materially. The wall of the left ventricle, including that part which forms the inter-ventricular septum, is nearly three times as thick as that of the right or pulmonary ventricle.

Interior of Right Auricle and Ventricle exposed by the removal of a part of their walls. (Allen Thomson.)

1. Superior vena cava. 2. Inferior vena cava. 2'. Hepatic veins. 3, 3', 3". Inner wall of right auricle. 4, 4. Cavity of right ventricle. 4'. Papillary muscle. 5, 5', 5". Flaps of tricuspid valve. 6. Pulmonary artery, in the wall of which a window has been cut. 7. On aorta near the ductus arteriosus. 8, 9. Aorta and its branches. auricle and ventricle.

10, 11. Left

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

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.

tendinous rings form the basis of attachment for the muscle bundles of the walls of both the ventricles and auricles.

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 chorda tendineæ ; 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 Another set of layers passes transversely around the ventricle lying between the inner and outer sets, and passing nearly at right angles to them.

around the arterial orifices.

The muscular fibres forming the thin auricular walls have their origin from the zones of the auriculo-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

zones.

MINUTE STRUCTURE.

The muscle tissue of the heart differs both in structure and mode of action from the other contractile tissues of the body. The elements are firmly united with one another to form irregular

close networks, which, however, can be broken up into masses easily recognizable as peculiar cells. These cells are irregular, prismoidal blocks, the blunt ends of which are often split, allowing connection with two contiguous cells. They contain a nucleus, situated in the central axis of the cell. The cells are not surrounded by a distinct sheath of sarcolemma.

Though striated, the action of the heart muscle is peculiarly independent of the higher nervous centres, being quite involuntary; it is characterized by a definite periodicity and is incapable of tetanus. The duration of its contraction is very long when compared with that of the skeletal muscles, but is much shorter than that of the contracting tissues of most hollow viscera.

VALVES.

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 (dd) and Aorta (a b c), showing attachments of one flap of mitral and the aortic valves: ( and g) papillary muscles; (e, e, and f) attachment of the tendinous cords. (Allen Thomson.)

Auriculo-ventricular Valves.-The mode of action of the flaps of the tricuspid and mitral valves is like that of a lateen sail of a boat, if we substitute the blood stream for the air current; the tendinous cords acting as the "sheet" or rope which restrains the sail when filled with wind.