3l6 TEXT-BOOK OF PHYSIOLOGY 



The effects of such a stimulation with induced currents of moderate intensity 

 are graphically shown inFig. 141 . The upper tracing shows that the heart was 

 first accelerated, the beats increasing from 15 per minute before stimulation, 

 to 30 per minute during stimulation. On the cessation of the stimulation, 

 the heart slowly returned to its former rate. Coincidently with this accelera- 

 tion of the rate there was an augmentation of the force of the ventricular 

 contraction as shown by an increase in the height of the ventricular con- 

 traction which before stimulation was 9 mm., but during stimulation 12 mm. 

 In addition to the foregoing changes in the heart-beat there is an altera- 

 tion in the sequence of the beat. The natural delay in the conduction of the 

 excitation process from the auricles to the ventricle is increased, in conse- 

 quence of which the auricle completely relaxes before the ventricular con- 

 traction begins. Moreover, the auricular contraction again occurs before 



FIG. 141. TRACINGS SHOWING THE EFFECTS ON THE HEART-BEAT OF THE FROG FROM STIMU- 

 LATION OF THE SYMPATHETIC NERVES PRIOR TO THEIR UNION WITH THE VAGUS NERVE. The 

 upper tracing shows an increase in the rate, which before stimulation was 15 per minute and 

 during stimulation 30 per minute. Before stimulation the height of the ventricular beat was 

 9 mm. and during the stimulation it was 12 mm. The lowest tracing shows a similar series of 

 effects, the differences being only of degree. (Brodie.) 



the ventricle has completely relaxed. After the effect of the stimulatfon 

 passes away, the acceleration diminishes, the augmentation declines and a 

 reverse change in the sequence occurs. The lower tracing shows a similar 

 series of effects. If the stimulus be applied to the pre-ganglionic sympathetic 

 nerves, an acceleration or augmentation of the heart follows, similar in all 

 respects to that which follows stimulation of the post-ganglionic or sympa- 

 thetic fibers proper; and the inference may be drawn that if the stimulus 

 could be applied directly to the nerve-cells in the medulla oblongata from 

 which the fibers take their origin, the same acceleration or augmentation 

 would follow; for this reason this collection of nerve-cells is known as the 

 cwdio-accelerator or augmentor center. Since stimulation of the nerve in 

 any part of its course, which in all probability exaggerates its normal function, 

 is followed by an acceleration or an augmentation, the sympathetic is said 



