THE ESSENTIAL PHENOMENA OF CIRCULATION. 



23 



The heart is a hollow muscle. It is a muscular mass 

 containing several cavities. 



It is situated in the thorax between the two lungs, 

 and is inclined from right to left and from back to front, 

 so that its point strikes against the sixth rib at each of 

 its movements. It is held in position only by the large 

 vessels, and is thus completely independent of other 

 organs. Lastly, like other important organs, it is sur- 

 rounded by a serous membrane, called the pericardium, 



FIG. 10. 



Fm. 11. 



A FRONT VIEW OF THE HEART. 1, the 



right auricle of the heart ; 2, the 

 left auricle; 3, the right ventricle; 

 4, the left ventricle; 5 r 6, 1, 8, 9, 10, 

 vessels through which the blood 

 passes to and from the heart. 



A BACK VIEW OF THE HEART. 1, the 



right auricle ; 2, the left auricle ; 3, 

 the right ventricle; 4, the left ven- 

 tricle ; 5, 6, 7, the vessels that carry 

 the blood to and from the heart ; 9, 

 10, 11, the vessels of the heart. 



The form of the heart is that of an inverted cone ; 

 its size is about equal to that of the fist ; its weight is 

 somewhat over eight ounces. Its interior is divided 

 vertically into two compartments that have no direct 

 communication; we might say that there is a right 

 heart and a left heart, having one wall common to both. 



