254 HUMAN PHYSIOLOGY 



so as to throw the valves into contact. Naturally, when 

 there is incompetency on the part of these valves and 

 some blood reenters one of the ventricles during its re- 

 laxation the second sound will be changed. It will lose 

 its crispness and be prolonged as a murmur. 



In any valvular disease of the heart the obvious result 

 is that more than the normal amount of work must be 

 done to maintain the full volume of the circulation. 

 More or less of the blood slips back and has to be pumped 

 twice instead of once. The extra labor is the same in 

 principle as that which must be performed with any 

 leaky pump when it is required to deliver a certain 

 amount of water. A heart which has to contend with 

 such a condition is exercised and trained like any other 

 muscle and it may act well for many years. Its walls 

 may be much thickened for their heavy duty. But it 

 will be seen that such a heart can scarcely be expected to 

 have as large a reserve power as the normal organ. It 

 is habitually working nearer to its limit. 



Some Properties of Cardiac Muscle. In the foregoing 

 description of the events of a heart cycle but little was 

 said of the contractile tissue at work. We must now 

 make some comparison between this and other forms 

 of muscle. We shall find that in many respects it is 

 intermediate between the smooth and the skeletal types. 

 This is true, first of all, of its microscopic organization. 

 Its cells 1 are short and have each a single nucleus, agree- 

 ing in this with those of smooth muscle. But, on the 

 other hand, they are transversely striated somewhat like 

 the skeletal fibers. In speed 9f movement they are 

 also intermediate; they can work more rapidly than any 

 smooth muscle but they do not equal the skeletal in its 

 capacity to contract and relax quickly. 



The most remarkable power of cardiac muscle is its 

 automaticity. We have seen that skeletal muscle seldom 



1 The lines of demarcation between these cells are ill denned and in 

 many respects they seem merged as a continuous structure a syn- 

 cytium. 



