86 



THE INDIVIDUAL ORGANISM 



cardiac muscle they branch and join together in such a way that a stimu- 

 lus spreads from fiber to fiber throughout the whole auricle or ventricle. 

 In a sense, the beating of the heart is automatic — i.e., it is not depend- 

 ent upon stimuli from the central nervous system. The heart will continue 

 to beat if all its nerves are severed. Furthermore, on account of its all-or- 

 none character, the contraction of any chamber of the heart is always 

 equally strong under a given set of conditions. This does not mean that 

 the strength of the heartbeat does not vary; as everyone knows, the 

 state of mind or body can modify both the rate and the power of the beat. 

 The controlling mechanisms responsible for these changes are discussed 

 below in connection with the mechanics of circulation. 



Fig. 6.2. The pumping heart. 



The heart muscle keeps up a rhythmic series of contractions through- 

 out the life of the individual. Each wave of contraction begins at the 

 upper end of the auricles, forcing the blood from them into the ventricles, 

 and then continues, more powerfully, into the walls of the ventricles. 

 The valves between the auricles and ventricles permit blood to flow 

 into the ventricles but prevent its being forced back into the auricles 

 when the ventricles contract. Instead, the blood from the left ventricle is 

 forced into the main systemic artery, the aorta, and that from the right 

 ventricle into the 'pulmonary artery leading to the lungs. 



Blood vessels are of three main types: arteries, which carry blood 

 from the heart to the tissues; veins, which carry blood from the tissues 

 to the heart; and capillaries, the minute blood vessels in the tissues that 

 connect the arteries with the veins. The walls of the arteries and veins 

 have much the same structure, but the walls of the arteries are markedly 



