536 THE OUTGO OF ENERGY 



out change of frequency, but the ventricle will 

 contract only when the excitation succeeds in 

 passing the block. 



2. Divide the auricles in two pieces con- 

 nected by a small bridge of auricular tissue. 

 Stimulate one piece. 



The stimulation of one piece will be followed 

 immediately by the contraction of that piece, 

 and, after an interval, by the contraction of the 

 other. The smaller the bridge, the longer the 

 interval. 



Gaskell has pointed out that a natural block 

 is furnished by the small number of the muscle 

 fibres joining the auricle to the ventricle, and 

 that this natural block explains the auriculo- 

 ventricular interval, i. e. the delay which the 

 excitation experiences in passing from the auricle 

 to the ventricle. 



3. Eepeat Experiment 1, but place the screw- 

 clamp across the middle of the ventricle. 



The passage of the excitation from one part of 

 the ventricle to another will be delayed or inter- 

 rupted by the lowering of the conductivity in 

 the compressed portion. 



Many irregularities in the frequency and force 

 of the heart can be explained by variation in the 

 conductivity of its several parts. They can be 



