126 HAND-BOOK OF PHYSIOLOGY. 



this part will remain motionless, and will not beat spontaneously, although 

 it will respond to stimuli. If the heart be divided lengthwise, its parts 

 will continue to pulsate rhythmically, and the auricles may be cut up into 

 pieces, and the pieces will continue their movements of contraction. It 

 will be thus seen that the rhythmical movements appear to be more marked 

 in the parts supplied by the ganglia, and that the apical portion of the 

 ventricle, in which the ganglia are not found, does not possess the power 

 of automatic movement. Although the theory that the pulsations of the 

 rest of the heart are dependent upon that of the sinus, and to stimuli pro- 

 ceeding from it, when connection is maintained, and only to reflex stim- 

 uli when removal has taken place, cannot be absolutely upheld, yet it is 

 evident that the power of spontaneous contraction is strongest in the 

 sinus, less strong in the auricles, and less so still in the ventricle, and 

 that, therefore, the sinus ganglia are probably important in exciting the 

 rhythmical contraction of the whole heart. This is expressed in the fol- 

 lowing way: "The power of independent rhythmical contraction de- 

 creases regularly as we pass from the sinus to the ventricles," and "The 

 rhythmical power of each segment of the heart varies inversely as its dis- 

 tance from the sinus." (Gaskell.) 



It has been recently shown that, under appropriate stimuli, even the 

 extreme apex of the ventricle in the tortoise may take on rhythmical 

 contractions, or in other words may be "taught to beat" rhythmically. 

 (Gaskell.) 



Inhibition of the Heart's Action. Although, under ordinary 

 conditions, the apparatus of ganglia and nerve-fibres in the substance 

 of the heart forms the medium through which its action is excited and 

 rhythmically maintained, yet they, and, through them, the heart's con- 

 tractions, are regulated by nerves which pass to them from the higher 

 nerve-centres. These nerves are branches from the pneumogastric or 

 vagus and the sympathetic. 



The influence of the vagi nerves over the heart-beat may be shown by 

 stimulating one (especially the right) or both of the nerves when a record 

 is being taken of the beats of the frog's heart. If a single induction shock 

 be sent into the nerve, the heart, after a short interval, ceases beating, 

 but after the suppression of several beats resumes its action. As already 

 mentioned, the effect of the stimulus is not immediately seen, and one beat 

 may occur before the heart stops after the application of the electric-cur- 

 rent. The stoppage of the heart may occur apparently in one of two 

 ways, either by diminution of the strength of the systole or by increas- 

 ing the length of the diastole. The stoppage of the heart may be brought 

 about by the application of the electrodes to any part of the vagus, but 

 most effectually if they are applied near the position of Remak's ganglia. 

 It is supposed that the fibres of the vagi, therefore, terminate there in 



