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Vencrticular Myocardial Band Structure
A-. The dissection of the band.
After the separation of the pulmonary artery and the aorta (fig. 12) some superficial fibres (the aberrant
or wrapping fibres, which will not be now described) are incised,
along the anterior interventricular sulcus, to move aside the right free wall (fig. 13). It can be observed, then, the
posterior limit of the right ventricular cavity, which is represented by the linear bottom
of the dihedral angle constituted by the free wall of the right ventricle and the septum (fig. 14). That anatomical fact, the one
represented by the linear posterior limit of the right ventriculkar cavity, has special
importance since it points out the beginning of the laminar trajectory that, when
followed, allows to unroll the
ventricular myocardial
band.
The beginning
of that trajectory is shown after pushing laterally the free wall of the right ventricle (fig. 15). Such trajectory leads, when its
pathway is followed contouring posteriorly the ventricles, to the left fibrous trigon,
which is then incised (fig. 16).
Afterwards,
coming back to the site of the posterior limit of the right ventricular cavity (fig. 14), two muscular strata, which fibres cross in a 90 degree
angle (fig. 17), are separated going in
between the vertical and the horizontal fibres (fig.
18). This separation points up then the right fibrous trigon, which is also incised (fig. 19).
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The ventricular miocardial band is then ready to be stretched out (fig. 20).
B-. The configuration of the ventricular myocardial band.
It was concluded that the ventricular myocardium consists of a single muscle band which starts at the origin of the pulmonary artery and finishes at the root of the aorta. This band, twisted two turns on itself in a helical fashion, defines the right ventricular cavity, by means of one half turn, and the ventricular left cavity by the subsequent one and a half turns (see figs. 9 and 10).
In order to make more clear the architectural reasons of the ventricular myocardium macroscopical structure, the winding process of the band has been reproduced with a paper strip, a rope model, an actual myocardial band and a silicone rubber mould.
a) A paper strip model.
In figure 21 has been represented, by means of a paper strip, the
winding process, of the ventricular band, by means of which are defined in the space the
ventricular cavities. At the centre (b) can be seen a fold between the two ends of the
strip, the pulmonary artery (a) and the aorta (b). The origin of that fold (it can be seen
also in the specimen of fig. 20) is not going to be described
now to avoid move the attention from the fundamental problem of this description.
b) A silicone rubber mould of the heart.
A silicone rubber mould has been also created with an elastic
material (fig. 22) to reproduce with high
fidelity the morphological particularities of the surface of
the band in all its lenght. The value of this silicone rubber mould lies in its
contribution to heart structure understanding, which comes from being able to handle it
freely, winding and unwinding the band as many times as needed (to ask for this model
address to: MODELOS CS, S.L., Apartado de Correos nš 20, 03700 Denia, Alicante, Spain.
Fax: 34 6 578 3082).
c) A sequence.
In figure 22 A, B, C, D and E, the successive stages for unwinding the ventricular myocardium are represented by means of a rope model, an actual anatomical specimen and the above mentioned rubber mould. Following this morphological sequence the architectural reasons of the ventricular myocardium structure can be easily understood ; the rubber mould utility, made with didactic purposes, is then verified.
C-. The anatomical basis for the segmentation of the ventricular band.
1. The loops.
Figures 21, 23 and 24 show how
the band, extending from the root of the pulmonary artery, a, to the root of the aorta, c,
is folded in its centre, b, a folding that divides the band in two loops ; a-b represents
the basal loop and b-c the apical loop.
2. The segments.
Each of these two loops, in its turn, can be divided into two
segments, as shown in figure
24. The posterior
interventricular sulcus, which coincides topographically with the posterior limit of the
right ventricular cavity, is shown at d (that posterior limit and the 
posterior interventricular sulcus are
pointed out by an arrow and PIS, respectively, in fig. 23 B).
The posterior interventricular sulcus thus divides the basal loop into two segments (fig. 24 B), the right one, which is represented by the free
wall of the right ventricle (RFW), and the left one, which is constituted by the
free wall of the left ventricle (LFW). Since the right segment (RFW) defines the outer
border of the tricuspid orifice (T) and the left segment (LFW) defines the outer border of
the mitral orifice (M), this two segmments can be regarded as the lateral segments.
The apical loop can be also divided into two segments. Since
the descending fibres of this apical loop, f in figure 24 A and
B, bend around e (scheme A), giving rise to the virtual orifice of the apex, to become in
ascending, g, the anterior papillary muscle (APM, dotted line)
conveniently becomes the dividing line between the descendent and the ascendent
segments. These two segments, f and g in figure
25 A, constitute the septum when the apical loop is closed as it is shown in B (both
schemes, A and B, must be compared with stages B and C of fig. 22).
3. The interventricular septum.
Since the descendent and the ascendent segment make up the ventricular septum it might be convenient to call them the septal segments ; on the other hand, as shown in figure 24 B, the left segment (LFW), that belongs to the basal loop, and the descendent segment (f), that belongs to the apical loop, could be regarded as the medial segments since theay are ubicated in the central region of the band.
Although it is not directly
concerned with the segmentation of the band, it should not be forgotten, of course, that
the septum incorporates a third component of fibres (they will be represented later on ;
r.s.f. in the scheme D of fig. 33) which
come from the free wall of the right ventricle, reflect at the anterior interventricular
sulcus, to become subendocardial, and ascend to the pulmonary artery ring, to the corda
pulmotricuspidalis and to the tricuspid ring, where they are inserted.
4. Summary.
Since the ventricular myocardium is made up of a band of
myocardial fibres which are configured into a
three dimensional helical way, some time spent handling and observing that teaching
elastic model mentioned above, can help any cardiologist to study, on one hand, the
anatomical subdividions or segments of the band (fig.
26) and, on the other hand, to understand and interpret the structural basis of the
motion patterns of the heart. We concede that the cooperation of all disciplines is
necessary to elucidate functional understanding of the myocardial band structure described
above.
