This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
Between each auricle of the heart and the ventricle of the same side are found valves which allow blood to pass from the auricle to the ventricle, but prevent any flow in the opposite direction. These valves are known as the tricuspid and mitral valves. The mitral valve (Fig. 59) consists of two flaps fixed by their bases to the margins of the opening between the left auricle and the left ventricle; their edges hang down into the ventricle when the heart is empty. These edges are not free, but have fixed to them a number of stout connective-tissue cords, the chordś tendineś, which are fixed below to muscular elevations, the papillary muscles, Mpm and Mpl, on the interior of the ventricle. The cords are long enough to let the valve flaps rise into a horizontal position and so to close the opening between auricle and ventricle, which lies behind the opened aorta, Sp, represented in the figure. The tricuspid valve is like the mitral, but with three flaps instead of two.
These are six in number ; three at the mouth of the aorta, Fig. 59, and three, quite like them, at the mouth of the pulmonary artery. Each is a strong crescentic pouch fixed by its more curved border, and with its free edge turned away from the heart. When the valves are in action their free edges meet across the vessel and prevent blood from flowing back into the ventricle.