216 ANATOMY FOR NURSES [Chap. XII 



numbtT and small bore of the capillaries, and (5) the large size 

 and non-contractile character of the walls of the veins. 



(1) Heart-beat. — So long as life lasts, the muscular tissue of 

 the heart contracts and relaxes unceasingly with a short interval 

 of rest. 



The contractions of the heart are rhythmical ; that is to say, 

 they occur in a certain order. First, there is a simultaneous con- 

 traction of the walls of both auricles ; immediately following this, 

 a simultaneous contraction of both ventricles ; then comes a 

 pause, or period of rest, when the cycle is repeated. 



The state of contraction is called the systole. 



The state of dilation, or rest, is called the diastole. During the 

 diastole, or period of rest, the blood is flowing into the auricles and 

 beyond into the ventricles (the auriculo-ventricular valves being 

 open). Then the auricles and ventricles contract again in the 

 same order as before, and their contractions are followed by 

 the same pause as before. 



The rhythmical succession of the systole and diastole constitutes 

 a cardiac cycle and corresponds to one beat or pulsation of the 

 heart. Reckoning on the number of heart-beats as 72 per minute, 

 the time required for a cardiac cycle will be about 0.8 of a second, 

 and half of this, or 0.4 of a second, represents the diastole or period 

 of rest. 



The sounds of the heart. — If the ear be applied over the heart, 

 certain sounds are heard, which recur with great regularity. The 

 first sound is a comparatively long, booming sound ; the second, 

 a short, sharp, sudden one. The sounds resemble the syllables 

 lubb-dup. The causes which produce the first sound are sup- 

 posed to be the closure of the auriculo-ventricular valves, the con- 

 traction of the ventricles, and the cardiac impulse against the 

 chest wall. The second sound is caused by the closure of the semi- 

 lunar valves. The first sound is heard best at the apex ; the 

 second sound is heard best at the base. 



In certain diseases of the heart these sounds become changed 

 and obscure, and are accompanied by various distinctive and 

 characteristic sounds called murmurs. 



Cause of the heart-beat. — The beat of the heart is caused 

 by the rhythmical contractions of its muscular fibres. These 

 contractions are automatic, and do not depend upon the cen- 



