212 ESSENTIALS OF PHYSIOLOGY. 



THE PROPERTIES OF CARDIAC MUSCLE. 



If a fine thread is tied round the apex of the frog's heart and 

 attached to a light lever supported by a spring, a graphic record of the 

 heart movements can be obtained ; this shows a small fall correspond- 

 ing with each auricular contraction, and a larger fall corresponding 

 with each ventricular systole (fig. 77). A thread, tightly tied at the 

 junction between the sinus and the auricles, will now bring the auricles 

 and ventricles to a standstill for a variable time, since the ligature 

 prevents the passage of the normal rhythmic stimuli from the sinus to 

 the rest of the heart; this is known as the Stannius ligature. The 

 quiescent ventricle may then be used to study the properties of cardiac 

 muscle as compared with those of skeletal muscle. 



The contraction of cardiac muscle shows the following characters : 

 (1) When the ventricle is stimulated with single shocks of gradually 



increasing strength, it is found that with a 

 certain strength of current the heart gives 

 a beat. If a stronger current is used, the 

 resulting beats are not increased in extent 

 or force. The observation that if the heart 

 beats at all in response to a stimulus its con- 

 traction is maximal, whatever the strength 

 FIG. 77. Tracing of the nor- . .. .. , . , 

 mal heart beat in the frog. tne stimulus, is known as the " all or none 



law." It is due to the fact that the heart 



muscle is a syncytium, and that a stimulus applied to any point will, if 

 it is effective, spread over the whole muscular tissue of the heart. The 

 heart gives the best beat of which it is capable at any moment, but the 

 force with which it beats will be influenced by various considerations, 

 including the nutrition of the fibres. The "all or none law" simply 

 means that the heart beat is maximal for the conditions under which 

 it is placed at any moment, and not that it remains constant through 

 life. In this respect cardiac muscle behaves like the individual fibres 

 of a skeletal muscle which also obey the " all or none law " (p. 22). 



(2) If the resting heart is stimulated by successive induction shocks 

 at an interval of 5 to 10 seconds, the height of the second contraction is 

 rather greater than that of the first. At the third or fourth contraction 

 a maximum is reached, and succeeding contractions are all of the same 

 height. This phenomenon is sometimes called the "staircase effect," 

 and is due to the beneficial influence of the first two or three stimuli 

 on the contractile power of the heart muscle. The same effect may be 

 observed in skeletal muscle in similar circumstances. 



(3) When the heart is made to beat by a single induction shock, 



