210 THE CIRCULATION OF THE BLOOD 



cells may contract and relax at as high as 400 a minute. These con- 

 tractions spread to the conducting bundle system at irregular intervals 

 and the ventricle contracts at unevenly spaced intervals. The arterial 

 pulse beats are also irregularly spaced, figure 1 786. When ventricular fibril- 

 lation occurs it quickly produces death, but auricular fibrillation only 

 reduces the efficiency of filling the ventricle. The physician controls 

 auricular fibrillation by reducing the irritability of the auricle and the 

 conductivity of the bundle system. 



THE REGULATIVE INFLUENCE OF THE CENTRAL NERVOUS 

 SYSTEM ON THE HEART. 



The.heart is capable of automatic rhythmic movement, yet while in the 

 body its beats are under the constant control of the central nervous system. 

 The influence which is exerted by the central nervous system is of two 

 kinds: first, in the direction of slowing or inhibiting the beats, and second, 



FIG. 1 79. Effect on the Heart Rate and on the Arterial Blood Pressure of Stimu- 

 lating the Right Vagus of the Dog. Stimulus applied at the mark "on" and removed 

 at "off." Pressure in millimeters of mercury shown by the scale to the left. Time in 

 seconds. (Hill and Chilton.) 



in the direction of accelerating or augmenting the beats. The influence 

 of the first kind is brought to bear upon the heart through the fibers of the 

 pneumogastric or vagus nerves, and that of the second kind through the 

 sympathetic nerves. 



The Inhibitory Nerves. It has long been known, indeed since the 

 experiments of the Weber brothers in 1845, that stimulation of one or 



