Human Anatomy Vol 1

margin of each cusp contains a central fibrous nodule

from each side of which a thin smooth margin

the lunule extends up to the base of the cusp. These

oalztes are closed during uentricular diastole when

each cusp bulges towards the ventricular cavity

(Fig. 18.17).

3 Opposite the cusps the vessel walls are slightly
dilated to form the aortic and pulmonary sinuses.
The coronary arteries arise from the anterior and the
left posterior aortic sinuses (Fig. 18.18).

PERICARDIUM AND HEART

Fibrous Skelelon

The fibrous rings surrounding the atrioventricular and

arterial orifices, along with some adjoining masses of

fibrous tissue, constitute the fibrous skeleton of the

heart. It provides attachment to the cardiac muscle and

keeps the cardiac valve competent (Fig. 18.20).

The atriouentricular fibrous rings are in the form of

the figure of 8. The atria, the ventricles and the

membranous part of the interventricular septum are

attached to them. There is no muscular continuity

between the atria and ventricles across the rings except

for the atrioventricular bundle or bundle of His.

There is large mass of fibrous tissue

-between

the

atrioventricular rings behind and the aortic ring in front.

It is known as the trigonum fibrosum dextrum.In some

mammals like sheep, a small bone the os cordis is present

in this mass of fibrous tissue.

Another smaller mass of fibrous tissue is present

between the aortic and mitral rings. It is known as the

trigonum fibrosum sinistrum. The tendon of the infundi-

bulum (close to pulmonary valve) binds the posterior

surface of the infundibulum to the aortic ring.

Musculolure of the Heorl

Cardiac muscle fibres form long loops which are

attached to the fibrous skeleton. Upon contraction of the

muscular loops, the blood from the cardiac chambers is

wrung out like water from a wet cloth. The atrial fibres

are arranged in a superficial transverse layer and a deep

anteroposterior (vertical) layer.

The ventricular fibres are arranged in superficial

and deep layers.

The superficial fibres arise from skeleton of the heart

to undergo a spiral course. First these pass across the

inferior surface, wind round the lower border and then

Anterior

Pulmonary

valve

Tendon of infundibulum

Aortic valve

Origin of left

coronary aftery Origin

of right

coronary

artery

Risht

Trigonu

Tricuspid

valve

Mitral valve Trigonum

Posterior fibrosum dextrum

Fig. 18.20: Heart seen from above after removing the atria. The

mitral, tricuspid, aortic and pulmonary orifices and their valves

are seen. The fibrous skeleton of the heart is also shown

(anatomical position)

The first heart sound is produced by closure of

the atrioventricular valves. The second heart

sound is produced by closure of the semilunar

valves (Figs 18.19a and b).

Narrowing of the valve orifice due to fusion of

the cusps is known as 'stenosis', yiz. mitral

stenosis, aortic stenosis, etc.

Dilatation of the valve orifice, or stiffening of the

cusps causes imperfect closure of the valve

leading to back flow of blood. This is known as

incompetence or regurgitation, e.g. aortic

incompetence or aortic regurgitation.

fibrosum

sinistrum

Figs 18.1 9a and b: (a) First heart sound, and (b) second heart

sound