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CHAPTER 1
Functional Anatomy of the Myocardium
1. Preliminary considerations

Classical anatomy of the heart considered that the muscle structure forming the myocardium was homogeneous and compact. Based on this concept, it was described by an external and internal surface limiting a solid, uniform muscle mass. Andreas Vesalius, in his work De Humanis Corporis Fabrica (1543), referred to the difficulty in identifying the layers forming the myocardium. He literally expressed “Whichever way you perform a dissection of the heart meat, either raw or cooked…, you can hardly remove a portion with only one type of fiber, because they have multiple and different directions, mainly transversal”. Three centuries after (1864), J. B. Pettigrew also referred to this situation: “About the complexity of the arrangement I need not say more than Vesalius, Haller and De Blainville, who all confessed their inability to elucidate it”. (81, 114)

This classical structural notion does not explain cardiac mechanics; hence, it is essential to establish its true internal anatomy. Historically, very little importance was attributed to the spatial arrangement of the muscle bundles forming the myocardium. (23) In1970 Francisco Torrent Guasp (102-108) defines the anatomy of the heart adapted to physiological reality. This study approach correlates with a cardiac structure presenting the remarkable characteristic of being adriving-suction pump with the size equivalent to a human fist and an average weight of 270 grams, which ejects 4-6 liters/minute at a speed of 300 cm/s, consuming only 10 watts, and working without interruption for 80 years without maintenance, almost without noise, and no smoke. Its work is equivalent to the daily withdrawal of 1 ton of water 1 m deep with a mechanical efficiency (work/energy relationship) of 50%, not achieved by man-made machines which only attain 30% efficacy. This allows ejecting 70% of the left ventricular volume with only 12% shortening of its contractile unit, the sarcomere.

Torrent Guasp demonstrated through numerous dissections of hearts from different species, including humans, that the ventricular myocardium is formed by an assembly of fibers coiled unto themselves similarly to a rope (rope model) (Figures 1.1 to 1.4) and flattened laterally as a band, which by giving two spiral turns defines a helix limiting both ventricles and setting their performance. (2, 3) This structure is supported by the evolutionary process taking place from the primitive circulatory duct of the annelids to that of mammals, whose arterial circuit forms a loop or fold that twists unto itself giving rise to the ventricular chambers. The primary duct lumen establishes a secondary communication between both adjacent chambers (ventricles) formed by the loop, assuming that the side of the duct where the interconnection takes place must have cleaved all along the duct to achieve this purpose.

Based on these facts, we find that the spatial organization and the rotational motion of the ventricular fibers in their anatomical arrangement, both at the base and apical region levels, find correspondence with the myocardial band. However, after its original description, this anatomy which allows unfolding the heart to form a muscle band has not been considered valid until now by the medical culture.

Figure 1.1. Myocardial band.

Figure 1.2. Origin of myocardial band unfolding.

Figure 1.3. Myocardial band prior to its

complete unfolding (See Figure 1.12).

Figure 1.4. Rope model of the myocardial band. It illustrates the different segments that form the band. In blue: Basal loop. In red: Apical loop.

In view of the criticism or indifference met by the helical myocardial band proposed by Torrent Guasp due to lack of information on the necessary anatomical technique to unfold it, we can currently obtain its confirmation through:

1 Anatomical and histological study of the heart. (128)

2 The evolutionary concept emerging from phylogeny.

3 New imaging procedures obtained with magnetic resonance imaging by diffusion tensor. (17, 24, 82, 142)

4 Echocardiography. (59, 70, 71)

5 Electrophysiological studies performed with three-dimensional electroanatomical mapping. (121-127, 131, 132)

Regarding the argued difficulty to dissect the myocardium, which is more apparent than real (128), we should consider that once the myocardial band originated as a loop in the arterial semicircle of amphibians and reptiles to adapt to the physiological demands of aerial life, the muscle bundles became firmly attached to their contact surfaces, hampering the necessary cleavage planes necessary for their anatomical dissection. The evolutionary goal was to develop a sufficiently solid hemodynamic structure with the strength to generate the suction and pumping of the blood volume that supplied the whole organism. Thus, every attempt to dissect an anatomical segment from the rest of the myocardium, avoiding the real cardiac arrangement, always turns into an obstacle due to the structural plan of the axes where the orientation of the myocardial band courses.

