180 STUDIES IN ADVANCED PHYSIOLOGY. 



thetic ganglia this nerve has usually been called the sym- 

 pathetic nerve. When stimulated it not only increases the 

 rate of the heart beat but for a little while also the strength 

 of the individual beat. In its influence on the heart it is 

 not so pronounced as the vagi, for if at the same instant 

 the vagi and the sympathetic nerves be stimulated the rate 

 of the heart beat is slowed, showing that the vagi are more 

 effective. Just under what circumstances in life these ac- 

 celerator nerves are brought into play is still not wholly 

 solved. It is probable, however, that the increase in the 

 rate of pulse which follows muscular exercise is due to such 

 sympathetic stimulation. 



The Accelerator Center. 



These accelerator nerves come from an accelerator cen- 

 ter which lies in the medulla, also. This center is called 

 the cardio-accelerator center. It is not in a state of con- 

 tinuous excitation, as in the case of the inhibitory center, 

 but like the latter, it may be excited by stimuli reaching it 

 from other sources. Thus psychic influences such as 

 strong emotions usually increase the rate of heart beat, 

 familiar to everybody in the common experience of having 

 " the heart rise into one's throat." Powerful stimulation 

 of sensory nerves in general may result in a marked quick- 

 ening of the heart beat. This quickening of the beat, of 

 course, at once raises the arterial pressure and so enables 

 the animal to make sudden and especially vigorous efforts. 

 This arrangement may be but the attempt of nature to put 

 at our disposal for sudden emergencies an increased circu 

 lation to make possible increased exertions, and so materi- 

 ally helD in the preservation of our safety. 



Summary. 



By way of summary the innervation of the heart is 

 briefly re-stated. 



The beat of the heart itself is due to the automaticity of 

 the muscular substance itself, and also to the automatic 



