ix CARDIAC MUSCLE AND NERVES 321 



pause which succeeds the extrasystole, attributed it to the 

 intrinsic nervous system of the heart, because in experiments with 

 the apex, where there were supposed to be no ganglia, the extra- 

 systole appeared, but not the compensatory pause observed upon 

 the intact heart supplied with ganglia (Dastre, Marcacci, Gley, 

 Keiser). Subsequently, however, Engelmann showed that the 

 compensatory pause can be obtained at the apex also, previous 

 observers having failed to detect it because they employed a 

 tetanising current as stimulus, instead of single make or break 

 shocks. He demonstrated that the compensatory pause may fail 

 in the entire ventricle also, with the constant current. P>ottazzi r 

 on the other hand, observed the compensatory pause on the 

 embryonic heart of the chick, which excluded the possibility of its 

 being essentially conditioned by the nervous elements. 



It .has frequently been noted, on stimulating the heart at the 

 sino-auricular junction, that not only is it possible to obtain an 

 extrasystole with weak currents that would be ineffectual beyond 

 these limits, but that a series of rhythmical beats, of greater 

 frequency than the normal, may also occur (Langendorff, Keiser). 

 This was explained by the presence of ganglion cells in these 

 parts. But in view of Gaskell's discovery that it is just these 

 parts which contain the more embryonic muscle cells, endowed 

 with a marked automatic rhythm, the difference observed in the 

 response may obviously depend rather upon these muscular 

 elements than upon the ganglia. 



The refractory phase of the cardiac cycle accounts for the 

 regular alternation of systole and diastole, and explains why it is . 

 difficult to produce a true tetanus of the heart by means of a , 

 tetanising current, i.e. to fuse a number of contractions into a 

 single very marked and persistent one, as in the case of ordinary 

 muscle (Kronecker and Stirling). 



The direct action of tetanising currents upon the surface of the , 

 mammalian heart produces the strange effect termed by Ludwig , 

 and Hoffa (1849) delirium cordis. This is a wholly unco-ordinated 

 activity of cardiac muscle, in which it contracts at isolated points, 

 and simultaneously relaxes at others, so that the mechanical work 

 of the heart becomes impossible. Co-ordinated activity may be 

 resumed after a delirium lasting for several minutes, but only 

 when the current has not been unduly strong, nor the stimulation 

 too prolonged. The origin of this phenomenon has been variously 

 explained. MacWilliam held it to be independent of nervous 

 influences, and merely the effect of direct excitation and altered 

 conduction in the muscle cells. Kronecker, on the contrary, inter- 

 prets delirium cordis as the functional disturbance of a nervous 

 centre of co-ordination for cardiac movements, situated in the 

 upper third of the interventricular septum. He showed that in 

 the dog, and frequently in the rabbit also, it was only necessary 



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