Human Anatomy Vol 1

THORAX

across the sternocostal surface to reach the apex of

heart, where these fibres form a vortex and continue

with the deep layer.

Superficial fibres are:

a. Fibres start from tendon of infundibulum (1) pass

across the diaphragmatic surface, curve around

inferior border to reach the sternocostal surface.

Then these fibres cross the anterior interventri-

cular groove to reach the apex, where these form

a vortex and end in anterior papillary muscle of

left ventricle (Fig. 19.2la).

b. Fibres arise from right AV ring take same course

as (2) but end in posterior papillary muscle

(Fig. 18.21a).

c. Fibres arise from left AV ring, lie along the

diaphragmatic surface, cross the posterior inter-

ventricular groove to reach the papillary muscles

of right ventricle (Fig. 18.21b).

d. Deep fibres are 'S' shaped. These arise from

papillary muscle of one ventricle, turn in inter-

ventricular groove, to end in papillary muscle of

other ventricle. Fibres of first layer circle RV, cross

through interventricular septum and end in

papillary muscle of LV. Layers two and three have

decreasing course in RV and increasing course in

LV (Fig. 18.21c).

CONDUCTING SYSTEM

The conducting system is made up of myocardium
that is specialised for initiation and conduction of the
cardiac impulse. Its fibres are finer than other
myocardial fibres, and are completely cross-striated.
The conducting system has the following parts.

I Sinuatrial node or SA node: It is known as the
'pacemaker' of the heart. It generates impulses at the
rate of about 70-1,00 beats/min and initiates the
heartbeat. It is horseshoe-shaped and is situated at
the atriocaval junction in the upper part of the sulcus
terminals. The iinpulse travels through the atrial wall
to reach the AV node (Fig. 18.14).

2 Atrioaentricular node or AV node: It is smaller than
the SA node and is situated in the lower and dorsal
part of the atrial septum just above the opening of
the coronary sinus. It is capable of generating
impulses at a rate of about 40 to 60 beats/min.

3 Atriooentricular bundle or AV bundle or bttndle of His: It
is the only muscular connection between the atrial
and ventricular musculatures. It begins as the
atrioventricular (AV) node crosses AV ring and
descends along the posteroinferior border of the
membranous part of the ventricular septum. At the
upper border bf tne muscular part of the septum, it
divides into right and left branches.

Fig. 18.21a: Superficial transverse fibres of atria and superficial

fibres of ventricles 1, 2

Vertical

flbres of atria

Fig.18.21b: Vertical fibres of atria and superficial fibres of

ventricle 3

Fig. 18.21c: Deep fibres of ventricles in three layers