THE CIRCULATION. 261 



of stimuli over the heart (dromotropy), as well as the contraction of 

 the cardiac muscle (inotropy), may be abnormally augmented or 

 decreased. The regular initiation of stimuli (chronotropy) may 

 vary, as well as the ability of the heart to respond to these stimuli 

 (bathmotropy.) 



Extracardiac Nervous System. 



The extracardiac nervous system is composed of the cardiac 

 branches of the vagus, together with the cardiac branches of the 

 sympathetic. 



Vagus-path. The vagus arises chiefly from the nucleus ambiguus 

 in the medulla oblongata. It descends in the neck, giving off near the 

 origin in the neck of the superior and inferior laryngeal nerves the 

 superior cardiac branch, and near the origin of the inferior laryngeal 

 in the thorax, the inferior cardiac branches, which contain the majority 

 of the cardio-inhibitory fibers. The inhibitory fibers belong to the 

 autonomic system, are pre-ganglionic and end around the sympathetic 

 ganglia in the heart, whilst the post-ganglionic fibers run from here 

 to the cardiac muscle-fiber. 



As has been previously stated, the immediate cause of the rhyth- 

 mical contractions of the heart lies in the protoplasm of the. muscle- 

 cells themselves, but the rate and force of its beats are influenced 

 by impulses reaching it through the central nervous system. The 

 effects of these impulses are twofold : inhibition, or diminution in 

 the rate or force of the heart-beat, and acceleration, or increase in 

 the rate or force. Both the inhibitory and accelerator centers are 

 located within the medulla, from which fibers leave the cranium and 

 reach the heart. Of these efferent fibers of the vagus, the inhibi- 

 tory ones are most prominent. 



Inhibitory Nerves of the Heart. Section of the vagus in the cat 

 and rabbit is not followed by much increase in the rate of the heart- 

 beat; in the dog and man the increase of rate is much greater. In 

 children the inhibitory power of the vagus is very weak, and they can 

 bear doses of atropin much better than adults because there is not 

 much increase of the rate of the heart after the use of it. These facts 

 lead to the conclusion that in warm-blooded animals the vagi are in a 

 state of constant activity with an inhibitory control over the heart. 



When all the extra cardiac nerves in the dog were cut on both 

 sides, and at the same time the inferior cervical and the superior 

 cervical ganglia were extirpated and the animal survived eight months, 

 the number of the beats of the heart was not markedly changed. If, 

 however, the dog was made to walk more than a kilometer, then serious 