An explanation for this muscular homogeneity hiding the myocardial band, implies considering the required functioning in birds and mammals to achieve blood ejection at a high speed in a limited time span by an organ that must supply two circulations (systemic and pulmonary). Despite these considerations and the classical concept about myocardial anatomy, its dissection finds a structure with defined planes where the successive and related physiological heart motions of narrowing, shortening-twisting, lengthening-untwisting and expansion take place depending on the propagation of the electrical stimulus along its muscle pathways (chapter 1). (125)

The myocardial fibers forming the myocardium cannot be considered as absolutely independent entities within a defined space. Despite the intricacy of fiber bundles with polygonal shape, which in addition receive and give off collateral fibers, a predominant course of central fibers is defined with sliding planes, which together form the myocardial muscle band. It should be recalled that the myocardium constitutes a spiraling continuum in its fibers responding to the helical pattern in its muscle bundles. This arrangement indicates the need of generating a mechanical work that dissipates little energy. Therefore, the fiber layers very gradually shift their orientation, with more or less acute angles, to avoid that abrupt changes in the spatial organization dissipate the necessary work for cardiac function. The fan of fibers that is formed reduces the stress among them.

This situation generates a tangle of fibers that allows the band to behave as a continuous transmission chain with the epicardial fibers taking an oblique direction, the intermediate fibers a transverse course and the endocardial fibers also an oblique direction, but contrary to that of the epicardial plane. The endocardial and epicardial plane access angle is approximately 60 degrees in relation to the transverse fibers. Fiber orientation defines function and thus the ejection fraction is 60% when the normal helical fibers contract and falls to nearly 30% if only the transverse fibers shorten. This occurs when the left ventricle dilates in cardiac remodeling and the fibers miss their oblique orientation, loosing muscular and mechanical efficiency.

It should therefore be acknowledged that a gradual change in orientation is generated from the superficial to the deep fibers that form the different segments of the muscle band. In the progression from the ventricular base to the apex, the number of horizontal fibers decreases in relation to the oblique fibers, showing that the heart is organized as a continuous muscle helix. The ventricular mechanical activity must be heterogeneous during diastole with subendocardial-subepicardial relaxation gradients. During systole, the muscle layers of the myocardial band evidence pronounced and opposite torsion in the subendocardium in relation to the subepicardium, whereas in the apex the subepicardial fiber rotation acquires more relevance.

Beyond this complexity it is necessary to establish the concept of linear and laminar trajectories. Myocardial muscle bundles and bands, which derive from phylogenetic development, essentially shape a master axis of precise dynamic requirement. The spatial muscle structure adopted by the myocardial muscle band has a double function: a) to limit the ventricular chambers and b) to fulfill the suction and driving action in its role of cardiac pump.

2. Myocardial architecture

Left ventricle. The entire apex belongs to the left ventricle. In the distal part of the left ventricle, called apical, a muscle layer with spiral trajectory extends from the surface to the center and undergoes a rotation that turns the subepicardial fibers into subendocardial fibers, overlapping like the tiles of a roof. Consequently, the left ventricular distal end, the apex, surrounds a virtual conduit with no muscular plane at its ultimate end, lined internally by the endocardium and externally by the epicardium, with no intermediary muscle. It is essential to consider that in the apex the fibers undergo a helical rotational motion with sphincter-like arrangement as they transform from subepicardial to subendocardial fibers, following a clockwise trajectory (apical view of the diaphragmatic surface of the heart in the anatomical position) (Figures 1.5 and 1.6). (105)

Figure 1.5. Apical view of the left and right ventricles.

Figure 1.6. Spiral arrangement of apical muscle layers.

