56 THE HEART 



negative to the rest of the muscle. The part which is the 

 earhest to become electro-negative is therefore the part 

 which is earhest to contract. Lewis, by systematically 

 exploring the auricle, placing the electrodes on various 

 points, found that the region which first becomes negative 

 is the sino-auricular node. 



Not only is the sino-auricular node the site of origin of 

 the impulse, but it is the part most sensitive to local 

 influences. Coohng slows the rhythm only when apphed 

 here. It is clear, therefore, that the sino-auricular node 

 plays the same part in the mammalian heart as the sinus, 

 from which the node is derived, plays in the amphibian 

 organ. 



The Auriculo-Ventricular Bundle 



It is now proved that the proper conduction of the 

 impulse from the auricle to the ventricle is dependent upon 

 the integrity of this structure. When the bundle is injured 

 the following effects are produced according to the degree 

 of the injury. 



1. Prolongation of the interval between the auricular 

 and ventricular contractions. 



2. An occasional ventricular lapse. 



3. Response of the ventricle only to alternate or to 

 every third auricular beat. (Partial Heart-block.) 



4. Complete failure of conduction from auricle to 

 ventricle, the latter chamber beating independently. 

 (Complete Heart-block.) 



The same changes occur when the bundle is diseased, the 

 condition being known as Stokes- Adams' disease. 



It will be seen that the effects produced by injury to the 

 bundle are the exact counterpart of those obtained in 

 Gaskell's experiment upon the heart of the tortoise. The 

 bundle performs the same function as the sino-auricular 

 canal from which it is developed. 



