304 PHYSIOLOGY CHAP. 



shall speak below. When the paroxysm is over there will in every 

 case be some isolated contractions, after which all spontaneous 

 movement ceases, although the ventricle preserves its reflex' 

 excitability, and responds to electrical and mechanical stimuli by 

 strong contractions. When the ligature falls on the junction of 

 the sinus, the periodic grouping of the cardiac contractions is 

 barely and irregularly indicated. When, lastly, it falls on the sinus 

 (1-1-5 mm. above its opening into the right auricle) no trace of 

 periodic grouping is visible. 



These facts obviously exclude the hypothesis that the periodic 

 rhythm observed by us during the above experimental conditions 

 depends exclusively on a specific toxic action of the serum, or on 

 asphyxia, due to its non- renewal, as was held by Rossbach 

 Langendorff and others ; while they show us just as convincingly, 

 that in the case of our experiments the fundamental determining 

 condition of the phenomenon consists in the physiological exclusion 

 of the sinus venosus effected by the ligature. 



They further show that the rhythmical activity of the heart is 



LHUILULUIL 



FIG. 135. Crisis of periodic rhythm in frog's heart itied 3 mm. above the auriculo- ventricular 

 groove, and filled with pig's serum. (Luciani.) Shows gradual dissolution' of groups into 

 isolated beats. 



most highly developed in the venae cavae and sinus venosus 

 (where lie the conspicuous ganglionic masses described by Eemak), 

 after the separation of which the rhythmical impulses probably 

 encounter resistance, and must summate before they can be 

 efficacious. Accordingly, there are long pauses, during which 

 tension is accumulated, and groups of contractions, during which 

 it is discharged again. 



In proportion as the activity of the heart exhausts itself the 

 pauses shorten, and the beats in the groups are separated by wider 

 intervals, till eventually all periodic grouping disappears. This is 

 the phenomenon we have termed the " crisis " (crisi), represented 

 by a longer or shorter series of single beats, which become con- 

 stantly rarer and weaker, until they disappear entirely, as soon as 

 the asphyxia and exhaustion of the heart are complete (Fig. 135). 



The crisis indicates that the resistances which determine the 

 periodic rhythm are gradually diminishing, and that the heart is 

 slowly adapting itself to the new conditions produced by the 

 ligature. Eenewal of the serum after the crisis has set in does 

 not reinstate the periodic rhythm. This is a striking fact, and in 

 our opinion indicates that the adaptation has been complete. 



IV. At the time at which we took up these physiological 



