THEORIES OF THE HEART-BEAT 177 



and its power of rhythmic contractions being greatest in the sinus, less in 

 the auricles, and least in the ventricles. By an arrangement of ligatures 

 or by a system of clamps, one part of the heart may be more or less isolated 

 from any other portion. With such a clamp the contraction waves can be 

 more or less completely interrupted in their passage from the sinus end of 

 the heart past the clamp toward the ventricular end. If the clamp is com- 

 plete, so as to interrupt the physiological continuity between the parts, then 

 any contractions in the apical portion will be entirely independent of those 

 in the sino-auricular portion. If the blocking is partial only, then the ventric- 

 ular end of the heart always contracts in unison with the sino-auricular, 

 although its rate may be as i to i, i to 2, i to 3, etc. In other words, only 

 every second or every third sino-auricular contraction will be able to pass 

 the block. 



The effects of blocking are due to the interruption of muscle continuity 

 rather than nerve continuity. This is beautifully demonstrated by an experi- 

 ment of zigzag cutting of the ventricle in the terrapin, since it cannot be 

 supposed that any nerves would pass through the ventricular mass by such 

 a zigzag course. In this experiment the contraction wave passes down 

 over the muscle and around the end of the cuts until it reaches the apex, 

 and the apex contracts in sequence with the auricle and base of the ventricle. 

 If the zigzag cuts are made almost complete so as to reduce to a minimum 

 the muscular tissue which bridges from one cut to the next, then it happens 

 that occasional contractions will be unable to pass and the apex contracts 

 in the ratio of i to 2, or i to 3, etc., as described above. Thus, division of 

 the muscle has the same effect as partial clamping in the same position. 



It was thought for a long time that in the mammal there is no mus- 

 cular continuity between the auricles and ventricles to conduct the contrac- 

 tion wave, but a well-marked muscular bridge, the bundle of His, has been 

 shown to pass between these two parts. This fact has proven to be of strongest 

 support to the myogenic theory. Erlanger has recently shown, by an in- 

 genious device for partially clamping this muscular band, that even the 

 mammalian ventricle exhibits the phenomenon of heart block. In his experi- 

 ments the ventricle contracts in unison with every auricular contraction, or 

 only every second or every third, according to the degree of blocking. 



It was shown long ago that the isolated apex of the ventricle of the frog 

 remains quiet when filled with blood, but readily gives good rhythmic con- 

 tractions in physiological saline and other artificial solutions. The inac- 

 tivity in blood is not necessarily, therefore, due to nervous isolation from 

 the ganglionated parts of the heart. Contractions occur in the small bits 

 of ventricular muscle as isolated by Gaskell, and these may continue for 

 hours. It is well known also that the embryonic heart contracts rhythmically 

 before nerve cells have reached the organ. 



The phenomena of heart block, the contractions of the ventricular apex 

 12 



