430 VETERINARY PHYSIOLOGY 



contraction wave is not regularly propagated from auricles 

 to ventricles is seen in the condition of auricular flutter 

 when, as the result of increased excitability of the sino- 

 auricular node, the normal " pacemaker," the rate of 

 contraction of the auricles is enormously increased, some- 

 times to two or three hundred per minute. In 

 such cases the contraction reaches the auriculo-ventricular 

 band while the ventricular fibres have not completed 

 their contraction while they are still in the refractory 

 phase. Hence, only one ventricular contraction for every 

 two or three auricular contractions may occur. In these 

 cases there is practically a heart-block, but the rate of the 

 ventricles is not decreased as in true heart-block. The 

 condition is fairly common, and it may be detected by the 

 study of the venous pulse, or still better, by the electro- 

 cardiogram. 



(3) Fibrillation. — While under normal conditions the 

 contractions are conducted in an orderly manner over auricles 

 and ventricles, interference with the coronary circulation with 

 the consequent decreased supply of oxygen to the wall of the 

 heart is apt to lead to marked inco-ordination of the contractions, 

 so that some bundles of fibres are contracting while others are 

 relaxing. Thus a peculiar fluttering and ineffective fibrillar 

 contraction or fibrillation is seen in the myocardium. This 

 may affect either the auricles or the ventricles. Since the 

 auricles are practically the cisterns of the heart, the con- 

 dition does not so seriously interfere with the circulation 

 when it affects them as when it affects the ventricles which 

 are the pumps. Ventricular fibrillation prevents the proper 

 expulsion of blood and soon leads to a fatal result. It may 

 be produced by powerful electrical stimulation of the 

 ventricles and is one factor in causing death in electrocution. 



