WORKING OF THE HEART AND BLOOD - VESSELS. 231 



a living animal. The origin of the first sound is still uncer- 

 tain: 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- 

 ventricular 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 " mur- 

 murs" which arise when the cardiac orifices are roughened 

 or narrowed or dilated, or the valves inefficient. 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 important information as to its 

 cause. 



Diagram of the Events of a Cardiac Cycle. In the fol- 

 lowing table the phenomena of the heart's beat are repre- 

 sented with reference to the changes of form which are seen 

 on an exposed working heart. Events in the same vertical 

 column occur simultaneously; on the same horizontal line, 

 from left to right, successively. 



Function of the Auricles. The ventricles have to do the 

 work of pumping the blood through the blood-vessels. Ac- 

 cordingly their walls are far thicker and more muscular than 

 those of the auricles; and the left ventricle, which has to 

 force the blood over the Body generally, is stouter than the 

 right, which has only to send blood around the comparatively 

 short pulmonary circuit. The circulation of the blood is in 

 fact maintained by the ventricles, and we have to inquire 

 what is the use of the auricles. Not unfrequently the heart's 



