CIRCULATION. 461 



rare. 1 The simultaneous stimulation of the nerves of both sides does not 

 give a greater maximum frequency than the stimulation of one nerve alone. 2 



The strength and the volume of the auricular contractions are also in- 

 creased. The increase in volume is not due to a rise of pressure in the veins 

 in fact, the pressure falls in the veins but to a change in the elasticity of 

 the relaxed auricle, a lowering of its tonus. This change is not related to the 

 increase in the force of the auricular contractions that stimulation of the aug- 

 mentor nerves also causes. It varies much in amount and is less constantly 

 met with than the change in force. 3 The changes in the ventricle and auricle 

 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. 4 



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

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

 experiment of Bayliss and Starling. 5 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 latent 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. 6 The 

 after-effect may continue two minutes or more. 7 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. 8 



The simultaneous stimulation of the inhibitory and the augmenting nerves 

 of the heart, either in the vagus or separately, causes, in warm-blooded ani- 

 mals, inhibition and not augmentation. The inhibition overcomes the aug- 

 mentation, 9 but the vagus effect is diminished nevertheless. 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. 10 



1 Franck, 1890, p. 819 ; Koy and Adami, 1892, p. 240. 2 Franck, 1880, p. 85. 



3 Roy and Adami, 1892, p. 240. * Ibid. 



5 Bayliss and Starling, 1892, p. 415. 6 Baxt, 1877, p. 529. 



7 von Bezold and Bever, 1867, p. 245; Schmiedeberg, 1870, p. 136; 1871, p. 43; Boehm, 

 1875, p. 273. 8 Baxt, 1877, p. 536. 



9 Bowditch, 1873, p. 273; Baxt, 1875, p. 204; Boehm, 1875, p. 278. 



10 Bayliss and Starling, 1892, p. 414. For further discussion of the effects of simultaneous 

 stimulation, see Meltzer, 1892, p. 376. 



