CHAP, iv.] THE VASCULAR MECHANISM. 145 



tetanus. A simple contraction or spasm cannot possibly produce 

 a sound having the characters of the first cardiac sound. And the 

 evidence, though perhaps not conclusive, goes to shew that the beat 

 of the heart is a slow long-continued single spasm, intermediate 

 between the contraction of an ordinary striated and that of an 

 unstriated muscle, and not a tetanic contraction. We cannot, it 

 is true, now rely in support of this view on the fact that when the 

 nerve of a rheoscopic muscle-nerve preparation is placed on the 

 beating ventricle, each beat is followed by a single spasm of the 

 muscle, and not by a tetanus ; for we now know that many forms 

 of tetanus (e. g. those caused by the constant current, by strychnia, 

 and probably all natural voluntary contractions) give rise, in a 

 rheoscopic muscle-nerve preparation, to a single initial spasm and 

 not to a tetanus. But the general features of the beat, its long 

 latent period and the gradation of the ventricular systole through 

 the auricular systole into the rhythmic contractions of the un- 

 striated fibres of the walls of the great veins, render it difficult 

 to suppose that the beat is really a tetanus. Moreover the long 

 duration of the ventricular systole is readily explained by the 

 wave of contraction passing in a complicated peristaltic manner 

 over the different fibres in succession. But if the beat be a simple 

 contraction, it cannot give rise to a muscular sound, unless we 

 suppose that this sequence of simple contractions over various 

 parts of the ventricle in succession is adequate to produce such a 

 sound. This, however, does not seem very satisfactory. 



On the other hand, if we reject the distinctly muscular origin of 

 the sound, we are almost driven to suppose that the abrupt systole 

 is able even in the absence of blood to produce such a sudden 

 tension of the valves, and of the ventricular walls, as to give rise to 

 a note. On such a view, the sound ought to vary in character 

 according as the ventricle is more or less filled, being low and 

 booming when it is full, and high and sharp when the contents are 

 scanty. And such is said to be the case. But the matter does not 

 at present seem ripe for any dogmatic statement. 



In the normal state of things, the beats of the two ventricles 

 are so far synchronous with each other that practically only one 

 first sound and one second sound is heard. It sometimes happens 

 however that the synchronism fails to such an extent and the 

 closure of the pulmonary and aortic valves respectively are sepa- 

 rated by such an interval as to give the second sound a double 

 character. 



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