THE CIRCULATION OF THE BLOOD 319 



one hour (Meyer); the heart of a turtle for four and a half hours (Mills). 

 The period of inhibition will depend on the strength of the electric current 

 employed, the nerve stimulated, the season of the year, etc. 



The effects on the heart-beat which will follow stimulation of the vago- 

 sympathetic in its course vary, however, because of the antagonistic action 

 of the inhibitor and accelerator nerve impulses. Thus stimulation of the 

 peripheral end of the divided trunk of the vagus in the frog or the toad with 

 weak tetanizing induced electric currents is followed by an increase in the 

 rate of the heart-beat because of the stimulation of the accelerator fibers, 

 which apparently respond before the inhibitor fibers; stimulation with some- 

 what stronger currents is followed by a diminution in the rate of the beat 

 because of the greater effect on the inhibitor nerve-fibers (Fig. 140). Stimu- 

 lation with strong tetanizing currents is followed by complete inhibition 

 (Fig. 144). 



The foregoing facts lead to the inference that the cardio-accelerator 

 and the cardio-inhibitor centers have as their function the discharge of nerve 

 impulses which are conducted by their related nerves, the efferent sympathetic 



FIG. 141. TRACING SHOWING COMPLETE INHIBITION FOLLOWING STRONG TETANIZATION 



OF THE VAGUS TRUNK. 



and vagal fibers, to the heart, and which, in a manner, as yet unexplained 

 accelerate or augment or inhibit, the action of the heart. The relation which 

 these two centers bear one to the other and the manner in which they are 

 influenced in their activities both directly and reflexly and thus regulate the 

 action of the heart from moment to moment will be considered in a subse- 

 quent paragraph. 



Changes in the Conductivity of the Heart. In addition to the 

 changes in the rate and force of the heart caused by stimulation of the inhib- 

 itor and the augmentor nerves, it is stated by Gaskell that there is also during 

 the inhibition a decrease in the conductivity of the heart at both the sino- 

 auricular and auriculo-ventricular junctions, and an increase in the con- 

 ductivity during acceleration of the beat. The decrease in conductivity 

 may be so pronounced that only every second or third contraction of the 

 auricle will be followed by a contraction of the ventricle. In other instances 

 both auricles and ventricles remain at rest while the sinus maintains its usual 

 rate. The increase in conductivity is shown by first artificially blocking the 

 contraction wave at the auriculo-ventricular junction with the clamp, until 

 only every second or third auricular contraction is conducted to the ventricle, 

 and then stimulating the sympathetic. At once the auricular contraction 

 forces the block, and passes to the ventricle, calling forth a normal contraction. 



The Physiologic Actions of the Sympathetic Nerves in Mammals. 

 In the mammal the actions of the sympathetic nerves closely resemble their 



