BLOOD AND CIRCULATION 



547 



adequate blood supply to the kidneys, for the low pressure arterial 

 system alone might not deliver enough blood to these vital organs. 



Primitive Tetrapods. When the shift was made from gills to lungs, 

 many changes occurred in the heart and aortic arches (Fig. 27.3). The 

 aortic arches were reduced in number, the first two and the fifth being 

 lost. Those that remain are no longer interrupted by gill capillaries. In 

 a primitive tetrapod, such as the frog, the third pair of aortic arches 

 forms part of the internal carotid arteries supplying the head; the fourth, 

 the aortic arches proper leading to the dorsal aorta; and the sixth, the 



Lefb 



Internal ca-Trotid: 



Dorsal a.orta.- 



Atriuni 



Common, 

 cardinal- 



ET Carotid. arcVi 



Conus a-rteriosaS 



Veintricle 



Subcla.'via.n 



artery 



Rioht a-trium. 

 Sinus venosus 



Ve-nae ca.va-e 

 Hepa±ic vein. 



External carotid 



Aortic a-rch. 



Trun-Cus 

 arteriosus 



Pulmocutaxieous 

 a-rcK 



"Left cLtrium 

 Pulmonary vein 



DorSa-1 n nrta. 



Sinus 



venosus 



PRI MITIVE FISH AMPHIBIAN (Fro^) 



- External CcLTotid.- 

 Internal Carotid.- 



Common, cajrotid. 

 — Su.bcIa3/iajrL- 



Pulmonary artery 

 Forrner conus 



ormer sinu.s \ 



venosus 



Venae ca.vajz. 



Venaz cavajs 

 Pulmonary vein, 

 rsal a_o-rta- 



Subclavian 

 Arch of aorta 



Embryonic 



ductus 



ai"teriosu.S 



Pulmonary vzin. 

 Dorsal aorta. 



REPTILE MAMMAL 



Figure 27.3. Diagrams of the heart and aortic arches to show the changes that 

 occurred in tlie evolution from primitive fishes to mammals. All are ventral views. 

 The heart tube has been straightened so that -the atrium lies posterior to the 

 ventricle. 



