320 



TEXT-BOOK OF PHYSIOLOGY 



action in the frog. Thus stimulation of the nerves in any part of their 

 course, either through the rami communicantes, or after their emergence 

 from either the stellate or the inferior cervical ganglion is followed by effects 

 similar to those observed in the frog: viz., an acceleration or augmentation, 

 or both, of the heart-beat. The percentage increase in the acceleration 

 varies in different animals. In some instances the increase varies from 

 58 per cent, to 100 per cent. (Hunt). If the heart is beating slowly before 

 stimulation, the acceleration is more marked than if it is beating rapidly. 



FIG. 142. ACCELERATION OF THE HEART-BEAT FOLLOWING STIMULATION OF THE CARDIAC 

 BRANCHES WHICH COME FROM THE ANNULUS OF VIEUSSENS. (After Morat.) 



The effect of the accelerator impulses is apparently a change in the inner 

 mechanism of the heart-muscle itself and not a change in the peripheral 

 portion of the inhibitor apparatus. This is indicated by the fact that 

 acceleration occurs after the full physiologic action of atropin, which acts 

 upon, and impairs the conductivity of the intra-cardiac nerve-cell (post- 

 ganglionic) terminals. 



A peculiarity of the sympathetic nerve is that it does not respond .to 

 stimulation as rapidly as do many nerves, so that a rather long latent period 

 intervenes between the moment of stimulation and the appearance of the 

 acceleration as shown in Fig. 142. A further peculiarity is that the accelera- 

 tion sometimes continues after the stimulus is 

 withdrawn, and sometimes ceases before it is 

 withdrawn. 



The force of the heart -beat may be in- 

 creased without there being any increase in 

 the rate (Fig. 143). The auricular contraction 

 is probably increased whereby the ventricle is 



FIG. 143. INCREASE IN THE 



FORCE or THE VENTRICULAR CON- , , ,.,, 



TRACTION (CURVE OF PRESSURE IN more completely tilled. 

 RIGHT VENTRICLE) FROM STIMU- 

 LATION OF AUGMENTOR FlBERS. 

 There is little or no change in 

 frequency. (Franck, reduced.) 



Though an increase in both the rate and 

 force frequently occur simultaneously, there is 

 no necessary relation or connection between 

 the two as they can and do occur independ- 

 ently of each other. For this reason it is generally assumed that the sym- 

 pathetic nerves contain two groups of fibers, viz., accelerators and augmen- 

 tors, the functions of which are respectively to accelerate the rate and 

 augment the force of the heart-beat. From the fact that both auricles and 

 ventricles exhibit these changes it is assumed that the nerve impulses stimu- 

 late both chambers. This is rendered probable also from the experiments 

 of Erlanger, who found that after complete heart-block, stimulation of 

 the sympathetic caused independent acceleration of both auricles and 

 ventricles. 



Division of the sympathetic nerves is at once followed by a diminution 

 in the rate, the degree of which will depend to some extent on the rate 

 at which the heart was beating prior to the division. The results therefore 

 that follow stimulation and division of these nerves indicate that they are 



