The orifices which lead into and out of the ventricles have peculiar arrangements of their lining texture, forming valves which allow the blood to pass only in a certain direction. These valves, which form a most interesting and important part of the economy of the heart, are of two kinds, each differing in its mode of action. One prevents the passage of the blood from the ventricles to the auricles, the other guards the openings into the great arteries.

The auriculo-ventricular valves have a sail-like action. They are made up of delicate curtains formed of thin sheets of connective tissue -arising from the margins of the auriculo-ventricular openings which form the fixed attachment of each of the curtains of the valves. The free edges and ventricular surfaces of the curtains are blended with the tendinous cords coming from the papillary muscles, and thus give points of tendinous attachment to some of the bundles of muscle fibres in the wall of the ventricle. At the right auriculo-ventricular opening there are three chief curtains; hence it is called the "tricuspid" valve (Fig. 117, RAV). The opening from the left auricle to the left ventricle, which is about one-third smaller, is guarded by two large valvular flaps, and is hence called the "bicuspid," or more commonly "mitral," valve (Fig. 116).

The aortic and pulmonary valves are made up of three deep semilunar pockets with free margins looking toward the vessel. The convex base of each pocket is attached to the arterial orifice of the ventricle, with the lining membrane of which it is continuous.

Portion of the Wall of Ventricle (d d') and Aorta (a b c), showing attachments of one flap of mitral.

Fig. 116. Portion of the Wall of Ventricle (d d') and Aorta (a b c), showing attachments of one flap of mitral and the aortic valves: (h and g) papillary muscles; (e;e, and.f) attachment of the tendinous cords. {Allen Thomson).