THE HKAKT 87 



formed of tissue of the same nature as connective tissue, 

 thin hut strong, and are covered by the endocardium, which 

 passes over them closely adherent to their thin tissue, so that 

 there is a layer of endocardium on both sides of the valves. 

 At the openings of the veins and arteries the endocardium is 

 continuous with a similar delicate membrane forming the 

 innrrmost layer of the veins and arteries. 



The Action of the Heart 



Contraction of the Muscle Fibres of the Heart. 

 The cardiac muscle fibres contract like other muscle fibres ; that 

 is, they become shorter and correspondingly thicker. Since 

 they form the walls of a cavity, when they contract they cause 

 a diminution in the size of the cavity, the wall of the cavity 

 becoming thicker. When an india-rubber ball is squeezed, the 

 cavity becomes smaller, and the air or water in it is driven out ; 

 the cavity becomes smaller because a portion of the wall is 

 pushed into it, but the wall itself does not change in extent or 

 in thickness. When the heart contracts the walls diminish in 

 extent and increase in thickness. Moreover, in each chamber 

 of the heart the fibres comprising the walls are arranged in 

 bundles in a complicated manner, passing from one part of the 

 wall round the chamber to another part, so that when they 

 contract they draw one side of the wall in till it meets the 

 opposite side. In this way the cavity of the chamber is obli- 

 terated, or nearly obliterated, and all, or nearly all the blood 

 in it driven out. 



A Beat of the Heart. A beat of the heart is the con- 

 traction of the walls of the auricles and of the ventricles. The 

 two auricles contract at the same time, and then immediately 

 afterwards the two ventricles contract at the same time, then 

 there is a pause during which the auricles and ventricles are 

 both relaxed ; then a contraction of the auricles occurs again, 

 followed immediately by a contraction of the ventricles, and 

 then there is another pause, and so on. The two auricles 

 always contract together, and the two ventricles always con- 

 tract together, so that the events taking place in the heart 

 on the left side are similar to those taking place on the right 

 ride. 



