THE CARDIAC NERVE CENTERS 195 



central end of the superior laryngeal, of the splanchnic and of the tri- 

 geminal. 



Acceleration is obtained by inflation of the lungs ; and in man it has been 

 found that any increase of intrabronchial air pressure, as in speaking, sing- 

 ing, rapid and forced respiration, etc., accelerates the heart beat. 



The effect on the heart of central stimulation of sensory nerves in the strict 

 sense, as well as nerves of the special senses, is twofold: either an acceleration 

 or a slowing. Only the trigeminal gives an invariable retardation ; stimulation 

 of the nasal mucous membrane stops the heart immediately. It is possible that 

 the result of stimulation depends upon its strength, inhibition following strong, 

 acceleration following the weak current. We may also suppose that such nerves 

 consist of two kinds of fibers, one of which brings about inhibition of the heart 

 beat, the other acceleration; but this is improbable. The afferent nerves from 

 the muscles appear to exercise but a slight influence on the heart. 



Although the heart reflexes have not been studied at all sufficiently, we 

 may affirm with a moderate degree of certainty that acceleration as well as 

 inhibition of the heart beat can be brought about reflexly by a great many 

 afferent nerves. 



Inhibition is certainly to be regarded as a reflex carried over to the vagus, 

 as appears unequivocally from the fact that when both vagi are cut the heart 

 beats faster. It is generally supposed also that acceleration is mediated by reflex 

 excitation of the accelerator nerves. However, Hunt has made some researches 

 from which he concludes that reflex acceleration is caused by a reduction in the 

 tonus of the inhibitory center, and he has endeavored with great skill to prove 

 that in most cases of augmented heart action the cause inheres in this rather 

 than in excitation of the accelerator, although he admits that augmentation is 

 stronger with uninjured accelerators, than when these are destroyed, because 

 with diminshed tonus of the vagus they can act on the heart more powerfully. 



13. THE CARDIAC NERVE CENTERS 



We designate as the center of a nerve, that place in the central nervous 

 system from which its activity is influenced either automatically or reflexly. 



Nothing definite is known about the location of the augmentor center for 

 the heart. But since a stimulus applied to the upper part of the medulla 

 produces acceleration of the heart beat, it is natural to locate this, with the 

 other vegetative centers of the body, in the medulla. 



It is perfectly certain that the inhibitory center is in the medulla. A 

 needle puncture halfway up the medulla and pretty well to one side, causes 

 slowing and stoppage of the heart. * 



The cardiac nerves are influenced also by portions of the brain anterior 

 to the medulla, including even the cerebral cortex. This is confirmed by our 

 daily experience that the psychic states joy, fear, hope, etc. increase or 

 diminish the frequency of the heart beat. Most persons, however, find it 

 impossible to influence these centers by direct effort of the will. 



The frequency of the heart beat can be changed in one direction or another 

 bv the so-called motor areas of the cerebral cortex and by different lower parts 

 of the brain. But it is not correct to regard these portions as the seat of the 

 active centers of the cardiac nerves. It seems preferable to regard the cere- 



