THE CAUSATION OF THE HEAKT-BEAT 1063 



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 ven- 

 tricles ; this band can be excised with all its contained nerves 

 without interfering in any way with the normal sequence of contrac- 

 tions. Moreover the pause observed between the contractions of 

 auricles and ventricles has been shown by Gaskell to be due to the 

 retardation of the excitatory wave which occurs in its propagation 



FIG. 427. Heart of tortoise with auricle slit up so as to cause a partial 

 block. (GASKELL.) 



through the muscular tissue in th uriculo-ventricular junction. A 

 similar retardation of the wave can be produced at any point either 

 in auricles or ventricle by diminishing the conducting muscular tissue 

 to a sufficiently small extent. Thus, if the auricle of the tortoise be 

 divided as in the diagram* (Fig. 427), it will be noticed that the sinus 

 first contracts, then the auricular half As ; a distinct pause then 

 occurs while the contractile process is passing over the * bridge/ and 

 finally Av contracts, followed by the ventricle. The apparent pause 

 between the contraction of the auricles and ventricle is due therefore 

 to a partial ' block ' at the auriculo-ventricular junction. If the 

 block be increased in the experiment just quoted, as, for instance, 

 by allowing the bridge of tissue to dry or by making it still 

 narrower, it may be found that only one out of every two con- 

 tractions passes across the bridge (Fig. 428), and the slightest 

 increase in the resistance to the propagation of the wave may lead to 



