At the attachment of each auricle to its corresponding ventricle there is situated a dense ring of tough connective tissue, which surrounds the openings leading from the auricles to the ventricles. Similar tendinous rings (zona tendinosa) exist around the orifice of the aorta and pulmonary arteries. These tendinous rings form the basis of attachment for the muscle bundles of the walls of both the ventricles and auricles.

The Left Auricle and Ventricle opened and part of their walls removed to show their cavities.

Fig.114. The Left Auricle and Ventricle opened and part of their walls removed to show their cavities. (Allen Thomson).

1. Right pulmonary vein cut short. 1'. Cavity of left auricle. 3. Thick wall of left ventricle. 4. Portion of the same with papillary muscle attached. 5, 5'. The other papillary muscles. 6. One segment of the mitral valve. 7. In aorta is placed over the semilunar valves.

In the ventricles many layers of muscles can be made out.

The outer fibres pass in a twisted manner from the base toward the apex, where they are tucked in so as to reach the inner surface of the ventricular cavity. They then pass back to be attached at the base; some passing into the papillary muscles are connected with the cardiac valves through the medium of the chordae tendineae; and the others, forming irregular masses of muscle on the inner surface of the cavity, pass in various directions toward the base, to be fused with the tendinous rings around the arterial orifices. Another set of layers passes transversely around the ventricle lying between the inner and outer sets, and passing nearly at right angles to them.

The muscular fibres forming the thin auricular walls have their origin from the zones of the auriciilo-ventricular orifices, and pass very irregularly around the cavities. The outer set of fibres have a transverse, the inner a longitudinal direction. Bands of fibres encircle the orifices of the great veins, and extend for some little distance along the vessels, particularly on the pulmonary veins, which have thick, circular, muscular coats after they leave the lungs.

The fibres of the auricles are not directly continuous with those of the ventricles, the auricular and ventricular fibres being only related to each other by their points of origin, viz., the auriculo-ventricular fibrous zones.