THE CARDIAC NERVES. 187 



the heart to beat more slowly. Such a continued action of a nerve 

 is known as a tonic influence. 



That the vagi can slow the heart or even stop it altogether is 

 shown by stimulation of these nerves with an electric current of 

 suitable strength (Fig. 25). If weak shocks are employed, the 

 heart is slowed, the blood pressure falls somewhat, and the 

 diastolic pressure becomes markedly decreased, because the ar- 

 teries have a greater period of time in which to empty between 

 the beats. If somewhat stronger stimuli be used, the heart will 

 stop beating entirely, and remain in the diastolic position for 

 several seconds, during which the blood pressure will sink to 

 zero (Fig. 26. No. I). It is scarcely possible to kill an animal 

 by stimulation of the vagus, however, since the heart will begin 

 to beat after a short time in spite of the continued vagus stimu- 

 lation. This phenomenon is known as escapement. The time of 

 its onset varies considerably in different animals. It has been 

 suggested that the vagi have much more effect on the auricles 

 than on the ventricles, which is suggestive of the auricles being 

 the pacemakers of the heart. 



Relation of the Sympathetic and Vagus Nerves to the Heart. 

 The antagonistic action existing between the cardiac fibers of 

 the sympathetic and vagus nerves allows the heart to respond 

 quickly to any need that the body may demand of it. These 

 demands are made through the brain, by various afferent or sen- 

 sory nerves. This is brought about in the following way : 



The Cardiac Centre. In the medulla, the hind part of the 

 brain, there is a collection of nerve cells from which the cardiac 

 branches of the vagus arise. Near by also are located the cells 

 from which the sympathetic nerves of the heart arise. Both of 

 these nerve centers, for by this term are known the important 

 cell stations of the brain, are supplied by extensive connections 

 with afferent or sensory fibers coining from all parts of the body, 

 the brain and even the heart. The centers become more or less 

 active in response to impulses reaching them along the sensory 

 fibers. 



The Cardiac Depressor Nerves. One of the most important 

 of the different cardiac nerves is that known as the cardiac deprcs- 