In the left ventricular basal half, at the level of its free wall (Figure 1.7), the fibers are arranged similarly to the apical half. A muscle layer with spiral trajectory extends from the surface to the center displaying its fibers from outside to inside (from paraepicardial to paraendocardial regions). At this level their orientation is opposite to that of the apex, following a counter-clockwise trajectory (apical view of the diaphragmatic surface of the heart in anatomical position). This arrangement of the muscle layer in its twist limits a cavity which at the base of the heart is real and not virtual as in the apex.

Figure 1.7. Basal third of the left ventricle.

It depicts the muscle layers of the free wall.

The apex should be considered as a tunnel with a muscular rim surrounding its entire ring, while at the ventricular base this ring has two parts. One part corresponds to the left ventricular free wall and the other to the interventricular septum. In addition, the most superficial basal fibers make contact with the fibrous mitral annulus, a setting that is absent at the apical level. Nevertheless, the essential functional difference between basal and apical regions is their opposite fiber motion. This characteristic determines myocardial torsion to achieve cardiac blood ejection and the untwisting that generates suction and diastolic filling.

Right ventricle. Theoretically, two types of fibers can be identified in its distal half according to their orientation: paraendocardial and paraepicardial fibers. The former extend from the pulmonary base backwards and downwards to the apical region, whereas the others extend from the anterior interventricular sulcus to the back approaching the base of the heart. This X-shaped crossover arrangement allows the fibers in the distal end of the right ventricle to adopt a helical arrangement, turning from subepicardial to subendocardial (Figure 1.8).

Figure 1.8. Right ventricular free wall.

Three segments can be identified at the basal half of the right ventricle (tricuspid orifice perimeter): free wall, supraventricular crest and interventricular septum. The free wall presents the same general configuration of spiraling fibers that extend from subepicardial to subendocardial positions. Similarly to the left ventricle, there is a difference in the rotating sense of the basal fibers with respect to the distal ones. They follow a counterclockwise trajectory at the basal end and a clockwise trajectory at the distal end (apical view of the diaphragmatic surface of the heart in anatomical position).

The comparison of the rope model (Figure 1.4) with the ventricular apical and basal regions (Figure 1.1) shows the analogy existing between the rope configuration and the myocardial fibers.

3. Segmentation of the myocardial band

In its course, the myocardial band adopts a helical configuration that defines the two ventricular chambers. The myocardial band is characterized by a descending and an ascending band. The former includes the right, left and descending segments, while the latter is formed by the remaining ascending segment (Figure 1.4). The figure in 8 outlined by this trajectory defines two loops: a basal and an apical loop. An interesting article by R. F. Shaner in 1923 (84) expresses that the myocardium is formed by “two flattened muscles arranged into a figure of 8. These muscles shorten in opposite directions in systole, emptying the blood content.”

The myocardial band describes two spiral turns with the insertion of its initial end along the line extending from the pulmonary artery to the orifice of the tricuspid valve, called the pulmo-tricuspid cord, in front of the aorta, while its final end attaches below the aortic root. Both ends are fixed by an osseous, chondroid or tendinous nucleus, depending on the different species (animal or human) used in the studies. This nucleus, which has been termed cardiac fulcrum, is the only perceptible edge where the muscle band fibers originate and end, and will be extensively analyzed in this chapter (Figures 1.16 and 1.19).

The basal loop extends from the base of the pulmonary artery to the central band twist. On the other hand, the apical loop courses from this point of inflexion to the base of the aorta. In turn, each loop consists of two segments. The basal loop consists of the right and left segments and the apical loop of the descending and ascending segments (Figure 1.4). In the general loop configuration, the basal loop envelops the apical loop, so that the right ventricular chamber presents as an open slit in the muscle mass thickness forming both ventricles (Figure 1.1). The segments, in turn, are defined by anatomical structures.

Basal loop. The posterior interventricular sulcus presents a trough that establishes the limit between the right and left segments of the basal loop. The right segment constitutes the right ventricular free wall and surrounds the tricuspid valve orifice on the outside. The left segment situated in the left ventricular free wall defines the mitral valve orifice externally. The fibers run from the subepicardium to the subendocardium following a counterclockwise helical direction (apical view of the diaphragmatic surface of the heart in the anatomical position).

