218 



nil INTLRNAL ENVIRONMENT OF THi: BODY 



Part III 



-aoHa 



pre- 

 cava 



righi 

 auricle 



semilunar 

 valves 



incuspid valve 

 righi ventricle 



Fki. 12.17. Diagram of the human heart with the front wall removed. Heavy 

 stipple, poorly oxygenated blood; light stipple, richly oxygenated blood. A, Auricles 

 filling from veins, i.e., right, precava and postcava; left, pulmonary veins. B, Blood 

 entering relaxed ventricles. C, Auricles contracting, ventricles relaxed and filling. 

 D, Ventricles contracting, blood forced into aorta and pulmonary arteries. (Cour- 

 tesy, Storer: General Zoology, ed. 2. New York, McGraw-Hill Book Co., Inc., 

 1951.) 



that conduct the heartbeat. The sinuauricular node, or pacemaker, is named 

 from its origin from the edge of the sinus chamber present in mammalian 

 embryos and adults of lower vertebrates (see frog heart); the similar auriculo- 

 vcntricular node is named from its position between these chambers. Muscle 

 fibers from the sinus node spread through the walls of the auricles but are not 

 shown in the diagram because of the thinness of the walls. Muscle fibers from 

 the auriculoventricular node extend through the septum between the ventricles 

 (auriculoventricular bundle) and spread throughout the walls of the ventricles. 

 The auriculoventricular bundle of muscle and nerve-cell fibers is the functional 

 bridge between the auricles and ventricles. 



The Pacemaker of the Heart. The neuromuscular mechanism is re- 

 sponsible for the conduction of the rhythmic contractions of the muscle of 

 the heart. The pacemaker is the dynamic center of the heart's action. In some 

 way not well understood, rhythmic stimuli develop from it and spread in 

 waves of contractions through the walls of the auricles. From there waves of 

 contractions spread through the auriculoventricular bundle of fibers in the 

 septum and thence throughout the ventricular walls. If this bundle is cut ex- 

 perimentally or damaged by disease, the ventricles either stop beating or beat 

 independently of the beat in the auricles. 



Central Nervous Control. The heart is profoundly a part of the body 

 as a whole. It can beat temporarily when separated from the body, but the 

 way it normally beats, weak or strong, slow or fast, is affected by conditions 



