324 THE NERVOUS REGULATION OF THE HEART 



period is less than one second. Clearly, therefore, the sympathetic or 

 accelerator fibers react more sluggishly but are less easily fatigued 

 than the inhibitor. The effect of their excitation consists either in an 

 acceleration or in an augmentation; in fact, in some cases both changes 

 are obtained simultaneously, the contractions becoming more frequent 

 as well as more forcible. In explaining this result, it is generally 

 stated that the accelerator mechanism is adjusted in such a way 

 that it may give rise to two reactions, namely, an increase in the 

 frequency, and an augmentation in the amplitude of the individual 

 beats. In analogy with this functional dissociation, it is also held that 

 the inhibitor mechanism is adjusted in such a way that the inhibition 

 may be accomplished either by lessening the frequency, or by decreas- 

 ing the amplitude of the cardiac contractions.^ 



While the experimental evidence is not very conclusive, it has been 

 suggested that the accelerator center discharges its impulses in rhyth- 

 mic succession, thereby establishing the so-called accelerator tonus in 

 antagonism to the inhibitor tonus. The removal of the former influ- 

 ence, ♦therefore, places the inhibitor discharges in complete control. 

 A slowing of the heart is the result of this disturbance of the cardio- 

 motor equilibrium. This end can be attained either by dividing the 

 accelerator fibers themselves, or by removing the intrathoracic ganglia. 

 Upon this basis cardio-acceleration may be explained by assuming that 

 the inhibitor tonus is temporarily diminished. ^ 



The increase in the rate of the heart is made possible by a shorten- 

 ing of each cardiac cycle, the duration of the diastolic period being 

 reduced first of all. It may be stated in general that the simultaneous 

 occurrence of accelerator and augmentor influences gives rise to a higher 

 blood pressure and more effective circulatory conditions than one 

 of these reactions alone could possibly produce. Thus, a simple 

 acceleration may fail absolutely in improving hemodynamical condi- 

 tions for the obvious reason that a greater number of ventricular 

 discharges alone does not suffice to increase the cardiac output per unit 

 of time, because the filling power or power of relaxation of the heart 

 may have been diminished in a measure to offset the increased rate. 



The Afferent or Cardiosensory Fibers. — These fibers are divided into 

 two groups, namely, those which bring the cardiac center into relation 

 with the various regions of the body, and those which connect it with 

 the heart and neighboring pericardial and mediastinal membranes. 

 The first group embraces a large number of nerves, because practically 

 any one of the afferent paths in our body may at times convey impulses 

 to central parts which here affect the activity of the heart in a reflex 

 manner. The second group includes the ordinary sensory nerves 



1 Bayliss and Starling, Jour, of Physiol., xiii, 1892, 407. 



2 Several cases have been recorded of persons who could voluntarily increase 

 their heart rate (West and Savage, Arch. Int. Med., 1918, 298). The acceleration 

 was accompanied by an augmentation of the respiratory movements and a dilatation 

 of the pupils. 



