304 Mr, J. A. Mac William. The Mechanism and 



contractions, but is greatly favoured by rapidity of sequence of the contractions. 

 Such depressing agencies are of various kinds — cooling, intra- vascular 

 injection of potassium salts, bile, over-doses of many drugs, etc., including 

 some substances that are in suitable doses useful as remedial agents 

 promoting recovery from fibrillation ; (2) by excessive rapidity of excita- 

 tion, e.g., by electrical stimulation. This (2) may be the sole cause of the 

 alteration in conduction, or it may co-operate with a depressing influence 

 acting directly on conduction, i.e. a combination of (1) and (2) is specially 

 effective. 



Change in Mode of Conduction due to Direct Depression. 



Fibrillar Beats. — That depression of conductivity is of fundamental 

 importance is evidenced by the fact that individual beats may be " fibrillar " 

 in character (fig. 1). This is strikingly realised on palpation ; instead of the 



M 



M 





3 J J ■ 



J i i i * * i 1 



A. B. 



Fig. 1. — The systolic movement of the lever is upward. A = normal beats. 

 B = fibrillar beats, which are strikingly wiry on palpation. 



usual sensation of uniform hardening at each systole, the contraction is felt 

 to be passing in asynchronous fashion along the different systems of fasoicuU 

 or bands of fibres, some feeling firm and contracted, with the characteristic 

 wiry feeling, while others are soft and relaxed. On the surface of the 

 ventricles the contraction wave is visibly slowed, and in the auricles this 

 may be very strikingly evident in its progress over the muscle.* In this 

 condition the nature of the ventricular beat is similar, whether it occurs in 

 response to an impulse travelling down the A-V. conducting system, or is 

 excited by a direct stimulus applied to the outer surface of the ventricles. 

 The fascicular dissociation is evident even when the impulse is distributed 

 through the endings of the Purkinje system of fibres (tig. 2). 



Fibrillar beats are often able to give considerable excursions of the 

 recording lever, and they are often able to pump out a very appreciable 

 amount of blood into the aorta. The contraction and relaxation phases are 



* In the ventricles waves can often be plainly seen entering at or emerging from the 

 vortex. 



