THE RATE OF THE HEART BEAT. 



605 



directly or through the cardio-inhibitory center of the medulla, 

 and distinct variations in the reaction or the inorganic constituents 

 of the blood may also affect the rhythm of the heart. Addition of 

 potassium salts in excess will stop the heart beat entirely, bringing 

 about the condition of potassium inhibition, but such conditions 

 are, for the most part, exceptional or experimental. In the usual 



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routine of life the heart beat is regulated and adapted to the needs 

 of the body chiefly, as far as we know, through the agency of its 

 extrinsic nerves rather than through modifications of the com- 

 position of the blood. 



