PHYSIOLOGICAL PROPERTIES OF CARDIAC MUSCLE 



339 



consists in fastening the apex of the ventricle to the long arm of a 

 writing lever, which is pulled upward beyond its horizontal position 

 by a counter spring (Fig. 177). In the latter case, the contracting 

 ventricle pulls the lever downward, while in the first two instances the 

 lever moves upward during systole and downward during diastole. 



Under normal conditions, the successive up and down strokes are 

 of equal size, but assume a smaller amplitude as soon as the prepara- 



FIG. 177. SCHEMA TO ILLUSTRATE THE METHODS OF RECORDING THE CONTRACTIONS OF 



THE FROG'S HEART. 



The writing lever (W) is pulled upward by a spring (S) against the action of the 

 heart. 



tion becomes fatigued or when it is made to act under less favorable 

 circumstances (Fig. 178). Very similar records may be obtained with 

 apex-preparations subjected to electrical stimuli or with strips of 

 ventricular muscle tissue immersed in a solution of the inorganic salts. 

 It is to be noted, however, that the amplitude of the contractions can- 

 not be changed by varying the strength of the stimuli. This fact 



FIG. 178. RECORD op THE CONTRACTIONS OF THE FROG'S HEART. 

 The time is registered in seconds. 



implies that a heart always contracts with full vigor irrespective of 

 the character of the stimulation. This result is somewhat different 

 from that ordinarily obtained with striated and non-striated muscle, 

 because the reactions of these tissues are directly proportional to the 

 strength of the stimuli. Cardiac muscle, therefore, is said to behave in 

 accordance with the "all or none" law, i.e., it always reacts maximally, 

 whether the stimulus be slight or strong. 



