256 PHYSIOLOGY OF THE HEART [CH. XXI. 



then from the other, all the nerves must be cut through at least once, 

 and the only remaining tissue not severed is muscular, yet the strip 

 still continues to beat ; in other words, the propagation is myodromic. 

 The passage of the wave from one chamber to another is also myo- 

 dromic. The slow rate of propagation indicates that this is so, and 

 the view has been fully proved by the discovery of muscular strands, 

 passing across from one chamber to the next. 



It appears that under normal conditions the wave of contraction 

 in the heart starts at the sinus, and travels over the auricles to the 

 ventricle ; the irritability of the muscle and the power of rhythmic 

 contractility is greatest in the sinus, less in the auricles, and still less 

 in the ventricles. Under ordinary conditions the apical portion of 

 the ventricles exhibits very slight power of spontaneous contraction. 

 The importance of the sinus as the starting-point of the peristalsis 

 can be shown by warming it. If a frog's heart is warmed by bathing 

 it in warm salt solution at about body temperature, it beats faster ; 

 this is due to the sinus starting a larger number of peristaltic waves ; 

 that this is the case may be demonstrated by warming localised portions 

 of the heart by a small galvano-cautery ; if the sinus is warmed the 

 heart beats faster, but if the auricles or ventricles are warmed there 

 is no alteration in the heart's rate. The sinus in the frog's heart, 

 and that portion of the right auricle in the mammal's heart which 

 corresponds to the sinus, is always the last portion of the heart to 

 cease beating on death, or after, removal from the body (ultima 

 moriens, Harvey). This is an additional proof of the superior rhyth- 

 mical power which it possesses. The foetal remnant of the sinus 

 in the mammal's heart has been called the sino-auricular ncde (Keith 

 and Flack), and this has been believed to act as the " pace-maker " 

 of the heart. But after its removal other parts in the same region 

 which contain no Purkinje's fibres have been found quite capable 

 of maintaining the ordinary cardiac rhythm. The sino-auricular 

 node cannot therefore be the specific or exclusive pace-maker, and 

 some observers regard the final part of the large veins outside the 

 heart altogether as more important from this point of view. 



It is supposed that the wave starting at the sinus is more or less 

 blocked by a ring of lower irritability at its junction with the auricle ; 

 again, the wave in the auricle is similarly delayed in its passage over 

 to the ventricle by a ring of lesser irritability, and thus the wave of con- 

 traction is delayed at its entrance into both auricular and ventricular 

 tissue. By an arrangement of ligatures, or, better, of clamps, one part 

 of the heart may be isolated from the other portions, and the contrac- 

 tion when aroused by an induction shock may be made to stop in the 

 portion of the heart muscle in which it begins. It is not unlikely 

 that the contraction of one portion of the heart acts as a stimulus 

 to the next portion, and that clamps and ligatures prevent or " block " 



