Biology of Man 479 



When a heart beats 70 times per minute, a cardiac cycle requires less 

 than one second. During contraction the heart undergoes electrical 

 changes, the active cardiac muscle is electrically negative to an inactive 

 cardiac muscle. These action currents can be recorded by a special 

 instrument called the electrocardiograph. The record known as an 

 electrocardiogram shows a series of waves which are correlated with 

 heart actions. 



Arteries (Figs. 237 to 241 ) are a series of vessels whose walls are rather 

 thick, contractile, and elastic. They consist of ( 1 ) an inner layer of 

 endothelial cells and elastic tissue, (2) a middle or intermediate layer 

 of muscle and elastic tissue, and (3) an external layer of elastic tissues. 

 Arteries carry blood away from the heart while veins carry blood back 

 toward the heart. 



Aorta 



Pulmonary arterij 

 AuLPicle Of 



left atrium 



Left 



Pulmonarq veins 



Breinch of lept 

 coronary art. 



Left ^ 

 ventricle 



per'ior vena cava 



RKjht pulmoneLrq 

 veins 



Left atrium 

 Coronary sinus 



Inferior vena cava 

 Ri^ht ventricle 



Fig. 238. — Human heart, posterior (back) view. (Compare with Figs. 237 and 

 239.) (From Francis: Introduction to Human Anatomy, The C. V. Mosby 

 Co.) 



Veins (Figs. 237 to 239) are a series of vessels whose structure re- 

 sembles that of arteries, being composed of three layers, but whose walls 

 are thinner and less elastic because of a poorly developed middle layer 

 and the presence of very little muscle and elastic tissue. Certain veins, 

 especially those of the lower extremities, have a series of semilunar valves 

 to prevent the backflow of blood. In general, the systemic veins accom- 



