170 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



probably account for the rise of blood-pressure in the systemic arteries and the 

 fall in both systemic and pulmonary veins observed by Roy and Adami. 



The speed of the cardiac excitation waive is increased. Its passage across 

 the auriculo-ventricular groove is also quickened, as is shown in the following 

 experiment of Bayliss and Starling. In the dog, the artificial excitation of 

 the ventricle may cause the excitation wave; to travel in a reverse direction, 

 namely, from ventricle to auricle. If the ventricles are excited rhythmically 

 and the rate of excitation is gradually increased, a limit will be reached beyond 

 which the auricle no longer beats in response to every ventricular contraction. 

 With intact vagi, a rate of 3 per second is generally the limit. If now the 

 augmentor nerve is stimulated, the "block" is partially removed, and the 

 auricle beats during and for a short time after the stimulation at the same 

 rapid rate as the ventricle. 



The Intent period of the excitation is long. In the dog, about two seconds 

 pass between the beginning of stimulation and the* beginning of acceleration, 

 and ten seconds may pass before the maximum acceleration is reached. The 

 after-effect may continue two minutes or more. It consists of a weakening of 

 the contractions and an increase in the difficulty with which the excitation 

 wave passes from the auricle to the ventricle. The return to the former fre- 

 quency is more rapid after short than after long stimulations. 



The effect upon the heart-rate of simultaneous stimulation of the vagi and 

 accelerator nerves, according to Hunt, is determined by the relative strength 

 of the two stimulating currents. For sub-maximal stimuli the result for 

 both systole and diastole is approximately the arithmetical mean of the re- 

 sults of stimulating the two nerves .separately.' The acceleration that is seen 

 after the stimulation of the vagus is due to the after-effect of the stimulation 

 of accelerating fibres in the vagus. 



The simultaneous stimulation of the augmentors and the vagi, the strength 

 of the current being sufficient to stop the auricular contractions, causes accel- 

 eration of the ventricular contractions. 



The acceleration of the heart may be more or less intermittent, although 

 the excitation of the augmentor nerves continues. It is probable that this is 

 due to irradiation from the bulbar respiratory centre. 2 



Other Centrifugal Heart-nerves. 



In the vago-sym pathetic trunk and the annulus of Vieussens fibres pass to 

 the heart that cannot he classed either with the vagus or the augmentor nerves. 

 The evidence for their existence is furnished by Hoy and Adami's observation 

 that when the intracardiac vagus mechanism is acting strongly, so that the 

 auricles are more or less completely arrested, the stimulation of the vago- 

 sympathetic trunk sometimes causes a decided increase in the force both of 

 the ventricles and the auricles, usually accompanied by an acceleration of the 

 rhythm of the heart. These changes are too rapidly produced to be aug- 

 mentor effects. 



1 I I nut : Aini'rinni Journal of l'lii/xio!o</it, 1899, ii. p. 422. 



2 Werthemier and Lepage : Journal de phygiologie et cle pathologic generate, 1899, p. 236. 



