CIRCULATION OF THE BLOOD. 35 



amount of blood equal to the total blood pf the body will pass 

 through the heart in about half a minute. 



What are influences of the nervous system upon the heart's 



action ? 



This matter is somewhat undecided at the present, for the reason 

 that many of the results must be obtained from experiments upon 

 the hearts of cold-blooded animals. We do know that the mechan- 

 ism of rhythmical contraction is contained within the heart itself. 

 Nerve-ganglia are demonstrated in the frog's heart which are essen- 

 tial to its action ; presumably similar ganglia exist in the human 

 heart. These ganglia are connected with fibres from the pneumo- 

 gastric (or vagus) nerve and with the sympathetic system. 



What is the effect of the pneumogastric nerve upon the heart? 

 It has an inhibitory or slowing effect upon the heart ; for if we 

 cut the nerve the heart becomes more rapid, and if we stimulate 

 the peripheral end of the nerve we slow the heart again. This 

 action may be traced to the medulla oblongata, where a cardio-inhib- 

 itory centre is located. 



What is the relation of the heart to the sympathetic nervous 



system ? 



Certain fibres of the sympathetic from the cervical and upper 

 dorsal spinal cord pass to the heart. If these fibres are left after 

 all other nerve-connections of the heart are cut away, stimulation 

 of the spinal cord will cause the heart to become rapid. These are 

 known as accelerator nerves. 



What is meant by the pulse? 



The wave which is felt in an artery after the systole of the 

 ventricle. 



What is a sphygmograph ? 



An instrument used to trace mechanically a diagram of the 

 pulse-wave. 



How is the heart nourished? 



By the coronary arteries, which arise in the sinuses of Valsalva V 

 behind the leaves of the semilunar valve. They do not receive blood 

 during systole, but during diastole from the pressure of blood in the 

 elastic arteries. The blood is returned to the right auricle through 

 the coronary vein. 



