162 The Vertebrate Organ Systems 



Efficient as this system may seem, it is hardly sufficient for active 

 vertebrates such as the reptiles and the homoiothermic birds and mam- 

 mals. In the reptiles further improvement is seen in the beginning of 

 a ventricular septum which ensures better separation of oxygenated and 

 unoxygenated blood. The conus arteriosus splits near its base into 

 the pulmonary artery and the dorsal aorta, and the sinus venosus be- 

 comes a part of the wall of the right auricle. 



In the birds and mammals, there is a complete separation of the 

 ventricle into the right and left halves, and the blood from the right 

 side of the heart cannot pass to the left except by taking a circuit 

 through the lungs. Thus mixing in the heart is now impossible. 



PULMONARY VEINS 



PULMONARY 

 ABTEBY 

 PULMONABY 

 ^ VEINS 



SEMILUNAR 

 VALVES 



TRICUSPID VALVE 



RIGHT VENTRICLE 

 CHORDAE TENOINEAE 



PAPILLARY MUSCLE 



LEFT AURICLE 



eiCUSPIO VALVE 



LEFT VENTRICLE 



Fig. 46. — The mammalian heart. 



The four-chambered mammalian heart (Figs. 46 and 47) is lo- 

 cated in the mcdiastini(m of the chest cavity and is surrounded by its 

 own protective membrane, the pericardium. The auricles are thin- 

 walled sacs which receive the blood coming into the heart. They act 

 as temporary retention chambers which are used only until the blood 

 can enter the ventricles. The heavy-walled muscular ventricles are the 



