552 



VERTEBRATE LIFE AND ORGANIZATION 



Arch, of 



-AcLult rcTnn.a.rLt of 

 embryonic ductus 



An^ (trior 

 ve-na cavaL" 



Semilunar valve, 

 Rioht atrium 



CoronsLry 



Posterior 

 ve-na. ca-va. 



Tricuspidv 



mona-ry vein 

 a.triu.m. 

 cuspid, va-lve 

 mi lunar va-lve. 

 inous cord 



ventricle 

 art muscle 



Sirio -a.tr ia.1 

 n.ode/ 



Atrioventricular 

 node 



^— At riove n.t rlcalaj;' 

 bundle 



Figure 27.6. The adult mammalian heart. Upper, Course of blood through the 

 heart; lower, distribution of the specialized cardiac muscle that forms the conducting 

 system of the heart. 



succession. Contraction of these chambers is known as systole; relaxation, 

 as diastole. Ventricular systole is very powerful and drives the blood 

 out into the pulmonary artery and arch of the aorta under high pres- 

 sure. Since the muscle fibers of the ventricles are arranged in a spiral, 

 the blood is not just pushed out, but is virtually wrinig out of them. 

 When the ventricles relax, their elastic recoil reduces the pressure 

 within them and blood enters from the atria. Atrial contraction does 

 not occur until the ventricles are nearly filled with blood; indeed, the 

 filling of the ventricles is nearly normal in cases where disease has 

 destroyed the ability of the atria to contract. The atria are primarily 

 antechambers that accumulate blood during ventricular systole. 



Blood being pumped by the heart is prevented from moving back- 



