The Circulatory System 173 



larities in these sounds. If instead of sharp snapping sounds, the lub- 

 dub becomes a lubb-shhh, it is apparent that the valves are not func- 

 tioning correctly and that blood is passing back through them. This 

 is known as a heart murmur, and is due to injuries of the heart valves. 



Heart Muscle Activity. — Some of the observable differences be- 

 tween the action of the heart muscle and that of other body muscles can 

 be accounted for by the unusual structure of the heart muscle fibers. 

 It will be recalled that cardiac muscle (Fig. 34, A) fibers are much 

 branched and all are interconnected. Unlike skeletal or smooth muscle, 

 there is no real separation between the individual fibers. Thus impulses 

 spread rapidly over the whole system. This results in the heart as 

 a whole obeying the all-or-none laiv. In skeletal muscle the individual 

 fiber obeyed this general rule, and greater strength of contraction 

 of a muscle resulted from increased strength of stimulation. This 

 was due to more fibers being brought into play and the phenomena of 

 summation could result. Summation cannot occur in heart muscle as 

 the entire heart acts as if it were but a single muscle iiber. 



Another peculiarity of heart muscle is its possession of a relatively 

 long refractory period. The refractory period is that period following 

 a muscular contraction when the muscle fails to respond to a second 

 stimulus. In the skeletal muscle of the frog, this is about 0.005 second. 

 The refractory period of heart muscle is from 0.1 to 0.2 second, just 

 about 100 times that of skeletal muscle. Due to this, a prolonged stimu- 

 lation of heart muscle will result only in a single contraction which then 

 becomes ineffective until the refractory period passes. The heart 

 cannot be thrown into a complete contraction or tetanus. Clearly, if the 

 heart were completely contracted for a prolonged period, it would be 

 ineffective as a pumping mechanism ; hence this long refractory period 

 has a definite survival value. 



From the above information, it may appear that the strength of 

 contraction of the heart muscle is constant. This, however, is not at 

 all true. While the contraction may not be varied by summation, its 

 strength is affected by increased load, that is, increased distention of 

 the auricles and ventricles stretches the muscle fibers. This increased 

 stretching due to load brings about increased strength of contraction. 

 This phenomenon was also noted in the contraction of skeletal muscle. 



Though the heartbeat itself is intrinsic in the muscle, its rate 

 is dependent upon other factors. There are two types of nerves supply- 

 ing the heart : one of these from the brain inhibits, the other from the 



