CHAP, iv.] THE VASCULAR MECHANISM. 181 



When a second induction shock is sent in at a certain interval 

 after a first, the beat due to the second shock is often larger than 

 the first, the beneficial effects of a "contraction (see p. 96) being 

 even still more manifest in the heart than in an ordinary skeletal 

 muscle. Frequently by successive shocks of equal intensity a 

 'staircase' of beats of successively increasing amplitude may be 

 produced. 



When a second induction shock follows upon the first too 

 rapidly, it is apparently without effect ; no second beat is produced. 

 So also when a series of rapidly repeated induction shocks are sent 

 in, a certain number of them are thus 'ineffectual'; the application 

 of the ordinary interrupted current gives rise not to a tetanus but 

 to a rhythmic series of beats. The ' refractory period,' which is so 

 brief in the skeletal muscle (see p. 87), is very prolonged in the 

 cardiac muscle. So also in a spontaneously beating heart, induc- 

 tion shocks sent in at a certain phase of a cardiac cycle, e.g. the 

 commencement of the systole, are ineffectual, though they produce 

 forced beats when sent in at the other phases of the cycle. 



As we shall immediately see, the beat of the heart, and even of 

 a part of the heart such as the ventricle, is not a mere muscular 

 contraction but a complex act, in which both nervous and muscular 

 elements intervene; and it is difficult in all cases to distinguish 

 the action of the one from that of the other. It is probable how- 

 ever that many of the features which we have just described are 

 due to peculiarities of the cardiac muscle. 



Nervous mechanism of the Beat. The heart of a mammal or 

 of a warm-blooded animal ceases to beat almost immediately after 

 being removed from the body in the ordinary way ; and though by 

 special precautions and by means of an artificial circulation of blood, 

 an isolated mammalian heart may be preserved in a pulsating con- 

 dition for a considerable time, our knowledge of the exact nature 

 and of the causes of the cardiac beat is as yet almost entirely 

 based on the study of the hearts of cold-blooded animals, which 

 will continue to beat for hours, or under favourable circumstances 

 even for days, after they have been removed from the body with 

 only ordinary care. We have reason to think that the mechanism 

 by which the beat is carried on, varies in some of its secondary 

 features in even the cold-blooded animals: that the hearts, for 

 instance, of the snake, the tortoise and the frog, differ as to the 

 exact manner of carrying out the beat, both from each other and 

 from the bird and the mammal; but we may, at first at all 

 events, take the heart of the frog as illustrating the main and 

 important truths confirm' Tig the causes and mechanism of the 

 beat. 



The heart of the frog, as we have just said, will continue to 

 beat for hours after removal from the body; and the beats are in 

 all important respects identical with the beats executed by the 



