172 The Vertebrate Organ Systems 



auricular refers to the fact that this mass of tissue is located at the point 

 where the sinus venosus has become incorporated into the walls of the 

 auricle in the mammalian heart. The second node, the auricular-ventricu- 

 lar node, is a mass of tissue located in the lower part of the separation 

 between the auricles and the ventricles. Fibers from this latter node 

 pass down the septum, separating the two ventricles, then separate into 

 a left and right branch which go up the walls of the heart, gradually 

 fusing with the muscle. The fibers of the sino-auricular node join with 

 the muscles of the auricle, and the stimulus from this node causes the 

 contraction of the auricular muscle tissue. These fibers do not connect 

 with the muscle tissue of the ventricle. This stimulus also affects the 

 auricular-ventricular node, and in turn affects the muscular contraction 

 of the ventricular tissue by means of the fibers extending from it. There 

 is a slight delay (0.15 second) in the conduction from the sino-auricular 

 node to the auricular-ventricular node, allowing the auricle to complete 

 its contraction before that of the ventricle begins. 



Careful inspection of a beating heart will clearly demonstrate this 

 orderly sequence of events. Contraction or systole is followed by a 

 longer period of relaxation or diastole. Further it may be noted that 

 the auricles contract slightly in advance of the ventricles and that the 

 right side contracts before the left. A heart beating at about the rate 

 of 70 times per minute has the following time relationship : 



Auricular systole 0.1 second 



Auricular diastole 0.7 second 



Ventricular systole 0.3 second 



Ventricular diastole 0.5 second 



The heart rate varies with a great number of factors such as age, 

 sex, body position, exercise, emotional state, etc. Any increase must 

 be accomplished at the expense of the diastolic phase of the beat. A 

 prolonged rapid heartbeat may ultimately injure the heart muscle as suf- 

 ficient time is not given for rest between systoles. 



Heart Sounds. — -The openings and closings of the two main pairs 

 of valves is in synchronization with the contraction of the parts of the 

 heart. These closings produce the familiar heart sounds, the "lub-dub." 

 The first heart sound occurs when the auricular-ventricular valves snap 

 shut. At this time, the ventricles are contracting and the semilunar 

 valves are open. The second heart sound, dub, occurs when the ven- 

 tricle relaxes, the auricular-ventricular valves open, and the semilunars 

 snap shut. Injuries to the valves may commonly be detected by irregu- 



