132 HANDBOOK OF PHYSIOLOGY. 



nius' experiments) however, complicate if they do not contradict the 



above explanation. 



If a ligature be tightly tied round the heart over the situation 



of the ganglia between the 

 sinus and the auricles, the 

 heart below the ligature stops 

 beating. The ligature might 

 be supposed to stimulate the 

 inhibitory ganglia, but for the 

 remarkable fact that the ex- 

 hibition of atropin does not 

 interfere with the success of 

 the experiment. 



Section of the heart at the 



f situation ha s f n 



ventricle without complete stoppage during irritation has (experiment xf, p. loO) a 

 of the vagus. (From Brunton, after Gaskell.) similar effect to ligature. 



Again, if the ventricle be 



separated from the auricles by ligature or by section, it will recommence 

 its pulsation and continue to beat rhythmically, but the auricles will con- 

 tinue at a standstill. It has been suggested as an alternative explanation 

 of these further experiments that the sinus contains the chief motor 

 ganglia of the heart, and that from it as a rule proceed the impulses 

 which cause the sequence of contraction of the other parts ; that 

 the auricles contain inhibitory ganglia which are not sufficiently power- 

 ful to prevent the motor impulses from the sinus ganglia, but that when 

 their influence is removed by section, by ligature, or by excessive stimu- 

 lation that the inhibitory ganglia are able to prevent the rhythmical con- 

 traction of the auricles and ventricle, but that the ventricle contains in- 

 dependent motor ganglia, since when it is removed from the influence 

 of the inhibitory ganglia of the auricles, it recommences rhythmical pul- 

 sation. 



Even if this theory cannot be absolutely maintained, yet it is evident 

 that the power of spontaneous contraction is strongest in the siuus, less 

 strong in the auricles, and less so still in the ventricle, and that, there- 

 fore, the sinus ganglia are important in exciting the rhythmical contrac- 

 tion of the whole heart. 



So far, the effect of the terminal apparatus of the vagi only has been 

 considered; there is, however, no doubt that the vagi nerves are simply 

 the media of an inhibitory or restraining influence over the action of the 

 heart, which is conveyed through them from a centre in the medulla ob- 

 longata which is always in operation, and, because of its restraining the 

 heart's action, is called the cardio-inhibitory centre. For, on dividing 

 these nerves, the pulsations of the heart are increased in frequency, an 

 effect opposite to that produced by stimulation of their divided (peri- 

 pheral) ends. The restraining influence of the centre in the medulla 

 may be reflexly increased, so as to produce slowing or stoppage of the 

 heart, through impulses from it passing down the vagi. As an example 

 of the latter, the well-known effect on the heart of a violent blow on the 



