208 THE CONTRACTION OF CARDIAC MUSCLE. 



Such curves are to be explained as follows : 



If the muscular fibres are initiating a rhythm, then the effect of 

 stimulating the fibres of the vagus which supply them is to diminish 

 both rate and contraction force, as is the case in the sinus muscle ; if 

 the muscle fibres are not initiating a rhythm, but only responding to a 

 regular series of contraction waves from the sinus, then the stimulation 

 of the fibres of the vagus which supply these muscle fibres cannot 

 influence rate, but only the force of their contraction, as is seen in the 

 case of the auricle when the coronary nerve is stimulated. 



This experiment proves that the nerve fibres of the vagus which 

 diminish the force of the auricular contractions act in the same 

 manner after they have passed the ganglia in the sinus as before. 

 Still it does not prove that the post-ganglionic fibres which pass directly 

 into the auricular muscle act in the same way as the pre-ganglionic 

 fibres, for at the junction of auricle and ventricle groups of nerve cells 

 exist with which the fibres of the coronary nerve or some of them for 

 many, if not all of them, are medullated do certainly have connection : 

 and it is possible to argue that these cells are part and parcel of an 

 inhibitory centre, just as much as those in the sinus. It is, however, 

 easy to show that the post-ganglionic nerve fibres in the muscle itself 

 still produce the same effect upon the auricular contractions as the 

 pre-ganglionic fibres. 



A strip of muscle is taken from the apex of the auricle, suspended and 

 attached by means of a thread at one extremity to a lever, so as to register its 

 contractions. Single induction shocks at regular intervals are sent in to the 

 muscle at the fixed end ; and by means of very thin wires, fixed at the 

 two extremities of the muscle, which do not interfere with the free movements 

 of the muscle strip, a weak interrupted current can be sent through the whole 

 length of the strip. When the strip is first suspended, each contraction is 

 able to travel only a little way, owing to the number of blocking places caused 

 by the section. 



These blocking places are gradually removed, their removal being assisted 

 by the application of a weak interrupted current at intervals, until at last 

 every single induction shock produces a strong contraction which travels 

 quickly and uniformly from one end of the strip to the other. When this 

 result has been obtained, the strip will often begin to beat spontaneously. If 

 now a weak interrupted current, too weak to cause any contraction, is sent 

 through the strip, whether the contractions are spontaneous or not, these 

 contractions are diminished in size, or even inhibited altogether, just as if the 

 vagus nerve had been stimulated. Further, atropine applied to the strip 

 removes this effect of the interrupted current, just as it removes the action of 

 the vagus nerve. Finally, if the strip be clamped in the middle, and the 

 single induction shocks applied to the lower half only, while the weak 

 tetanising current is applied to the upper half, it is possible to clainp so tightly 

 that only the lower half will respond to the single induction shocks, and yet 

 these contractions will be diminished in size by the stimulus of the weak 

 interrupted current applied to the upper half only. Clamp tighter, and then 

 the weak interrupted current produces no effect upon the contractions of the 

 lower half. In other words, the upper part of the strip behaves like a nerve 

 to the lower half of the strip. 



These experiments are, it appears to me, very strong evidence that 

 the post-ganglionic inhibitory fibres in the auricular muscle itself act in 

 the same way as the pre-ganglionic, and that such fibres are stimulated 



