

 system; <S2, vessel from system. 



