THE CIRCULATION OF THE BLOOD 141 



The apex of the heart beats against the chest wall 

 between the fifth and sixth ribs, about an inch and a half 

 to the left of the middle line of the body. 



198. The Pericardium. The heart lies within a strong, 

 fibrous membrane forming a kind of bag or purse called 

 the pericardium. This bag is really double, with two layers, 

 one over the other. The inner layer closely covers the 

 surface of the heart and is reflected upon itself to form 

 a sack without an opening. 



Place your hand in a stocking which has the foot turned 

 inwards, and your hand will be covered by a double coat 

 somewhat as your heart is by its double bag. 1 Between 

 the two layers of the pericardium there is a small quantity 

 of clear fluid which permits the parts to move one upon 

 the other with little friction. 



199. The Chambers of the Heart. The heart has a mus- 

 cular partition running down its center from top to bottom, 

 separating the right side from the left. Each of these 

 sides has two hollow chambers or cavities : an upper one 

 called an auricle, from the fancied resemblance of one cor- 

 ner to the ear of a dog ; the other and lower one called a 

 ventricle. Hence there are two upper chambers called auri- 

 cles, and two lower chambers called ventricles. 



The heart is a muscle; hence it can contract. When 

 each of its chambers contracts, blood is forced to flow into 

 the next chamber or a blood vessel, as the case may be. 

 The walls of the ventricles are stouter and stronger than 



1 We must remember, however, that the leg of the stocking is open at 

 the end while the pericardium is closed. We may think of the pericardium 

 shielding the heart somewhat as the leather case of a football protects the 

 bladder within it. Again, the pericardium is arranged about the heart as a 

 boy's toboggan cap covers his head. 



