CII. XX.] THE STANNIUS HEART 255 



The Stannius Experiment. This consists in applying a tight ligature 

 to the heart between the sinus and the right auricle ; the sinus 

 continues to beat, but the rest of the heart is quiescent. The quiescent 

 parts of the heart may be made to contract in response to mechanical 

 or electrical stimulation. If a second ligature is applied to the 

 junction of the auricles with the ventricle, the ventricle begins to 

 beat again; the auricles may also beat, but they usually do not. 

 According to Gaskell, the effect of the first ligature is simply an 

 example of blocking ; it is, however, difficult to wholly accept this 

 view, for if instead of applying a ligature at the sino-auricular 

 junction, the heart wall is simply cut through at this spot, the 

 auricles and ventricle are not thereby always rendered quiescent. It 

 appears probable, therefore, that there is some truth in the older 

 view that the ligature acts as a stimulus irritating the vagal termi- 

 nations in Eemak's ganglion, and so eliciting a condition of prolonged 

 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 | 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 (see p. 117), but it is more marked in the heart. 

 The following tracing shows the result of an actual experiment : 



FIG. 262. Staircase from frog's heart. This was obtained from a Stannius preparation ;{ an 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 



