26 PHYSIOLOGY FOR NURSES 



sists of two distinct parts; i.e., auricular systole, ven- 

 tricular systole, and diastole, or rest. The closure of 

 the auriculoventricular valves takes place at the same 

 instant; that of the semilunar valves a little later. The 

 sound of the first, dull and low pitched and confused 

 with the sound made by contracting muscle, is called 

 the first sound of the heart; the sharp, high, short click 

 of the semilunar valves, make the second sound. Cardiac 

 systole lasts about the tenth of a second, diastole about 

 five tenths; so that the heart is at rest about sixty per 

 cent of the time. 



Exact closure of all the valves is necessary to prevent 

 leakage. It follows, therefore, that if any valve is too 

 short or too long, has a rough place on it either through 

 the presence of a foreign body or a wound, it will not 

 meet its fellows exactly and there will be a falling back 

 of blood into the chamber which that valve guards. Or 

 if the opening is too small from any cause, there will be 

 greater difficulty in driving the blood through, or it 

 would take a longer time. Either defect would cause 

 a change in the heart sounds. 



Various infectious processes are apt to cause such 

 damage to the valves through the production of "endo- 

 carditis," an acute inflammatory process involving the 

 inner lining of the heart. When the inflammation sub- 

 sides, scar tissue appears and, as is always the case with 

 such tissue, contraction causes a distortion of the leaf- 

 lets with a resultant leakage. Rheumatic fever, tonsil- 

 litis, chorea, and pneumonia are the commonest predis- 

 posing causes for endocarditis. 



The heart is the chief, but not the only, force of the 

 circulation. The arteries are lined by a thin coat the 

 Mima continuous with the lining membrane of the 

 heart ; but around this their w r alls are composed of fi- 



