3 o 4 TEXT-BOOK OF PHYSIOLOGY 



ably the central part of the conducting system, the muscle bundle of 

 His. This has been accomplished in the dog by Erlanger by means 

 of a specially devised hook clamp (Fig. 134), which consists of an L- 

 shaped hook of steel wire the arm of which can be made to approach 

 a brass block by means of a bolt and screw. The L-shaped hook is 

 inserted into the right wall of the aorta, then passed downward and 

 backward into the left ventricle, then pushed through the ventricular 

 septum into the right ventricle. In this position it lies under the 

 auriculo-ventricular bundle. Compression is now brought about by 

 approximating the hook to the brass block by means of the nut. 

 When the compression is brought about suddenly and completely the 

 ventricles at once cease beating, though the auricles continue to beat 

 with their customary rate and regularity. After a variable period of 

 time, varying from a few seconds to 70 seconds, during which the 

 ventricles are relaxed and gradually filling with blood from the auri- 

 cles, the ventricular beat returns, at first slowly but with a gradually 

 increasing frequency until a definite but a comparatively slow rate is 

 attained. The rhythm thus developed is termed the idio-ventricular 

 rhythm. 



In experiments on the dog heart performed by Erlanger the following 

 results were obtained when the auriculo-ventricular bundle was com- 

 pletely crushed. 



Aur. rate per minute. Ven. rate per minute. Ratio of Aur. to Ven. 



Max. 216 Max. 69.8 3-9 



Min. 117.8 Min. 34.8 3-38 



Ave. 166.9 Ave. 52.3 3.19 



The reason assigned for the cessation of the ventricular contraction 

 is the non-arrival of the excitation process at the ventricular end of the 

 conducting system, because of the blocking or compression. Under 

 physiologic conditions the ventricular beat is directly dependent on the 

 arrival of the excitation process from the auricles and if it fails to arrive 

 the ventricle does not contract for some seconds. The return of the 

 beat during complete blocking is attributed to the development of a 

 hitherto dormant inherent rhythmicity. When this is established both 

 auricles and ventricles continue to beat though with totally different 

 rhythms. 



The effects which follow gradual compression of the muscle-bundle 

 are somewhat different from those which follow sudden compression. 

 If the clamp is accurately adjusted and the compression gradually 

 applied, the first perceptible effect is a lengthening of the normal pause, 

 the inter-systolic, between the auricular and the ventricular contraction. 

 With an increase in the compression there will come a moment when 

 one of the auricular contraction waves fails to reach the ventricle, or if it 

 does, it is so enfeebled that it is incapable of exciting the ventricle, 

 which in consequence fails to contract. This dropping out of a ven- 

 tricular contraction may occur once in every 10, 9, 8, 7, 6, etc., auricular 

 beats, in accordance with the degree of compression. With a further 

 tightening of the clamp, the blocking of the excitation process may be 

 still further increased so that only every second, third, or fourth auricular 

 beat is capable of developing a ventricular beat, establishing what has 



