MULTICELLULAR ANLMALS 



239 



The beating of the vertebrate heart is effected by a harmonious 

 contraction and relaxation of the muscle cells, the impulse traveling 

 from cell to cell so that the order of contraction is first sinus 



A B 



Fig. 165. — A, branch o£ the privet; B, axillary buds developing after removal of the 



terminal bud. 



venosus (or its remnant, the sino-auricular node in the mammalian 

 heart), then the atria, and lastly the ventricles (Fig. 164). Although 

 the rate of heart beat is under control of the nerves, the beat itself 

 does not originate by nerve stimuli. Coordinated beating occurs in 



