Atrio-ventricular Orifices
The right and left atrio-ventricular orifices are oval apertures. The openings lie in a plane that is almost vertical with a slight downward inclination. Each opening is directed forwards then left and slightly downwards. Each orifice is strengthened by a variable amount of fibrous tissue around it. It is guarded by a valve which allows flow of blood, only from atrium to ventricle but not in the reverse direction. The valves are made up of cusps which are thin leaflets of tissue. Each cusp consists of a double fold of endocardium within which is some fibrous tissue. It has two surfaces, ventricular and atrial. It has a base which is attached to the ring of fibrous tissue around the Atrio-ventricular orifice. It has an apex and free margins. The margins of adjoining cusps are fused to each other for some distance so that the cups form a continuous membrane. The apex and the margins give attachment to tendinous strands called the chordae tendinae. The chordae tendinae are also attached to the ventricular surfaces of the cusps which are therefore, rough in contrast to the atrial surface which are smooth. At their other end the chordae tendinae are attached to the apices of the papillary muscles. Each papillary muscle through the chordae tendinae is attached to the adjoining part of the two cusps. As a result the adjoining margins of the two cusps are drawn together when the papillary muscle contracts.
The features peculiar to each orifice are: the right orifice is larger than the left; it admits the tips of three fingers. In contrast the left orifice is smaller and admits the tips of two fingers. The arrangements of cusps and papillary muscles also differ.