THE CAUSATION OF THE HEART BEAT 1011 



occurs in the latter condition, must therefore be ascribed to non-gaseous 

 metabolites produced by the contracting muscle. Thus the heart contains 

 in itself a mechanism for increasing the flow of blood through its tissues, 

 whenever this becomes inadequate and the muscle is suffering for lack of 

 oxygen. Mechanical and physiological factors thus co-operate in providing 

 the most important muscle in the body with oxygen sufficient for its needs. 



If a coronary artery be ligatured, the heart very often beats for one or 

 two minutes with unimpaired force, then a beat is dropped occasionally, 

 and very shortly afterwards the heart stops altogether and the blood 

 pressure falls to zero. On inspection of the heart immediately after the 

 blood pressure has fallen, its muscular wall is seen to be in a state of fibrillar 

 contractions, or 'delirium cordis.' All the strands of muscle fibres are 

 contracting more or less rhythmically, but the rhythms of no two parts 

 coincide, so that the heart dilates and is incapable of carrying on the circula- 

 tion. It is probably in this way that sudden deaths occur in cases where 

 the coronary arteries are diseased or calcified. In such cases a man may 

 drop down dead, having previously shown no symptoms of heart mischief. 



Delirium cordis may be explained, as the result of block, produced by 

 interference with the nutrition of a large part of the cardiac wall. The con- 

 tractile wave arriving at this part, in some directions will not spread at all, 

 in others will spread at a lower rate, so that different parts of the heart 

 receive the impulse to contract at different times and a state of inco- 

 ordination results. The same condition can be produced by freezing the 

 apex of the ventricle, so causing a block, or by stimulating the surface of the 

 ventricle at a rate which is greater than can be taken up by the ventricle as 

 a whole, as, e.g., by tetanising currents. Such a condition in the higher 

 animals, as the dog and man, is practically irrecoverable, . although in the 

 rabbit, and very rarely in the dog, it is sometimes possible to bring the heart 

 back to a state of rhythmic contraction by kneading it rhythmically. 



According to Mines delirium cordis is susceptible of a simpler explanation. This 

 condition is easily brought on in the mammalian heart by stimulation of its surface with 

 strong faradic currents. The effects on the heart muscle of increased frequency of con- 

 traction are to decrease the rate of propagation and to decrease the length of the wave 

 of excitation. Ordinarily in the naturally beating heart the wave of excitation is so long 

 and spreads so rapidly, that it excites the whole of the ventricle at a considerable 

 interval before it has ceased in any one part. When however the muscle is stimulated 

 more frequently, the wave becomes slow and short, so that more than one wave can 

 exist at one time in a single chamber. The main factor then, in the production of 

 delirium cordis after obstruction of the coronary artery, would probably be a diminished 

 rate of conduction through the affected part. 



