CH. XX.] THE CARDIAC SYMPATHETIC 249 



inhibitory fibres than the left, but this is by no means a constant rule. 

 One can always obtain good inhibition if the stimulus is applied to 

 the wall of the sinus ; here one stimulates the post-ganglionic fibres 

 which originate from the nerve-cells in the sinus ganglion around 

 which the vagi terminate. 



The effect of the stimulus is not immediately seen ; one or more 

 beats may occur before stoppage of the heart takes place, and slight 

 stimulation may produce only^ slowing and not complete stoppage of 

 the heart (fig. 256). The stoppage may be due either to prolongation 

 of the diastole, as is usually the case, or to diminution of the systole. 

 Vagus stimulation lessens the conductivity of the cardiac tissue, 

 but it does not abolish the irritability of the heart-muscle, since 

 mechanical stimulation may bring out a beat during the stand-still 

 caused by vagus stimulation. The inhibition of the beats varies in 

 duration, but if the stimulation is a prolonged one, the beats reappear 

 before the current is shut off. This is known as " vagus escape," and 

 occurs in all animals; it is probably due to fatigue of the vagal 

 endings. 



The Sympathetic. The influence of the sympathetic is the 

 reverse of that of the vagus. Stimulation of the sympathetic 

 produces acceleration of the heart-beats, and according to most 

 observers, section of the nerve produces no slowing. Hence the 

 nerve is not in constant action like the vagus. The acceleration 

 produced by stimulation of the sympathetic fibres is accompanied by 

 increased force, and so the action of the nerve is also termed 

 augmentor. It is probable that the augmentor fibres are distinct 

 from the accelerator fibres, because in mammals one or two of the 

 small nerves leaving the stellate ganglion on stimulation produce 

 augmentation without acceleration. 



The fibres of the sympathetic system which influence the heart- 

 beat in the frog, leave the spinal cord by the anterior root of the 

 third spinal nerve, and pass by the ramus communicans to the third 

 sympathetic ganglion, then to the second sympathetic ganglion, then 

 by the annulus of Vieussens (round the subclavian artery) to the first 

 sympathetic ganglion, and finally in the main trunk of the sympa- 

 thetic, to near the exit of the vagus from the cranium, where it joins 

 that nerve and runs down to the heart within its sheath, forming the 

 joint vago-sympathetic trunk. These fibres are indicated by the dark 

 line in fig. 257. The fibres of the sympathetic seen running up into 

 the skull are for the supply of blood-vessels there. It should be noted 

 that the frog has no spinal accessory nerve. 



In the mammal the sympathetic fibres leave the cord by the 

 second and third dorsal nerves, and possibly by anterior roots of two 

 or more lower nerves ; they pass by the rami communicantes to the 

 ganglion stellatum, or first thoracic ganglion, and thence by the 



