THE CARDIAC NERVES. 535 



on the contrary, that only one kind of fiber is present, and that 

 its influence on the metabolic changes in the heart muscle expresses 

 itself differently upon the several different properties of the tissue 

 according to the extent of its action. 



Engelmann has made the most complete attempt to analyze the influence 

 exerted by the cardiac nerves (inhibitory and accelerator). He designates 

 these influences under four different heads with the further supposition that 

 they are mediated by different fibers : (1) The chronotropic influence, affecting 

 the rate of contraction, positive 'chronotropic actions causing an acceleration 

 and negative chronotropic actions a slowing of the rate. (2) The bathmo- 

 tropic influence, affecting the irritability of the muscular tissue; this also may 

 be positive or negative. (3) The dromotropic influence, positive or negative, 

 affecting the conductivity of the tissue. (4) The inotropic influence, posi- 

 tive or negative, affecting the force or energy of the contractions.* 



Does the Vagus Affect Both Auricle and Ventricle? The 



inhibitory action of the vagus is most marked upon the venous 

 end of the heart, and the question has arisen as to whether it affects 

 the ventricle directly or not. Gaskell gave evidence to indicate 

 that in the terrapin the auricle only is inhibited, the ventricle stop- 

 ping because it fails to receive its normal impulse from the 

 auricle. When this heart is inhibited the contractions of the 

 auricle after cessation of inhibition gradually increase in amplitude 

 until the normal size is reached; in the ventricle, on the contrary, 

 the first contraction after inhibition is of normal size or greater 

 than normal (see Fig. 219). When a block is produced in the 

 mammalian heart between auricle and ventricle by clamping the 

 connecting muscular bundle, for instance stimulation of the 

 vagus stops the auricle only (Erlanger), and this result would seem 

 to indicate that the vagus affects only the auricle, unless it is as- 

 sumed that the clamp has interrupted the inhibitory paths to the 

 ventricle. On the other hand, in favor of the view that the vagus 

 fibers reach the ventricle and influence its beats directly, we have the 

 fact, emphasized by Tigerstedt, namely, that when the connection 

 between auricle and ventricle is severed suddenly the ventricle 

 frequently continues to beat at its own rhythm without any obvious 

 pause. It would seem from this fact that when the whole heart 

 is inhibited by stimulation of the vagus the ventricle does not 

 stop simply because the auricle fails to send on its usual contraction 

 wave, since, if that were so, cutting off the auricle or clamping the 

 connection between it and the ventricle should also bring on a 

 ventricular pause, as happens in the case of the terrapin's heart. 

 It seems, however, to be the general belief of those who have experi- 

 mented with the subject that the action of the vagus is exerted 

 mainly upon the auricles. 



* Engelmann, "Archiv f. Physiologic," 1900, p. 313, and 1902, suppl. 

 volume, p. 1. 



