The tissues that form the skeletal structure of the heart are four annular fibrous rings, two small bones, the muscle, serous membrane, vessels and nerves.
Fibrous rings
- The fibrous rings are four in number-two at the atrio-ventricular openings and two at the arterial openings.
- The former separate the muscle fibres of the atria from those of the ventricle.
Bone
- A small bone called the Os cordis (right and left)is found in the aortic ring of old animals.
- The right is larger than the left and is triangular in form. It is related to the attachment of the right posterior cusp of the aortic valve.
- The left is at the junction on the anterior and left posterior cusps.
- They appear to be entirely cartilaginous in young animals.
Musculature
- The myocardium or heart muscle is of a peculiar striated variety arranged in a somewhat complicated manner.
- The muscular tissue of the atria is almost completely separated from that of the ventricles by the fibrous rings around atrio-ventricular orifices.
- The two groups of muscular tissue are however, connected by a bundle of muscle fibres called the atrio-ventricular bundle.
- It is a rudimentary structure and pale in colour, which forms part of the conducting system of the heart.
- The fibres of the myocardium fall into two main groups, atrial and ventricular muscle fibres which are independent of each other except for the atrio-ventricular bundle.
- The atrial fibres are of two varieties, superficial or common and deep or special. The superficial fibres are common to both the atria.
- They begin and end in the atrio-ventricular rings, passing transversely across the atria and a few of them dip into the atrial septum.
- The deep fibres are peculiar to each atrium and of two varieties; annular fibres surround the ends of veins, which open into the atria and the fossa ovalis while looped fibres pass over the atria from ring to ring.
- The ventricular fibres are stronger and form two sets. The superficial or common fibres arise from the atrio-ventricular fibrous rings in successive layer and pass in a spiral direction from above downward around the ventricles towards the apex. Here they form a coil bend upon themselves and pass deeply upward to terminate in a papillary muscle of the ventricle opposite to that in which they arose.
- The loop so formed at the apex constitutes a vortex cordis. The deep fibres begins from one side, curve around the wall of the ventricle between the two parts of the superficial set of fibres then pass in the septum to the opposite side and curve around the other ventricle. Thus their arrangement is scroll- like.
Serous membrane
- The myocardium is covered by the visceral layer of the serous part of the pericardium called the epicardium.
- The cavities of the heart are lined by endocardium.
