CIRCULATION OF THE BLOOD. 41 



the pericardium and around the heart ivS a small quantity 

 of liquid, called the pericardial fluid. 



The Size of the Heart. A person's heart is about the 

 size of his clenched hand. 



The External Features of the Heart. The heart is 

 cone-shaped and the bulk of it is made up of the ventricles, 

 the auricles being two ear-like flaps at the base, one on 

 each side. There is a deep notch between the auricles 

 and the ventricles. The line of division between the two 

 ventricles is marked by a groove, which runs obliquely 

 along the ventral surface. In this groove are blood tubes 

 and usually considerable fat. 



The Internal Structure of the Heart. The two halves 

 of the heart are completely separated from one another 

 by a partition. Each half, in turn, has valves which, 

 part of the time, separate the cavity of each auricle (at 

 the base) from the cavity of the corresponding ventricle 

 (at the apex). 



The Valves of the Heart. Between the auricles and 

 the ventricles are curtain-like valves, whose upper edges 

 are attached to the inner surface of the walls at the upper 

 margin of the ventricle. These flaps are somewhat tri- 

 angular, and have strong white, tendinous cords extending 

 from their edges and under surfaces to the walls of the 

 ventricle below. In the right half of the heart there are 

 three flaps, and this valve is called the tricuspid valve. In 

 the left side there are two flaps, which, together, constitute 

 the mitral valve. In the resting heart these flaps hang 

 down along the walls of the ventricles so that on opening 

 the heart one would see only a single cavity in each half 

 of the heart. 



