128 



PRINCIPLES OF ANIMAL BIOLOGY 



and the constant vigor of its contraction. Several other features of its 

 beating are of the utmost importance. 



The heart has a long refractory period. Any muscle, after it has con- 

 tracted, will refuse to respond to a subsequent stimulus until a certain 

 time has elapsed. This interval of rest, known as the refractory period, 

 is exceedingly short (0.005 second) in skeletal muscle, but very long in 

 the heart. This prevents the heart from responding to any abnormal 

 nervous condition by remaining continuously contracted. It contracts 

 once, then must wait an appreciable time, during which it relaxes, 

 before it can contract again. 



Contraction of the heart is initiated by a mass of rather embryonic 



tissue located in the right auricle, near 

 the point where the great veins enter. 

 This tissue is known as the sinus node 

 (Fig. 102). When this node is stimu- 

 lated, the right auricle starts to con- 

 tract, and a wave of contraction spreads 

 to the left' auricle. This wave is mo- 

 mentarily blocked at the margins of the 

 ventricles but is carried over to them by 

 another node located on the partition 

 between the two auricles, a bundle of 

 whose tissue is distributed through the 

 ventricle walls. 



The sinus node is the "pacemaker" 

 of the heart. It responds to an increase 

 of carbon dioxide in the blood by caus- 

 ing the heart to beat faster. An in- 

 crease of temperature, acting through 

 the sinus node, also leads to faster 

 beating. For both of these reasons, exercise accelerates the circulation 

 of the blood. 



The pacemaker is in turn partly regulated by nerves. A pair of 

 accelerator nerves comes to it from the spinal cord in the chest region 

 and a pair of inhibitor nerves from the medulla of the brain. The 

 inhibitors are working constantly, exerting a continual drag on the heart. 

 Against this braking effect the accelerators act to variable degree. 

 Excitement and various reflexes (page 146) stimulate heart beat through 

 the nervous control of the sinus node. 



Blood Pressure. — Tli(> pressure of the blood against the walls of the 

 vessels is greatest in the arteries near the heart, declines moderately 

 in the more distant arterial branches, diops markedly in the minute 

 arterioles and capillaries, then declines slightly in the veins (Fig. 103). 



Fig. 102. — Pacemaker of human 

 heart, the sinus node (SN). AVN, 

 auriculoventricular node, with its 

 extension in auriculoventricular bun- 

 dles (AVB). V, valves between left 

 auricle and left ventricle. 



