BO VALVES OF THE HEART. 



or; the other descending with the blood of the head, 

 chest, and upper extremities, called the vena cava supe- 

 rior. The two veins discharge their contents by a com- 

 mon opening into the right auricle. This cavity being 

 excited by the presence of the blood which completely 

 fills it, contracts suddenly and strongly upon the mass 

 within. Being pressed in all directions it will escape 

 wherever it can find an outlet. Now there are but two 

 openings into the auricle the one by which the blood 

 gained admittance, and the other leading into the ven- 

 tricle. Through the former it is impossible for it to 

 escape, except in very small quantities, because a long 

 column of blood is in these veins, presenting too great a 

 resistance to its escape in that direction. The latter 

 opening that into the ventricle, is now the only one 

 left, and this, as soon as the auricle begins to dilate, pre- 

 sents a free and open passage into the ventricle. 



Emily. But why does not the blood flow directly 

 through this opening while the auricle is filling ? I see 

 nothing now, to prevent it. 



Dr. B. You will see in a moment why it does not ; 

 but first we must trace it into the ventricle. This cavi- 

 ty when filled, like the auricle contracts upon the blood, 

 which must find an outlet either back through the open- 

 ing into the auricle, or into the vessel which carries it to 

 the lungs, called the pulmonary artery. A very in- 

 genious though simple contrivance effectually prevents 

 it from passing back again into the auricle. From every 

 part of the circular opening between these two cavities, 

 arises a thin, light membrane cleft almost down to its 

 roots, in three places, thus dividing it into three sepa- 

 rate pieces. These pieces are still farther divided, giv- 

 ing the membrane a fringe-like appearance. These 

 fringes are connected to the walls of the ventricle by 

 many little tendinous strings sufficiently long to allow 

 the fringes a considerable extent of motion. Now 

 when the ventricle contracts, the blood driven in all di- 

 rections insinuates itself behind these fringes,, and raise? 