Apical loop. The descending apical segment extends from the twist in the band to the apex, where it becomes the ascending segment to end mainly below the base of the aorta in the cardiac fulcrum, a structure to which the myocardial band fibers attach. Both segments mainly form the interventricular septum. Similarly to the basal loop, the fibers run from the subepicardium to the subendocardium, but in this case following a helical clockwise direction (apical view of the diaphragmatic surface of the heart in the anatomical position).

These concepts indicate that the right ventricular free wall is formed by one loop (basal) and the left ventricular free wall by both loops (basal and apical). The fundamental point for cardiac mechanics is that the base and apical muscle fibers course in different directions. This disparity finds correlation with the fiber trajectories and the helical pattern of the muscle band limiting the ventricles.

4. Anatomy of the cardiac band. Dissection

To dissect the heart it must be boiled in water with acetic acid (15 cc per liter) for approximately two hours and a half, except that a pressure cooker is used, in which case the time is reduced by half. Prior work before unfolding the muscle band consists in separating the atria from the ventricles in a very simple maneuver that demonstrates the different evolutionary origins between both types of chambers. Then, the aorta and the pulmonary artery are cut three centimeters from their origin, separating the attachment between them, and finally, a longitudinal incision is done on the superficial fibers (interband or aberrant fibers) (Figure 1.58) (105, 123, 128) extending transversally along the anterior wall of the ventricles. Prior boiling of the anatomical piece allows the easy execution of all these steps. Between the atrial and the ventricular walls there is only connective tissue, allowing the easy separation of these chambers due to the denaturation produced by heat.

The key maneuver to unfold the myocardial band consists in entering the anterior interventricular sulcus with a blunt instrument, leaving on the left side of the operator the end of the band corresponding to the pulmonary artery and its continuity with the right ventricular free wall (right segment). Next, traction is applied towards the same left side (Figure 1.2), completely releasing the pulmonary artery from the rest of the myocardial band (Figure 1.27). Below the aorta we find the cardiac fulcrum, where the origin and end of the band attach (Figures 1.9 and 1.10). (128)

Figure 1.9. Cardiac fulcrum. Septal part of the aortic annulus between the right and left trigones.

Figure 1.10. Cardiac fulcrum in the bovine heart.

It should be understood that as the myocardial band is unfolded, separating the pulmonary artery and the pulmo-tricuspid cord (anterior) from the ascending segment (posterior), the vision of the homogeneous anatomical reality is lost. This concurrence of the beginning and end of the muscle band in the cardiac fulcrum constitutes a meeting point between the right segment and the ascending segment, origin and end of the myocardial band. Thus, both band ends are situated in the same point, with the origin placed anteriorly to the end of the band. Once the initial end of the band is separated from this meeting point, of great stiffness and certain resistance to the maneuver, the heart loses its functional anatomy (Figures 1.3, 1.11 and 1.27).

The progression of the myocardial band dissection implies finding the whole extent of the right segment, the beginning of the left segment and, at the posterior margin of the right ventricular chamber, the dihedral angle formed by the interventricular septum and the right ventricular free wall (right segment).

The next step (the most delicate one) consists in entering the dihedral angle between the right ventricular and intraseptal fibers. This separation from the right ventricle allows entering a cleavage between the anterior septal band and the intraseptal band (final segment of the myocardial muscle band), at the ventral part of the septum (Figure 1.11 C). Then, the dorsal part of the septum is dissected between the posterior septal band and the left descending segment to remove and separate the aorta.

Finally, the trajectories of the muscle planes belonging to the descending segment are separated in blunt fashion from those of the ascending segment leading to the cardiac fulcrum, contiguously with the aorta at the opposite end of the muscle band, to the right of the operator, allowing the band to be unfolded in all its length (Figure 1.11 D). Figures 1.11 and 1.12 show the sequence of myocardial unfolding until it becomes a muscle band. Being able to unfold the myocardium with a similar thickness in all its extension proves that the band is real and not a “heuristic” construction.

