THE CIRCULATION 155 



21. Action of the Heart. The substance of the heart is of a 

 deep red color, and its fibres resemble those of the voluntary 

 muscles by which we move our limbs. But the heart's move- 

 ments are entirely involuntary. The advantage of this is 

 evident ; for if it depended 

 upon us to will each move- 

 ment, our entire attention 

 would be thus engaged, 

 and we would find no time 

 for study, pleasure, or even 

 sleep. The action of the 

 heart consists in alternate 

 contractions and expan- 

 sions. During contraction 

 the walls come forcibly to- 

 gether, and thus drive out 

 the blood. They then ex- 

 pand and receive a renewed 

 supply. These movements 

 are called respectively sys- 

 tole and diastole. The latter PlG> 3T _ SECTION OF THE HEART 

 may be Called the heart's A ' Ki ht Ventricle E, P, Inlets to the Ventricles 

 . , B, Left Ventricle G, Pulmonary Artery 



period ot repose; and al- c, Eight Auricle H, Aorta 



though it lasts only during D ' Left Auricle 



two-fifths of a heart-beat, or about a third of a second, yet 



during the day it amounts to more than nine hours of total rest. 



The ebbing current steals away, 



And, red with Nature's flame, they start 



From the warm fountains of the heart. 



"No rest that throbbing slave may ask, 

 Forever quivering o'er his task, 

 While far and wide a crimson jet 

 Leaps forth to fill the woven net, 

 Which, in unnumbered crossing tides, 

 The flood of burning life divides ; 

 Then, kindling each decaying part, 

 Creeps back to find the throbbing heart." 



Dr. 0. W. Holmes. 



21. Substance of the heart? Its fibres? Its movements? The advantage of such 

 movements ? Action of the heart ? Its period of repose ? 



