516 CIRCULATION OF BLOOD AND LYMPH. 



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. 212). When a block is produced in the 

 mammalian heart between auricle and ventricle by clamping the 

 uniting muscular bundle, for instance stimulation of the vagus 

 stops the auricle only (Erlanger), but this result may be due to 

 the fact that the clamp has interrupted the inhibitory fibers on 

 their way to the ventricle, or to some other reason connected with 

 the peculiar properties of the auriculo- ventricular muscle bundle. 

 As far as the mammalian heart is concerned, one must believe 

 logically that the vagus affects the ventricle directly, as Tiger- 

 stedt has well said, from the mere fact that, when the con- 

 nection between auricle and ventricle is severed or paralyzed 

 the ventricle continues to beat at its own rhythm without any 

 obvious pause. It is evident 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 experimented with the subject that the action of the vagus 

 is exerted mainly upon the auricles. 



* Engelmann, "Archiv f. Physiologie," 1900, p. 313, and 1902, suppL 

 volume, p. 1. 



