306 PHYSIOLOGY CHAP. 



at different times, i.e. first in the ventricle, then in the auricles, 

 lastly in the sinus and the venae cavae, all go to prove that 

 automaticity in the various parts of the heart decreases gradually ' 

 from sinus to auricles, from auricles to ventricle. To these facts, 

 as determined on the heart of the frog, we may add those observed 

 by Mac William on the fish's heart. When it is divided into 

 segments each of these continues to beat, but each with its own 

 proper rhythm the more slowly in proportion as the segment is 

 farther away from the veins opening into the auricles. 



In mammals, again, similar phenomena may be observed. By 

 an ingenious operative method Tigerstedt succeeded in the rabbit 

 in completely separating the auricles from the ventricles without 

 interrupting the circulation, and found that the latter continued 

 to beat, although the rhythm was markedly slower. 



Porter, on extending the experiments with artificial perfusion 

 through the coronary system, found that the rhythm persisted even 

 in isolated sections of the ventricles connected with the rest of 

 the heart by a single branch of the coronary artery. 



It may therefore, generally speaking, be concluded that every 



'- segment of the heart, whether in poikilothermic or in homothermic 



I' 1 animals, is endowed with rhythmical automatic excitability, which 



decreases from the sinus venosus downwards, from the mouths of 



the venae cavae to the auricles, from the auricles to the ventricles, 



Other experiments show that the rhythmical activity of the 

 | more automatic determines the rhythm of the less automatic 

 ' segments. Gaskell found that changes of temperature localised at 

 the sinus in the frog's heart modified the frequency of the beats of 

 the whole heart, as if the heart had been warmed or cooled in toto. 

 Adam experimented by the same method on the heart of cat and 

 rabbit. He saw that when an area between the mouth of the two 

 venae cavae near the lowest segment of the wall of the right 

 auricle was warmed or cooled, the frequency of the beats of the 

 whole heart was modified. No effect was produced on varying 

 the temperature of the venae cavae or pulmonary veins, of the 

 two auricular appendages, or the walls of the left auricle. It is, 

 '/ therefore, evident that the sinus venosus in cold-blooded animals 

 and the orifice of the venae cavae as indicated by Adam on the 

 mammalian heart, represent the most automatic parts, in which 

 the pulsatory cycle is initiated, and on which the frequency of 

 rhythm of the entire heart depends. 



When automatic excitability is played out, reflex excitability 

 usually persists for some time ; when the heart is stimulated either 

 by a localised mechanical shock or by an induction shock, it reacts 

 with a beat or a series of beats, which are either transmitted to the 

 other segments of the heart, or remain circumscribed in the 

 segment stimulated. 



When reflex excitability is extinguished, the reaction to 



