THE CIRCULATION. 259 



block is produced, the ventricles cease to beat for a time, the auricles, 

 however, beating their usual rate. But after a pause from one to 

 eighty seconds the ventricle resumes its beat and gradually increases its 

 frequency. The stoppage of the ventricles is due to a want of 

 impulses from the auricles to fire them off. The subsequent slow beat 

 of the ventricle is due to an inherent power in the muscle of the 

 ventricle to beat in a rhythm. In cases of complete heart-block by 

 compression of the auriculo-ventricular bundle, irritation of the 

 accelerator nerves increases the rate of the auricles and ventricles, 

 whilst irritation of the vagus inhibits the auricle but not the ventricle. 

 Hering states that the vagus also slows the ventricle in this case. 



Similar cases of heart-block in man have taken place in the cases 

 of the disease known as Stokes-Adams. In a case of this kind Dr. 

 Alfred Stengel has discovered an atheromatous lesion in the bundle of 

 His. Clinically we may estimate the time taken in the conduction of 

 the wave of contraction from the auricles to the ventricles by the 

 duration of the "A-C" interval from the venous pulse in the jugular, 

 where "A" stands for the auricular contraction and "C" for the pulse 

 in the carotid (contraction of ventricle). Normally the A-C interval 

 is about 0.20 of a second, but where the conductivity is poor it may 

 be lengthened to 0.40 of a second or more. 



If conduction of the impulses from the auricle cannot travel 

 to the ventricles, as in the heart-block of Stokes-Adams disease, then 

 we have a beat of the auricle but not of the ventricle. The next wave 

 of stimulus, however, passes over the bundle with unusual rapidity 

 and the A-C period is shortened. Only occasional impulses may 

 fail to pass the bundle of His, and if heart-block is complete we 

 have the ventricles beating 30 per minute independent of the auricles, 

 which beat 84, as Erlanger observed in Stokes-Adams disease. H. E. 

 Hering, in contradiction to several observers has shown that the 

 papillary muscles contract before the apex of the heart, as the bundle 

 of His runs first in the papillary muscles and then gives off branches. 



A depression of conductivity results in a lengthening of the A-C 

 interval, an occasional dropping out of the ventricular systole, a 2 to 

 1 rhythm or a 3 to 1 rhythm, and a complete block, the auricular 

 rhythm being independent of and differing from the ventricular. 



Pathological and Pharmacological Actions. The alternating 

 pulse, pulsus alternans, in which large and small pulse-beats alternate 

 is a good example of defect in contractility. In -the cases of Stokes- 

 Adams disease digitalis can produce partial heart-block, whilst atropin 

 increases the auricular beats but leaves the ventricular beats unchanged. 



