CH. XX.] 



THE STANNIUS HEART. 



257 



the sinus is warmed the heart beats faster, but if the auricles or 

 ventricle are warmed there is no alteration in the heart's rate. 



The sinus in the frog's heart, and that portion of the right 

 auricle of the mammal's heart which corresponds to the sinus, is 

 always the last portion of the heart to cease beating on death, or 

 after removal from the body (ultima moriens). This is an addi- 

 tional proof of the superior rhythmical power which it possesses. 



The fact that the Stanuius 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. 124), but 

 it is more marked in the heart. The following tracing shows the 

 result of an actual experiment : 



Fig. 254. 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 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 heart-beat. 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 faradisation. Indeed, 



K.P s 



