316 PHYSIOLOGY CHAP. 



the septum (across the right auricle). Each time the incision into 

 the bundle of His is complete (as can subsequently be ascertained 

 anatomically) a sharp functional dissociation between the beats of 

 the auricles and those of the ventricles, a true allorhythmia, 

 appears when the cardiac cycle recommences. Hering studied this 

 effect on ten dogs' hearts, with results as follows : 



(a) The ventricles beat more slowly than the auricles (Fig. 140). 



(&) The wave of contraction is neither transmitted from the 

 auricles to the ventricles, nor vice versa, whether it be spontaneous, 

 or determined by external artificial stimuli. 



(e) Both auricles and ventricles possess independent automatic 

 activity. 



From these experimental data Hering deduces a new argument 

 in favour of niyogenic conduction, adopting the ideas previously 



FIG. 140. Tracing of beats of auricle (A) and ventricle (F) in a dog's heart, in which 

 the bundle of His had been cut. Time in seconds. (H. E. Hering.) 



brought forward by Gaskell in connection with the amphibian 



heart. 



According to Gaskell, communicating fibres between the sinus 

 and auricles, and the auricles and ventricle, present certain 

 morphological peculiarities in which they approximate to embryonic 

 cardiac fibres. These less differentiated fibres, which in arrange- 

 ment and structure resemble those of the primitive cardiac sheath, 

 have also from the physiological point of view preserved a more 

 embryonic character, since they are endowed with a far higher 

 degree of automatism, and probably (according to Gaskell) conduct 

 the excitation from cell to cell far more slowly. This is sufficient 

 to explain in the simplest possible manner, why the contraction 

 wave arising in the sinus is not of uniform velocity, but is delayed 

 at the limits of the several segments, breaking up into contractions 

 of sinus, auricles, ventricle, and bulbus arteriosus. 



The highly developed automatic excitability of the muscle cells 

 of the venae cavae and sinus explains why these parts govern the 

 rhythm of all the remaining segments of the heart, where automatic 



