192 



CIRCULATION OF THE BLOOD 



(Baxt). The increase is accomplished mainly by shortening the diastole. 

 When stimulation has ceased an after-effect remains which in favorable cases 

 lasts for as much as two minutes. 



Just as with the inhibitory nerves, the accelerators appear to exercise a 

 tonic influence on the heart. Evidence for this we have in the fact that 



Seats 



20 



Sec. 



20 



FIG. 68. Graphic representation of the pulse rate: v, on stimulating the vagus; a, on stimu- 

 lating the accelerator nerves ; a v, on stimulating both simultaneously, after Hunt. Stimu- 

 lation lasted fifteen seconds in each case, s s. 



bilateral extirpation of the lowermost cervical and uppermost thoracic ganglia, 

 after section of both vagi,, diminishes the pulse frequency. The normal rate 

 of the heart beat therefore is determined by the accelerator nerves as well as 

 by the inhibitory, and it appears from FriedenthaFs results, given on page* 

 188, that the former are just as necessary for the heart's activity as the 

 inhibitory nerves. 



The size of the auricular and the ventricular contraction in most cases 

 increases upon stimulation of the accelerator. But it may happen also that 

 the extent of the contraction increases, while the heart frequency is not influ- 

 enced at all, and, vice versa, acceleration may take place without any increase 

 in extent. In any case these nerves improve the execution of the heart and 

 heighten the dissimilatory processes going on within it; hence the proposed 

 designation of them by Hofmann as the promoting nerves in contradistinc- 

 tion to the inhibitory nerves, very aptly characterizes their properties. 



By way of analogy with his conception of the inhibitory nerves of the heart, 

 Engelmann supposes that the accelerator nerves also have several kinds of fibers : 

 positively chronotropic (increasing the rate) ; positively inotropic (increasing 

 the force) ; positively dromotropic (increasing the conductivity) ; and positively 

 bathmotropic (increasing the excitability). The same comment would apply to 

 this conception as to that concerning the inhibitory fibers (page 189). 



It has been found, mainly by the use of the nicotine method, that the 

 " promoting fibers " also unite with intracardial ganglion cells. 



With regard to the antagonistic relations of the vagus and accelerator, 

 experiments show that the one nerve or the other predominates according 

 to the relative strength of its stimulation, and that with a stimulus of suitable 

 strength for each, the two effects may be made to balance each other so that 



