THE PHYSIOLOGY OF THE HEARTBEAT 187 



SITE OF ORIGIN OF THE BEAT 



The working hypothesis from which we may proceed to attack this 

 problem is that the beat originates in the sinoauricular node, and to 

 put this to the test, various methods have been employed: (1) Warming 

 or cooling or injuring the node and noting the effect on the heartbeat. 

 Such procedures greatly affect the rate of the heartbeat, whereas they 

 produce no change when applied to other parts of the heart. (2) De- 

 termination of the comparative rhythmic power of strips cut out from 

 different regions of the auricular walls. It is greatest in those taken 

 from the region of the node. (3) Determination by the use of galvan- 

 ometric curves of the relation of the node to the seat of origin of cardiac 

 impulse. By all these methods the results indicate clearly that the beat 

 originates in the sinoauricular node, but on account of the great im- 

 portance in connection with the interpretation of electrocardiograms in 

 man, it is particularly with the result of the third group of experiments 

 that w r e will concern ourselves here. 



Evidence Furnished by Studying the Current of Action Which 

 Accompanies the Heartbeat 



To start with, it is essential that we make ourselves familiar with 

 the principles of the methods employed. These principles are briefly as 

 follows: When a wave of contraction passes along a muscle, it is im- 

 mediately preceded by a -change in electrical potential, which can be 

 detected by means of a galvanometer connected with the muscle through 

 so-called nonpolarizable electrodes. The galvanometer employed must 

 be extremely sensitive, and must not vibrate after the current has ceased 

 to pass. The form generally in use today is known as the string galva- 

 nometer of Einthoven. It differs from the galvanometer ordinarily em- 

 ployed in physical laboratories in that the current instead of passing 

 through a coil of wire surrounding a magnetic needle, passes through a 

 silverized quartz thread suspended in the strong magnetic field which 

 exists between the two opposing poles of a horseshoe electromagnet. 

 The string is thus surrounded on all sides by innumerable lines of force 

 extending between the two poles of the magnet. When a current, how- 

 ever small, passes along the string, it will generate lines of force of its 

 own, and these by reacting with the stationary lines of force of the field 

 will cause the string to move. The string is placed in the pathway of a 

 strong beam of light, and its shadow, after being magnified by lenses, 

 is projected 011 a moving photographic plate or paper arranged in a 

 suitable holder. The nonpolarizable electrodes referred to are employed 

 in place of ordinary electrodes in order to obviate the generation, of elec- 



