THORAXDissection
Cut off the pulmonary trunk and ascending aorta,
immediately above the three cusps of the pulmonary
and aortic valves. Remove the upper part of the left
atrium to visualise its interior (Fig. 18.29b). See the
upper surface of the cusps of the mitral valve. Revise
the fact that left atrium forms the anterior wall of the
oblique sinus of the pericardium (Fig. 18.5).Posilion
The left atrium is a quadrangular chamber situated
posteriorly. Its appendage, the left auricle projects
anteriorly to overlap the infundibulum of the right
ventricle. The left atrium forms the left two-thirds of
the base of the heart, the greater part of the upper
border, parts of the sternocostal and left surfaces and
of the left border. It receives oxygenated blood from
the lungs through four pulmonary veins, and pumps it
to the left ventricle through the left atrioventricular or
bicuspid (Latin two tooth point) or mitral orifice (Latin
like bishop's mitre) which is guarded by the valve of the
same name.Feotures
L The posterior surface of the atrium forms the anterior
wall of the oblique sinus of pericardium (Fig. 18.5).
2 The anterior wall of the atrium is formed by the
interatrial septum.
3 Two pulmonary veins open into the atrium on each
side of the posterior wall (Fig. 18.8).
4 The greater part oI the interior of the atium is smooth
walled. It is derived embryologically from the
absorbed pulmonary veins which open into it.
Musculi pectinati are present only in the auricle where
they form a reticulum. This part develops from the
original primitive atrial chamber of the heart tube.
The septalwall shows the fossa lunata corresponding
to the fossa ovalis of the right atrium. In addition to
the fourpulmonaryveins, the tributaries of the atrium
include a few venae cordis minimi.
Table 18.2 compares the right atrium and the left
atrium.
DISSECTION
Open the left ventricle by making a bold incision on the
ventricular aspect of atrioventricular groove below left
auricle and along whole thickness of left ventricle from
above downwards till its apex. Curve the incision
towards right till the inferior end of anterior inter-
ventricular groove. Reflect the flap to the right and clean
the atrioventricular and aortic valves (Fig. 18.10).
Remove the surface layers of the myocardium. Note
the general directions of its fibres and the depth of the
coronary sulcus, the wall of the atrium passing deep to
the bulging ventricular muscle. Dissect the musculature
and the conducting system of the heart.Position
The left ventricle receives oxygenated blood from the
left atrium and pumps it into the aorta. It forms the
apex of the heart, a part of the stemocostal surface, most
of the left border and left surface, and the left two-thirds
of the diaphragmatic surface (Figs 18.7 and 18.8).Feotures
L Externally, the left ventricle has three surfaces-
anterior or sternocostal, inferior or diaphragmatic,
and left.
2 The interior is divisible into two parts.
a. The lower rough part with trabeculae carneae
develops from the primitive ventricle of the heart
tube (Fig. 18.15).
b. The upper smooth part or aortic vestibule gives
origin to the ascending aorta: It develops from the
mid portion of the bulbus cordis. The vestibule
lies between the membranous part of the inter-
ventricular septum and the anterior or aortic cusp
of the mitral valve.
3 The interior of the ventricle shows two orifices.
a. The left atrioventricular or bicuspid or mitral
orifice, guarded by the bicuspid or mitral valve.
b. The aortic orifice, guarded by the aortic valve
(Fig. 18.15).Table 18.2: Comparison of right atrium and left atrium
Right atrium
Receives venous blood of the body
Pushes blood to right ventricle through tricuspid valve
Forms right border, part of sternocostal and
small part of base of the head
Enlarged in tricuspid stenosisLeft atrium
Receives oxygenated blood from lungs
Pushes blood to left ventricle through bicuspid valve
Forms major pad of base of the heartEnlarged in mitral stenosis