CH. XXI.] THE STANNIUS EXPERIMENT 259 



inhibition; this, however, passes off after a variable time, and the 

 auricles and ventricle once more beat rhythmically. It is impossible 

 to explain the effect of the second Stannius ligature except on the 

 hypothesis that it acts as a stimulus, and there is no a priori reason 

 why the two ligatures should act in opposite ways. 



The fact that the Stannius heart is quiescent has enabled 

 physiologists to study the effects of stimuli upon heart muscle. A 

 single stimulus produces a single contraction, which has a long latent 

 period, is slow, and propagated as a wave over the heart at the rate 

 of f to f inch, or 10 15 mm. a second. A second stimulus causes 

 a rather larger contraction, a third one larger still, and so on for 

 some four or five beats, when the size of the contraction becomes 

 constant. This staircase phenomenon, as it is called, is also seen in 

 voluntary muscle, but it is more marked in the heart. The accom- 

 panying tracing (fig. 233) shows the result of an actual experiment. 



FJ<;. 233. Staircase from frog's heart. This was obtained from a Stauuius preparation; au induction 

 shock being sent into it with every revolution of the cylinder (rapid rate). The contractions 

 became larger with every beat. To be read from right to left. 



There are, however, more marked differences than this between 

 voluntary and heart muscle. The first of these is, that the amount 

 of contraction does not vary with the strength of the stimulation. A 

 stimulus strong enough to produce a contraction at all brings out as 

 big a beat as the strongest. The second is, that the heart muscle 

 has a long refractory period ; that is to say, after the application of 

 a stimulus, a second stimulus will not cause a second contraction 

 until after the lapse of a certain interval called the refractory period. 

 The refractory period lasts as long as the contraction period. The 

 third difference depends on the second, and consists in the fact that 

 the heart-muscle can never be thrown into complete tetanus by a rapid 

 series of stimulations ; with a strong current there is a partial fusion 

 of the beats, but this is entirely independent of the rate of faradisa- 

 tion. Indeed, as a rule, the heart responds by fewer beats to a rapid 

 than to a slow rate of stimulation. 



Although nearly all our information on this subject is derived 

 from the examination of the hearts of cold-blooded animals, and 

 mainly from the heart of the frog, there is no reason to suppose 

 that what is true for one vertebrate is not true for all-; such differences 



