THE CAUSATION OF THE HEART-BEAT 953 



the great veins, and along the auricular muscle in all directions. When it 

 arrives at the auriculo- ventricular node, the impulse is carried on to the 

 ventricles along the auriculo- ventricular bundle, spreading along the branches 

 of this bundle to almost all parts of the ventricular muscle. Although 

 normally the sino- auricular node initiates each heart-beat, this node can be 

 put out of action by injury or cooling without stopping the rhythmic 

 sequence of the heart-beat, the office of pace-maker being now taken up by 

 the auriculo-ventricular node. A specialisation of function accompanies 

 the differentiation in structure which we find in the auriculo-ventricular 

 bundle and its branches. Lewis has shown that the conduction of the 

 excitatory process along the auriculo-ventricular bundle of the Purkinje 

 tissue occurs about ten times as fast as the conduction through the ordinary 

 muscular tissue of the heart, the rates being about 5000 mm. and 500 mm. 

 per second respectively. Although all parts of the ventricles receive the 

 impulse to contraction almost simultaneously, the contraction wave, as 

 judged by the electrical changes, is found to commence slightly earlier at two 

 j3ojiits, namely, on the anterior surface near the apex to the right of the 

 groove separating right and left ventricles^ and at the extreme apex of the 

 heart where the endocardiac tissue comes very close to the surface. On 

 the other hand, the conus arteriosus is the last part of the heart to begin 

 contracting. 



The limitation of the muscular continuity to a single narrow bundle which is en- 

 dowed with greatly increased conducting powers, and ends in a network of tissue 

 endowed with similar powers, is evidently designed (to use an old-fashioned but 

 convenient word) to ensure that all parts of the ventricles contract practically 

 simultaneously. If this were not the case the sudden contraction of the muscle fibres 

 near the base of the ventricles would simply bulge out the still uncontracted portion 

 near the apex, and there would be a risk of injury or even rupture of the uncontracted 

 part of the ventricle under the stress of the pressure produced by the contracting part. 

 On the other hand, if all parts of the heart were endowed with a similar rapid power of 

 conduction, any part slightly more excitable or irritated than the rest, might serve as 

 a centre for emitting excitatory waves which would interfere with those transmitted 

 from the auricles, and the tendency to heart delirium would be enormously increased. 



THE HUMAN ELECTROCARDIOGRAM 



The passage of the 'excitatory wave over the different heart cavities is 

 associated with corresponding electrical changes resulting in differences of 

 potential. If we lead off any two parts of the heart's surface to a string 

 galvanometer or capillary electrometer, we obtain movements which are 

 caused partly by changes occurring in the muscle just underlying the elec- 

 trode, partly by changes occurring at a distance and transmitted by the inter- 

 vening muscle acting simply as a moist conductor. These two kinds of 

 effect may be alluded to as direct and indirect. If we lead off, not from the 

 heart itself, but from neighbouring tissues in contact with the heart, we 

 shall still obtain the indirect effect of the electrical changes at each heart- 

 beat, and these can be obtained, as Waller has shown, when the intact 

 animal is led off to the electrometer or galvanometer by his limbs. In an 

 animal such as the dog the two fore limbs may form one lead and the two 



