IX 



CAEDIAC MUSCLE AND NEK YES 



315 



ventricular fibres forms a diffuse and complex muscular ramifica- 

 tion, which continues uninterruptedly from auricles to ventricles. 

 From this system a short bundle of muscle goes out at the back to 

 the coronary sinus, where it joins the ordinary muscles of the 

 auricle. Another bundle runs inwards from the muscular system, 

 towards the muscles of the ventricle, and bifurcates at the two 

 walls of the septum. At the base these two branches break up 

 into a number of small bundles, some of which enter the musculi 

 papillari, while others spread over the whole internal surface of 

 the endocardium, passing either to the apex or the base of the 

 ventricles. Throughout its course the auriculo-ventricular bundle 

 is separated by connective tissues from the cardiac muscle proper, 

 and connects with the fibres of the ventricle by its terminal 

 branches only. 



It would be interesting to study the effects on cardiac rhythm 

 of interrupting the 



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Fin. 139. Tracing of beats of auricle (A) and ventricle (B) of small, 

 isolated dog's heart. At 7 a thread was tied round the bundle of 

 His, after which a decided alloihythmia of the two cardiac seg- 

 ments is apparent. (Humblet.) 



conduction of ex- 

 citation along this 

 system of muscle 

 fibres, which passes 

 u n i n t e r r u ptedly 

 fr< im auricles to 

 ventricles. Some 

 physiologists have 

 attempted the ex- 

 periment on the 

 isolated surviving 

 heart of mammalia : e.g. His, jun., Graupuer, Erlanger, and especially 

 H. E. Hering. Gentle compression of the principal bundles increases 

 the interval between presystole and systole, without altering their 

 normal sequence. Stronger compression produces the allorhythmia, 

 in which 2, 3, or 4 presystoles correspond with a single systole 

 (Fig. 139). Marked and sudden compression when the cardiac 

 rhythm is very frequent and intense may produce a longer or 

 shorter arrest in systole, after which the ventricles begin to beat 

 with a rhythm of their own, independent of that of the auricles. 

 The same results occur when the conductivity of the auriculo- 

 veiitricular bundle is impaired by cooling, while the excitability 

 of the auricles is simultaneously raised by warming. 



The experiments which most definitely bring out the im- 

 portance of this bundle as the bridge across which the wave of 

 contraction passes from auricles to ventricles, are those of H. E. 

 Hering (1905) on the dog's heart. He kills the animal, and 

 then revives the heart (without isolating it from the body), 

 perfusing Ringer's physiological solution from the aorta through 

 the coronaries. While the heart is still motionless, he divides the 

 auriculo-ventricular bundle by a comparatively small incision in 



