478 Animal Biology 



The heart Is enclosed by a double-walled membranous sac known as 

 the pericardium whose surfaces are kept moist by a secretion of serum. 

 for lubricating" purposes (Figs. 237 to 239). The walls of the atria and 

 ventricles consist of ( 1 ) an inner epithelial lining known as the endo- 

 cardium, (Gr. endo, within; cardium, heart), (2) a middle muscular 

 layer called the myocardium (Gr. myo, muscle), and (3) an outer, single 

 layer of mesothelial cells called the pericardium (Gr. peri, around). In 

 a heartbeat the contraction phase is called the systole, while the relaxa- 

 tion phase is called the diastole, and the two constitute a ''cardiac cycle/' 



Aor+a 



Superior 

 vena. caveL 



Auricle op 

 Rl^ht eitrlum 



Ri(4ht 

 QLtriunn 



Ri(4ht 

 ventricle 



Coronarq 

 arterij 



Inferior 

 vena cava 



PulmonarLj ar+erij 



\-^»^ Auricle of lefi ««Ltrlum 



' V ^ In -terven-trlcular 



fi *^~ branch of left 

 I V coronary artertj 



Lept ventricle 



'■<'^;^ 



ii>>7^ 



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^^'^^'^^ — Apex Of heart 



Fig. 237. — Human heart, front view, showing some of the blood vessels. The 

 aorta (artery) carries blood from the left ventricle to all parts of the body (ex- 

 cept lungs). The pulmonary arteries carry blood from the right ventricle to the 

 lungs. The coronary arteries arise from the aorta to supply blood to the walls 

 of the heart. Both the superior vena cava (also known as the precaval vein) and 

 the inferior vena cava (known as the postcaval vein) return blood from the body 

 (except lungs) to the right atrium. The atrium has an outpouching called the 

 auricle because of its earlike shape. The pulmonary veins (two right and two 

 left) return oxygenated blood from the lungs to the left atrium. The blood from 

 the heart walls is returned by way of the coronary sinus (not shown) to the 

 right atrium. The opening between the right atrium and right v'cntricle is closed 

 by the right atrioventricular valve (tricuspid valve, because it has three cusps). 

 The left atrium and left ventricle are separated by the left atrioventricular valve 

 (mitral valve, has only two cusps). (From Francis, Knowlton, and Tuttle: Text- 

 book of Anatomy and Physiology, The C. V. Mosby Co.) 



