44 THE CIRCULATION OF THE BLOOD 



Valves and Openings. 



Right Auricle. Leading off from the right auricle anter- 

 iorly 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. 

 Guarding 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 open- 

 ing. 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 pulmonary 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, be- 

 cause 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 bi- 

 cuspid valve closes the left auriculo-ventricular opening. It 

 is somewhat like the tricuspid except that it has' only two 

 flaps instead of three. 



