212 



THE CONTRACTION OF CARDIAC MUSCLE. 



to the first Stannius ligature the auricle and ventricle at first remain 

 absolutely quiescent naturally, and then begin to contract with their 

 own independent rhythm. It is then found that the contractions of the 

 auricle are diminished in force when the vagus nerve in the neck is 

 stimulated, showing that the vagus in the neck can affect the auricular 

 muscle, through the coronary nerve. 



This experiment shows that the vagus produces an effect upon the 

 muscle of the auricle, which is manifested as a diminution of the force 

 of its contractions, as soon as the auricle begins to beat. It follows 

 clearly that this action of the nerve on the muscle must still exist 

 during the period of quiescence, before the spontaneous contractions 

 begin, although the stimulation of the nerve at this time produces no 

 visible change of form. 



This invisible action of the nerve on the muscle can be rendered 



FIG. 117. Method of preparation of the heart of the tortoise in order to 

 demonstrate the action of the vagus nerve on the quiescent auricles. S, 

 sinus and part of auricle ; C, the coronary nerve and vein ; V, the 

 right vagus nerve ; E, electrodes connected with interrupted current ; 

 G, non-polarisable electrodes connected with the galvanometer. 



visible by means of the galvanometer, and it is then seen that the action 

 of the inhibitory nerve is of the opposite character to that of a motor 

 nerve, in that it produces a positive and not a negative variation of the 

 demarcation current. 1 



The experiment is carried out in the following way : 



The heart is cut through between the sinus and the auricles so as to leave 

 the coronary nerve intact, and the apex of one auricle and that of the ventricle 

 are attached to the levers in the usual way. Fig. 117 shows the preparation. 

 The apex of the auricle is heated, so as to cause a strong demarcation current, 

 and non-polarisable electrodes are placed on the heated apex and on the, 

 uninjured auricular tissue near its base. The electrodes are attached to the 

 galvanometer, and the demarcation current is registered every five seconds by 

 an observer stationed at the galvanometer, who is therefore not necessarily aware 



1 Gaskell, Beitr. z. Pliysiol. C. Ludwig z. s. 70 Geburtst. gew., Leipzig, 1887, S. 114; 

 also Journ. PhysioL, Cambridge and London, 1887, vol. viii. p. 404. 



