162 



THE CIRCULATION 



of the heart. The difference in the thickness of the walls of 

 the two ventricles is accounted for by the fact that the right, 

 having thinner walls than the left, has only to pump the blood 

 through the lungs, whereas the left ventricle has to pump the 

 entire volume of blood through the whole body. The valves 

 separating the different chambers of the heart are named as fol- 

 lows : that between the right auricle and right ventricle is called 

 the tricuspid; that separating the left auricle from the left 

 ventricle, the mitral. There are also two sets of valves guard- 

 ing the exit from the ventricles, called the semi-lunar valves. 



23. 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, c . 

 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 FlG . 37> _ SECTION OF THE HEART 



maybe Called the heart's A, Right Ventricle E, F, Inlets to the Ventricles 

 n . B, Left Ventricle G, Pulmonary Artery 



period of repose; and al- c, Right 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. 



23. Substance of the heart? Its fibres? Its movements? 

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



The advantage of such 



