92 THE CIRCULATION. 



capillaries, where it again becomes dark and impure, and 

 thence into the branches of the systemic veins, which, forming 

 by their union two large trunks, called the superior and in- 

 ferior vena cava, discharge their contents into the right auricle, 

 whence we supposed the blood to start (Fig. 33). 



FIG. 33. 



Diagram of the circulation through the heart (after Dalton). a, a. Vena cava, su- 

 I erior and inferior, b. Right ventricle, c. Pulmonary artery, d. Pulmonary vein. 

 e. Left ventricle. /. Aorta. 



Structure of the Valves of the Heart. 



It will be well now to consider the structure of the valves 

 of the heart, and the manner in which they perform their func- 

 tion of directing the stream of blood in the course which has 

 been just described. The valve between the right auricle and 

 ventricle is named tricuspid (Fig. 34), because it presents three 

 principal cusps or pointed portions, and that between the left 

 auricle and ventricle bicuspid or mitral, because it has two such 

 portions (Fig. 35). But in both valves there is between each 

 two principal portions a smaller one; so that more properly, 

 the tricuspid may be described as consisting of six, and the 



