ix CARDIAC MUSCLE AND NERVES 327 



Some observers have stated that during the arrest of cardiac 

 movements produced by vagus stimulation the heart becomes 

 inexcitable to direct artificial stimuli (Schiff, Eckhard, Mill). 

 Mac William, on the other hand, observed that when, in mammals, 

 vagus excitation produces not arrest, but pronounced weakening 

 of the systole (negative inotropic effect), the value of the threshold 

 of excitation, or least minimal efficacious stimulus applied directly 

 to the auricles, rises, i.e. the excitability of the myocardium is 

 lessened. Engelmann found, on the contrary, in the frog's heart 

 that during the inotropic negative effects due to stimulation of 

 the vagus, excitability of the auricles to direct stimuli may remain 

 unaltered, and even sometimes be augmented, which he terms the 

 positive bathmotropic effect. Engelmann, however, admits (experi- 

 menting always with the frog's heart) a great variety in the results 

 of his researches. The most frequent case is the association of 

 negative inotropic with negative bathmotropic effects ; but other 

 cases are to hand of simultaneous positive bathmotropic and 

 positive inotropic effects. At other times vague excitation gives 

 rise now to inotropic and now to bathmotropic actions. 



On the strength of this last fact more particularly, Engelmann 

 holds that the changes of excitability in cardiac muscle (bathmo- 

 tropic influence) are of a primary nature independent of the 

 simultaneous inotropic influences. This opinion is, however, 

 contradicted by H. E. Hering, who holds the bathmotropic effects 

 to be secondary and dependent on changes in the duration of the 

 systole. 



IX. The discovery that the heart receives accelerator or 

 systolic branches of the sympathetic in addition to the inhibitory 

 or diastolic fibres of the vagus was made in 1862 by V. Be/cold, and 

 worked out more accurately by Bevor (1866). On dividing the 

 two vagi and cervical syrnpathetics in rabbit, excitation of the 

 medulla oblongata and cervical cord produced a rise of blood 

 pressure, with acceleration of cardiac rhythm. On repeating the 

 same experiment after dividing the cord between the first and 

 second vertebra (with the object of cutting out the influence of 

 the bulbar vaso-motor centre of Ludwig and Thiry), acceleration 

 was obtained without rise of pressure. There must accordingly be 

 accelerator nerve fibres running from the cervical cord through 

 the rami communicantes of the sympathetic to the heart. Von 

 Bezold afterwards demonstrated that these accelerator fibres pass 

 through the last cervical ganglion, and thence to the heart. 



The brothers Cyon obtained the same results in 1866, on 

 dividing the splauchnics instead of the cord ; and further observed 

 that when the first thoracic ganglion was destroyed, there was no 

 longer acceleration of cardiac rhythm. 



Schmiedeberg (18*70) detected the presence of accelerator fibres 

 in the frog also, running with the vagus ; after a mild dose of 



