90 CONTROL OF CIRCULATION. 



Gangliated Nerve Chain. By picking away a little tissue it 

 may readily be seen that each spinal nerve gives a branch to 

 this system. Find the sympathetic chain of the other side. 

 Trace the sympathetic system through the diaphragm to the 

 Solar Plexus back of the stomach. Some of the other ganglia 

 and plexuses may be discovered. Turn back to the thorax, 

 and trace the chain on one side up into the neck. Some large 

 ganglia will be found by following in this direction, 



Control of the Heart and Blood Tubes by the Ner- 

 vous System. We know that certain emotions affect the 

 circulation of the blood ; for instance, blushing and pallor, to 

 which reference has been made. Certain emotions may also 

 quicken or retard the action of the heart. Excessive grief or 

 joy has produced sudden death by stopping the beat of the 

 heart. 



Let us look a little more closely at that part of the nervous 

 system that has such intimate relation to the blood system. 



In the first place, the action of the heart is automatic. 

 The heart of the frog continues to beat a long time after it is 

 removed from the body. This is regarded by many as due to 

 the action of certain ganglia imbedded in the walls of the 

 heart, especially in the auricles ; while others say that since 

 the ventricle, in which no ganglia have been found, may beat 

 independently of the auricles, rhythmic contraction is charac- 

 teristic of heart muscle, and that we are at present unable to 

 explain it. 



But while the impulses that originate the action of the 

 heart arise within the heart itself, still the beat of the heart 

 is constantly modified by nerve impulses reaching it from 

 without. 



The heart receives its nerves from two sources, the Sym- 

 pathetic System and the Pneumogastric Nerves. 



The Sympathetic Nervous System consists of two rows of 



