i 9 4 THE CONTRACTION OF CARDIAC MUSCIE. 



hood of the electrodes ; then, as the single stimuli continue, and especially 

 if at intervals a weak tetanising current is sent through the whole strip, 

 we see that the contractions are not all of the same strength ; some are 

 stronger, some weaker, although the strength of the induction shocks 

 remains the same. The reason of this variation is clear on inspection : 

 those contractions which are stronger travel further along the strip 

 from the point of stimulation than the shorter contractions ; we see, in 

 fact, that every now and then a contraction succeeds in forcing a 

 previously existing blocking place, so that different contractions travel 

 to different degrees along the strip. As time passes, more and more of 

 the blocking places are overcome, until at last a regular series of con- 

 tractions takes place, every contraction passing rapidly and easily from 

 one end of the strip to the other. When this has occurred, when the 

 conduction power has been fully restored along the whole strip, then is 

 the time when the strip begins to contract rhythmically of its own 

 accord, without the necessity of any regular external stimuli. 



On the Tonicity of the Caediac Muscle. 



So far we have considered various properties of the cardiac muscle, 

 such as rhythmicity, excitability, conductivity, and have found that 

 different parts of the heart differ in the extent of their possession of 

 these different properties. It is impossible at present to lay down 

 absolute laws of the relations which exist between these several 

 properties in the different parts of the heart, but still we may with 

 advantage sum up our knowledge by suggesting that the present 

 direction of research seems to point roughly to some such series of 

 laws as the following : — ■ 



1. The rhythmicity of the cardiac muscle varies inversely as its 

 conductivity. 



2. The rhythmicity of the cardiac muscle varies directly as its 

 excitability. 



3. The rate of recovery of the excitability, after a contraction, varies 

 directly as the rate of recovery of the conductivity after a contraction. 



In other words, the most quickly contracting parts of the heart are 

 the least rhythmical, and, conversely, a block in the passage of the con- 

 traction wave is most likely to occur in the more rhythmical tissue. 



The end result of the different modifications of the different parts is 

 not only to form an efficient force-pump, the different chambers of 

 which shall always act in regular order, but also, as Engelmann l has 

 pointed out, to render as harmless as possible to the economy any 

 accidental irregularity in the beat of the pump. For we see from his 

 experiments, that not only does a contraction travel at different rates 

 over the different muscular tissues of the heart, but also the rate of 

 travel in any one tissue depends upon the phase of that tissue at the 

 time. For just as the excitability of the tissue is restored gradually 

 after a contraction, so also is the restoration of the full power of 

 conductivity a gradual one after a contraction. It follows, therefore, that 

 an interpolated wave of contraction, starting at the sinus soon after a 

 normal contraction, will travel to the ventricle over tissue which has 

 not yet regained its full power of conduction, and consequently will 

 take a longer time to reach the ventricle than the previous normal 

 1 Arch./, d. gcs. Physiol., Bonn, 1897, Bd. lxv. S. 153. 



