THE CIRCULATORY SYSTEM: THE COMMON CARRIER FOR THE BODY 



85 



skeletal muscle in the strength and rapidity of its contractions, but with 

 one very important difference. A skeletal muscle is capable of graded 

 contractions, the strength of which is dependent upon the intensity of 

 the nerve stimulus and the resultant variation in the number of individual 



right 



pulmonary 



artery 



vena cava 



right auricle 



aunculo- 



ventricular 



valve 



right 

 ventricle 



monary artery 



monary 

 ins 



ft auricle 



riculo- 

 ventricular valve 



mi-lunar 

 valves of aorta 



eft ventricle 



Fig. 6.1. A section through the heart, from the front. 



muscle fibers stimulated to contract. In the heart, on the other hand, any 

 stimulus strong enough to cause the heart muscle to react at all will cause 

 complete contraction of an entire auricle or ventricle. The heart chambers, 

 as units, follow the all-or-none rule, whereas in skeletal muscle it is only 

 the individual muscle fibers that behave in this way. The reason for the 

 difference is that in skeletal muscle the fibers are separate, whereas in 



