THE STORY OF THE HEART 67 



by and through the medium of the nervous system, 

 with which it exists in very close and intimate 

 relationship. A plain but sufficiently correct idea 

 of the heart may be gained by comparing it to two 

 semi-detached houses. In other words the heart is 

 completely divided into two sides. As there is no 

 possibility of the tenant in one of the houses 

 visiting his neighbour through the partition wall, so 

 no blood can pass directly from one side of the 

 heart to the other. Each side of the heart, as we 

 have seen, consists of two chambers, called auricle 

 and ventricle, and we have also noted that it is the 

 duty of the left side of the heart to propel pure 

 blood everywhere through the body, the function 

 of the right side being to send impure blood brought 

 to it by the veins to the lungs for purification. 

 Having regard to the duties performed by the two 

 sides of the heart we see that the left side (Fig. 21) 

 has by far the greater share of the work to do. 

 Hence, as might be expected, the left heart is the 

 much stronger side of the two, its muscle being three 

 or four times the thickness of that of the right side. 

 THE WORKING OF THE HEART. In studying the 

 manner in which the heart works we find that the 

 two auricles contract together, and the two ventricles 

 similarly act in unison. Each cavity of the heart 

 has to dilate to receive blood, the auricles from the 

 bloodvessels pouring blood into them, and the ven- 

 tricles from the auricles themselves. The expansion 

 or dilatation of the cavities receives the name 

 diastole, whilst when each cavity of the heart con- 

 tracts, that is, grows smaller, it naturally sends out 

 its blood; the act of contraction being known as systole. 

 Returning for a moment to the course of the circu- 

 lation, *we must bear in mind that as each auricle 



