EVENTS IN A CARDIAC CYCLE. 



223 



heart. The first is of lower pitch and lasts longer than the 

 second and sharper sound: vocally their character may be 

 tolerably imitated by the words lubb, diip. The cause of 

 the second sound is the closure, or as one might say the 

 " clicking up," of the sem'ilunar valves, since it occurs at 

 the moment of their closure and ceases if they be hooked 

 back in a living animal. The origin of the first sound is 

 still uncertain: it takes place during the ventricular systole 

 and is probably due to vibrations of the tense ventricular 

 wall at that time. It is not due, at least not entirely, to 

 the auriculo-vcntricular valves, since it may still be heard 

 in a beating heart empty of blood, and in which there could 

 be no closure or tension of those valves. In various forms 

 of heart disease these sounds are modified or cloaked by 

 additional " murmurs" which arise when the cardiac orifices 

 .are roughened or narrowed or dilated, or the valves ineffi- 

 cient. By paying attention to the character of the new 

 .sound then heard, the exact period in the cardiac cycle at 

 which it occurs, and the region of the chest-wall at which it 

 is heard most distinctly, the physician can often get impor- 

 tant information as to its cause. 



Diagram of the Events of a Cardiac Cycle. In the 

 following table the phenomena of the heart's beat are rep- 

 resented with reference to the changes of form which are 

 seen in an exposed working heart. Events in the same 

 vertical column occur simultaneously; on the same horizon- 

 tal line, from left to right, successively. 



