BIBLIOGRAPHY 



767 



rhythm then dominates the contraction wave which spreads forward over the 

 heart. The area of the sinus continues to be the "pacesetter" of the heart beat 

 throughout life, although in birds and mammals, the sinus is taken up into 

 the posterior wall of the right atrium. In the mammal (fig. 340F), the sinus 

 node, located in the right atrium, initiates, under normal conditions, each 

 heart beat. The contraction stimulus spreads distally to the peculiar fibrous 

 bundle, located in the atrial septum and the atrioventricular area. This bundle 

 is known as the atrioventricular node, and its fibers descend into the muscles 

 of the ventricular area, conveying the heart beat to the ventricles. 



Though fibers from the autonomic nervous system reach the heart in the 

 region of the right atrium and stimuli from these nerves may greatly affect 

 the rhythm of the heart beat, the essential control of the beat lies within the 

 heart's own nodal system (fig. 340F). 



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