THE PHYSIOLOGY OF THE HEART 



133 



and eventually stands still in systole; in lactic acid (1 in 10,000 normal 

 saline) the contractions become less and less, and finally the ventricle 

 stops in a state of complete relaxation. The drugs digitalis and 

 veratrine yield results similar to alkalies, muscarin similar $o that 

 of acids. Chloroform lessens, and adrenalin increases, the tonicity 

 of the heart. After death from chloroform, the heart is dilated and 

 the muscle flabby. 



The heart possesses the properties of rhythmic automaticity, of 

 starting a stimulus. It also possesses the property of conduction 



FIG. 42. ELECTRO-CARDIOGRAM FROM A CASE OF COMPLETE HEARTBLOCK. 

 (W. T. Ritchie, from Cowan's "Diseases of the Heart.") 



The auricular rate is 97, the ventricular rate 38, per minute. 



of an impulse. It is a vexed question whether these properties 

 reside in the heart-muscle itself, or in the nervous tissue abounding 

 there, or in the intermediate nodal tissue. The electrolytes in solution 

 in the tissue lymph are essential factors in the maintenance both of 

 rhythmic automaticity and tonicity. 



The Electrical Change of the Heart. The contraction of the heart, 

 like that of other muscle, is accompanied by an electrical change. 

 The part in contraction is at different potential to the part at rest. 



FIG. 43. ELECTRO-CARDIOGRAM SHOWING REGULARLY RECURRING VENTRICULAR 

 EXTRA SYSTOLES. (W. T. Ritchie, from Cowan's " Diseases of the Heart.") 



Thus, an electrical wave accompanies the wave of contraction. This 

 is studied by means of either the capillary electrometer or the string 

 galvanometer. The principle of the string galvanometer is that a 

 movable conductor, a very fine silvered glass thread or a quartz 

 fibre, suspended between the poles of a powerful electro-magnet at 

 right angles to the lines of force of the magnetic field, tends to be 

 deflected to one or other side according to the direction of the current. 

 The degree of deflection is directly proportional to the intensity of 

 the current and to the strength of the magnetic field, and inversely 

 proportional to the weight and tension of the fibre. The changes of 



