THE CAUSATION OF THE HEART BEAT 987 



muscle cell to another, or by means of nerve fibres which would excite the 

 muscular tissue of each cavity to contract. 



The distinct pause which intervenes between the contractions of auricles 

 and ventricle was long regarded as evidence for the nervous character of the 

 contraction, and as showing the operation of a nerve centre in the co- 

 ordination of the contractions of different cavities. A contraction wave 

 may however be started at any part of the heart and may travel from this 

 to all other parts. Thus, although the normal direction of the contractions 

 is from sinus to ventricle, it is possible, by stimulating the apex of the 

 ventricle, to excite contractions in the reverse order, viz. from ventricle to 

 sinus. Such a fact is at variance with all our present knowledge of excitation 



FIG. 434. Heart of tortoise with auricle slit up so as to cause a partial 

 block. (GASKELL.) 



of motor nerves. Excitation of the nerve going to the sartorius, or of any part 

 of the nerve, may excite contractions of all the fibres of which the muscle 

 is composed. On the other hand, excitation of a part of the muscle which 

 is free from nerve fibres causes a contraction which is limited to the muscle 

 fibres directly excited and does not extend to the nerves. If motor nerves 

 arose from the hypothetical motor ganglion of the heart and passed to the 

 ventricular muscle, one would not expect that contraction of the ventricular 

 muscle could excite these nerves and so cause the propagation of a wave of 

 contraction in the reverse direction. 



That the propagation cannot be due to any nerve trunks running from 

 sinus to ventricle is shown by various experiments of Engelmann and 

 Gaskell. Thus, if the auricle is slit up by a series of interdigitating cuts, the 

 contraction wave starting from the sinus travels along the auricular muscle 

 around the end of each section and finally, on arrival at the ventricle, 

 causes a contraction of this cavity. In the heart of the tortoise the nerve 

 trunks run, not in the interauricular septum, but in a band of tissue joining 

 the sinus to the ventricle ; this band can be exercised with all its contained 



