n THE NERVES OF THE HEART 73 



spring. On the other hand, other emotions cause 

 that extreme rapidity and violence of action which we 

 call palpitation. These facts suggest at once that the 

 heart, like the arteries, is subject to control by the 

 central nervous system, and we must now consider the 

 more important details of this control. 



The heart is well supplied with nerves. There are 

 many small ganrilia, or masses of ner\e cells, lodged in 

 the substance of the heart, more especially in the auricles, 

 and nerves spread from these ganglia over the walls, both 

 of the auricles and ventricles. Moreover, several nerves 

 reach the heart from the outside (Fig. 23). Of these the 

 most important are branches of a remarkable nerve which 

 starts from the spinal bulb, and supplies not only the 

 heart, but the lungs, alimentary canal, and other parts, 

 and which is called the pneumogastric, or from its 

 wandering coui-se, the vagus (see Lesson XI.). Other 

 nerves reaching th,e heart seem to come from the sym- 

 pathetic, but may be traced back thi'ough the sjTupathetic 

 to the spinal cord, and, for reasons which will presently 

 become apparent, are called accelerator nerves. 



The heart, as already exjjlained (p. 51), contracts 

 rh}i;hmically, but the regular i-hythmical succession of the 

 ordinary contractions is not primarily dependent upon the 

 ganglia lodged in its substance, as was at one time 

 supposed to be the case. Neither does it depend on the 

 action of the nerves connected with the heart, since the 

 movements continue even after the heart is removed from 

 the body. Hence we must conclude, and experiment 

 bears out the conclusion, that the muscle substance of ivhich 

 the heart is made is itself endowed xmth the power of con- 

 tracting and relaxing at regular intervals. On the other 

 hand the influences which alter the hearts action, as in 

 fainting or palpitation, do as a rule come to the heart from 

 without, and are carried to the heart along the vagus 

 and accelerator nerves. This may be demonstrated on 

 animals, such as frogs, with great ease. 



