330 PHYSIOLOGY CHAP. 



The acceleration may rise from 7 per cent to a maximal 

 70 per cent, according to the prevailing frequency of rhythm. 

 The difference in effect depends principally on the frequency of 

 rhythm previous to excitation. The maximum of acceleration is 

 not increased when the accelerators on both sides are excited 

 simultaneously. The duration of the after-effect is in proportion 

 with the duration of the stimulus. 



The positive inotropic and dromotropic effects must be dis- 

 tinguished from acceleration or positive chronotropic effects. 

 Heidenhain and Gaskell observed on the frog that stimulation of 

 the sympathetic fibres increases the height of systole and shortens 

 diastole by raising the tonicity of cardiac muscle (inotropic effect) ; 

 the capacity of the latter to transmit the excitation from one segment 

 to the next (dromotropic effect) also increases ; the effects from 

 every point of view are antagonistic to those of the vagus. These 

 facts are confirmed in essentials by the investigation carried out 



FIG. lf-2. Augmentation of ventricle beats of dog after electrical tetanisation of first left acceler- 

 ator nerve, as traced on abscissa. (Franc.ois-Franck.) 



with various experimental methods on mammals by Francois- 

 Franck, Boy and Adami, Bayliss and Starling. They appear to 

 justify the physiological name of systolic nerves given by us to 

 the cardiac branches of the sympathetic, in opposition to the 

 diastolic nerves or cardiac fibres of the vagus (Fig. 152). 



When these two nerves (which seem to be antagonistic in 

 function) are excited simultaneously, the effects are not algebraic- 

 ally summed up and cancelled, but both are expressed, first those 

 proper to the vagus, then those from the sympathetic. This 

 remarkable fact was discovered on the dog by Baxt (1875) in 

 Ludwig's laboratory. It can also be observed when the vagus is 

 excited with minimal induction currents, and the sympathetic with 

 strong currents to produce maximal effect (Fig. 153). The probable 

 interpretation, according to Baxt, is that the two kinds of nerve 

 fibres act upon the heart at two different points. During the 

 excitation of the diastolic nerve the fibres of the systolic nerve 

 cannot act, the excitability of the cardiac muscle being modified ; 

 they will confine themselves to storing up the latent excitability 

 in the ganglion, to appear as an after-effect at the close of 



