THE CIRCULATION OF THE BLOOD. 295 



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 3.09 



Min. 117.8 Min. 34.8 3.38 



Ave. 166.9 Ave. 52.3 3^9 



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 

 been termed the 2 :i, 3 :i, 4:1, rhythms respectively; and finally 

 when the blocking is complete no excitation process can reach the 

 ventricle. 



Owing to the capability of the mammalian ventricle to develop an 

 independent rhythm when not stimulated by the auricles for a few 

 seconds or more, it is not always possible to state at what particular 

 moment in the successive stages of compression the independent ventric- 

 ular rhythm becomes manifest. Usually when the rhythm is of the 3 : i 

 type, i.e., when the third auricular contraction fails to reach the ventricle, 

 it will begin to beat of itself. Under such circumstances the auricles 

 and ventricles become dissociated even though the block is not quite 

 complete. 



These experimental facts have afforded an explanation of the altered 

 rhythm between auricles and ventricles often found in that pathologic 

 condition known as Adams-Stokes disease. In this disease the rhythm 

 may be any one of the rhythms stated in the foregoing paragraph. In 

 two instances the following ratio of the ventricle to the auricle was 

 observed by Erlanger. 



