THE EFFERENT CARDIAC NERVES 189 



vagus stimulation than otherwise, but this might be caused by the longer 

 pause, affording more time for relaxation (0. Frank, F. B. Hofmann). 



The following observations have been made with regard to the way in which 

 the vagus acts upon the different divisions of the mammalian heart. The inhib- 

 iting influence extends not only to the heart itself, but also to the central veins, 

 so that their contractions may completely cease on vagus stimulation (Knoll). 

 With reference to the auricles it is unanimously asserted that it is the force of 

 contraction which is primarily diminished, and that it may even happen, in spite 

 of a considerable decrease in the extent of the contraction, that the rhythm 

 remains entirely unaltered. On the other hand, it invariably happens that a 

 fall in frequency is accompanied by a reduction in the size of the contraction. 



Results differ somewhat as to the behavior of the ventricles. It seems, how- 

 ever, to be pretty certain that with weak stimulation where the heart beats are 

 not retarded very much, the contractions of the ventricles are somewhat stronger 

 than otherwise ; and that with stronger stimulation and considerable retardation 

 the contractions become weaker. The augmentation in the first instance need 

 not be a direct effect of the vagus, for it may be due to the fact that with a 

 slower cadence a greater volume of blood is at the disposal of the heart at each 

 systole ; besides, it must not be forgotten that with the longer diastole the blood 

 pressure in the arteries must fall, so that the heart has less resistance to overcome. 



The cause of the reduction in frequency, or the complete standstill of the 

 ventricle effected by the vagus is to be sought in a direct effect on the ventricles 

 themselves. One would think that when the auricles are stopped they would 

 no longer discharge impulses to the ventricles. While this is possible it does 

 not seem very probable, at least for the mammalian heart, for under certain 

 circumstances the ventricles may beat at the rhythm of the great veins while 

 the auricles are perfectly quiescent (Knoll). Besides, the power of the ven- 

 tricles to beat rhythmically when isolated from the auricles is so great that 

 mere stoppage of the auricles may not necessarily affect the ventricles. Vagus 

 retardation may, however, be brought about in such a way that the impulse 

 cannot be propagated from auricles to ventricles. Thus there are cases where 

 the auricles beat at a more rapid rhythm than the ventricles, although the 

 excitability of the latter is not diminished in the least. Finally, it has been 

 shown that when the heart is brought to a complete standstill by strong vagus 

 excitation, the cardiac muscle is less excitable to direct stimulation and can- 

 not be roused to contractions so extensive as is normally the case. All of 

 which bears out the statement that the vagus acts directly on the ventricular 

 muscle. 



Engelmann describes these effects of vagus excitation as negatively chrono- 

 tropic (retarding), negatively inotropic (weakening), negatively dromotropic 

 (diminishing the conductivity), and negatively bathmotropic (reducing the irri- 

 tability) ; and is inclined to the assumption that they are brought about by four 

 special sets of nerve fibers. This conception, based on the frog's heart, receives 

 some support from Pawlow's observations on the stimulation of separate fibers 

 in the cardiac plexus of the dog, according to which either the force or the fre- 

 quency of the heart beat could be influenced either in a positive or negative 

 direction, according to the fibers stimulated. Other authors take the view that 

 the inhibitory nerves consist of only one set of fibers, and that the different 



