HEART 427 



2. Conduction of Contraction. — There is little evidence 

 that nerve structures play a part in the conduction of the 

 impulse when once started. The syncytial structure of heart 

 muscle is specially well fitted to secure the propagation of 

 the contraction, and poisoning the nerve structures with 

 chloral does not abolish this. 



While conduction is a function of the muscular tissue of 

 the heart, it is undoubtedly modified through the action of 

 nerves (p. 418). 



The propagation of the wave of contraction over the 

 auricles and ventricles and the part played by the primitive 

 tissue have been already considered (p. 395). 



V. The Nature of Cardiac Contraction. 



The contraction of the ventricle lasts for a considerable 

 fraction of a second. Is it of the nature of a single contrac- 

 tion, or of a tetanus ? 



(i.) A single stimulus applied to heart muscle produces a 

 single prolonged contraction {Practical Physiology). 



(ii.) It is impossible to tetanise the heart by rapidly 

 repeated induction shocks. This is due to the long refractory 

 period after contraction has occurred. The resistance to 

 further stimulation gradually wanes, till, just before the onset 

 of the next contraction, a very small stimubis is effective. 

 By a slower sequence of stimuli it is therefore possible to 

 produce an incomplete fusion of contractions. 



(iii.) The steady passage of the contraction wave along 

 the heart is against the idea that the normal action of the 

 heart is a tetanus. 



(iv.) That it is really a single contraction is demonstrated 

 by taking advantage of the fact that the contracting part of 

 a muscle is electro-positive (" zincy ") to the rest. By the 

 use of the string galvanometer, it is possible to show that 

 the region of the sino-auricular node first becomes " zincy," 

 and that this variation then travels over the auricle and 

 onward to the ventricle. 



In man, by leading oft* from the right hand and left foot 



