THE HEART AND THE BLOOD VESSELS 137 



outer layer is loose, the two moving freely over each other. 

 These pericardial layers are covered by glandular epithelium 

 which secretes a fluid into the space between them, this liquid 

 being naturally called the pericardial fluid. Were it not for 

 this fluid, the ever-moving heart would rub against the sur- 

 rounding tissues, producing much friction and inflammation. 



A person's heart is about the size of his fist. In shape it is 

 something like a strawberry and lies with the small end, or 

 apex, pointing downward and toward the left; the upper end 

 is called the base and here the large arteries leave and the 

 veins enter it. It is a hollow organ, the walls of which are com- 

 posed mainly of muscle; on the outside more or less fat is 

 usually deposited, especially in certain depressions where the 

 arteries and veins emerge, and along grooves which extend 

 lengthwise or obliquely on the organ over places whert par- 

 titions run in the interior. 



The cavity of the heart is divided by a vertical wall into 

 right and left chambers; and each of these is again partially 

 divided into an upper portion, the auricle, and a lower, the 

 ventricle. Each of these four chambers is lined with a smooth 

 glistening sheet of membranous epithelium, which keeps the 

 blood from direct contact with the muscle tissues of its walls. 

 The walls of the auricles are very much thinner than those of 

 the ventricles and the wall of the left ventricle is thicker than 

 that of the right. 



THE EVENTS OF A HEART BEAT 



We can best learn the structure and action of the heart if 

 we trace the flow of blood through it, noticing how the valves 

 are closed and opened so that the blood always flows onward. 

 The blood is brought from the body to the heart through two 

 large veins, which open into the right auricle the superior 

 vena cava, bringing the blood from the upper part of the body, 

 and the other, the inferior vena cava, bringing it from the 



