44 THE CIRCULATION OF THE BLOOD 



Valves and Openings. 



Right Auricle. Leading off from the right auricle anteriorly 

 and superiorly is a sinus that bears the name of the auricular 

 appendix. It is a little hollow pouch capable of distention with 

 blood. 



Opening into the right auricle we find the coronary veins, the 

 two venae cavae, and the auriculo-ventricular opening. Guard- 

 ing these openings are valves to prevent the backward flow of 

 the blood current. 



Right Ventricle. Opening into the right ventricle are the 

 pulmonary artery and the right auricle. 



The tricuspid valve guards the auriculo-ventricular opening. 

 It is composed of three triangular shaped membranes attached 

 to the base of the circumference of the opening and the apices 

 of the triangles coming together when closed. 



The semi-lunar valves guard the pulmonary opening. They 

 are three entirely separate segments of semi-lunar shape and are 

 attached by their long curved margins to the circumference of 

 the artery just where it springs from the muscular substance of 

 the ventricles. 



Left Auricle. Like the right auricle, this cavity has a small 

 sinus leading off from it anteriorly and superiorly the auricular 

 appendix. The openings into the left auricle are the four pul- 

 monary veins and the left ventricle. 



Left Ventricle. This ventricle has the thickest walls and 

 does the most work of any of the chambers of the heart, because 

 it forces the fresh arterial blood out into the aorta and thence 

 through the entire systemic circulation. 



The aorta and the left auricle open into this ventricle. The 

 aortic semi-lunar valves guard the aortic opening. They are 

 three distinct semi-lunar shaped membranes to close the aortic 

 opening at the end of the systole. The mitral or bicuspid 

 valve closes the left auriculo-ventricular opening. It is some- 



