166 PHYSIOLOGY FOR DENTAL STUDENTS. 



It is of interest to know that there has been quite an advance 

 recently in the knowledge of the conduction of the cardiac im- 

 pulse from the auricles on to the ventricles. It has been known 

 for a long time that when a muscle contracts, a small but definite 

 electric current is set up between the relaxed and the contract- 

 ing portions of the muscles. New methods of detecting and re- 

 cording the direction of the flow of such currents produced in 

 the heart in man have shown that cases of heart block are by no 

 means rare. The instrument used for this purpose is a highly 

 "sensitized galvanometer, and the tracings are known as electro- 

 cardiograms. By this method it can be shown that in certain 

 cases of heart disease the auricles beat twice to the ventricles 

 once, or again that the auricles may beat very fast while the 

 ventricles are beating very irregularly and slowly. 



The Action of Inorganic Salts on the Heart Beat. A very 

 interesting theory has recently been advanced concerning the 

 cause of the heart beat. It will be remembered that the blood 

 contains salts of sodium, potassium and calcium in solution. If 

 these salts are replaced by other non-poisonous salts in the same 

 concentration as the salts removed, the heart will not beat. If 

 the heart is perfused with a solution of sodium chloride alone, 

 the beat becomes very weak and finally stops. If, however, a 

 small amount of calcium' and potassium salts is added to the 

 sodium chloride solution, the heart will again begin to beat, but it 

 stops after a while in a state of relaxation, or diastole, if calcium 

 chloride is removed from the solution, or in systole, or contrac- 

 tion, if the potassium salts are removed. These experiments sug- 

 gest that the salts of the blood offer a solution to the problem of 

 the cause of the heart beat, the potassium favoring relaxation, 

 and the calcium contraction. If the proper balance of these 

 salts is present in the blood, it is conceivable that a regular se- 

 quence of contraction and relaxation of cardiac muscle will take 

 place because of the action of the salts. 



The Vascular Mechanism of the Heart. 



Definition of Terms. A definition of the terms applied to 

 the different phases of the heart's activity will help in the de- 



