66 HUMAN PHYSIOLOGY 



The heart contains within itself the processes which 

 stimulate it to its rhythmic activity, for it beats for some 

 time after it has been cut out of the animal immediately after 

 death. Concerning the nature of this stimulation nothing 

 is known. 



Perhaps the cardiac muscle is stimulated directly by the 

 normal stimulus and not through the intervention of the 

 ganglionic cells and nerve-fibres found in the walls of the 

 heart. 



In the mammalian heart the ganglionic cells lie in the auriculo- 

 ventricular groove, in the partition between the auricles, and in the 

 auricle near the mouth of the superior vena cava. Their function 

 is not known. 



The embryonic heart has no ganglia and yet beats rhythmically; 

 in thai* case the cause of the rhythmic activity must certainly lie 

 in the muscle itself. 



Concerning the influence of the central nervous system 

 upon the heart, see page 74. 



3. The circulation of the blood in the heart. 



(a) In the ventricle. During the ventricular systole, the 

 cavities of the ventricles are reduced in size; the blood 

 which it contains is pressed out into the aorta and pulmonary 

 artery. The ventricles do not completely empty themselves 

 since, even in the strongest contraction, the cavities of the 

 heart are not entirely obliterated. 



The auriculo-ventricular valves which float upon the blood 

 during the ventricular diastole are closed during the ventric- 

 ular systole so that the blood cannot regurgitate into the 

 auricle. When the pressure in the ventricle is increased by 

 the systole, the blood flows behind the valves and presses 

 their surfaces together so that they are completely closed. 

 The valves do not bulge into the auricles because they are 

 fastened to the papillary muscles which contract simultane- 

 ously with the walls of the ventricle. 



The closure of the valves seems to follow the beginning 

 of the systole so quickly that no blood whatever re-enters 

 the auricle. 



During the ventricular diastole the blood does not flow 