Figure 1.11. Unfolding of the myocardial band.

The diagram in Figure 1.13 details the different segments comprising the band, as well as the final insertions constituting the origin and end of the band, very close to each other in the intact heart.

Histological analysis of the myocardial band. The histological analysis sequence of the unfolded myocardial band demonstrates its linear orientation according to the segmental continuity of its spatial organization when the band is coiled, both in its internal and external surfaces (Figure 1.14). These orientations are identical in both surfaces (internal and external). And if two things are identical it means that they present a unique orientation.

Figure 1.12. Myocardial band unfolded in all its extension.

Figure 1.13. Descriptive diagram of the muscle band segments.

It can be seen that the myocardial structure is not a lattice but a band. The lattice concept used was developed due to the band folding resulting in overlapping segments, which are functionally independent and with friction between their surfaces. The sliding motion between segments implies that they have a lubricating system that facilitates autonomous motion with lower energy expenditure despite being overlapped, as will be analyzed in this chapter. This arrangement is essential to achieve myocardial torsion, a fundamental action of cardiac mechanics that would be impossible with the lattice structure (crisscross of myocardial fibers).

No segment of the band histological sequence explored in our investigations presents a lattice organization. As the external surface of the distal descending segment (Figure 1.14, lower panel) twists to become the ascending segment, the cardiomyocytes generate in the planimetric histological sections a different architecture in their orientation from that of the internal surface, only site (cardiac apex) where this situations occurs. The rest of the orientation is always parallel. In the apex, the spiral course of the myocardial fibers, which shift from the periphery towards the center, determine a torsion where the subepicardial fibers become subendocardial, overlapping like the tiles of a roof, as evidenced in this image.

5. Interpretation of the origin and end of the muscle band.
The cardiac fulcrum

As previously expressed, the opposite orientation and rotational movement of the left ventricular fibers, both at the basal and distal apical segment levels, explain Torrent Guasp’s myocardial band model. The author considered that the ventricular myocardium is formed by an assembly of muscle fibers coiled unto themselves as a rope (rope theory) (Figure 1.4) and flattened laterally to form a band, which by giving two spiral turns describes a helix limiting the two ventricles and defining their function. In turn, the phylogenetic study allows to theoretically understand, through the 600 million-year evolution of the circulatory system, that the ends of the band are located at the root of the great vessels, as it is formed from a loop of the primitive circulatory duct. (109)

Figure 1.14. Segment sequence from the myocardial band histological analysis.

The muscle shaping the right ventricle corresponds to the origin of the myocardial band (right segment), which begins both in the cardiac fulcrum as in the fibrous structures related with the pulmonary artery and the tricuspid annulus (pulmo-tricuspid cord) (Figures 1.15 to 1.17). The ascending segment, which is part of the autochthonous muscle bundles constituting the left ventricle, ends at the base of the aorta, and the cardiac fulcrum constitutes a solid point of attachment of the end of the myocardial band.

Figure 1.9 demonstrates that the aortic annulus is not continuous. Its circumference is interrupted between the ends of the trigones, in the region where the mitral valve anterior cusp is inserted. Our research has demonstrated that in the course of the aortic annulus septal segment, extending from the left to the right trigone, there is a thickening we have called cardiac fulcrum (below and in front of the right trigone) where most of the myocardial band is attached, since as any muscle needs a supporting point to develop the leverage required to fulfill its function.

The insertion of the myocardial fibers in the fibrous skeleton of the heart has been considered for three centuries. The development of the myocardial band reported in 1970 by Torrent Guasp (106-108) indicates in the anatomical research that it originates and ends at the root of the great vessels, but that the fibers do not insert in the atrioventricular annuli. The myocardium attaches to these annuli but does not insert in them. In our investigations we have not found insertion of cardiomyociytes in the collagen matrix of the trigones (Figure 1.39).

However, Torrent Guasp considered that the myocardial band lacked a fixed supporting point as those present in the muscular system to develop force. In this sense, he assumed that it behaved as the circular muscle of the arteries, finding support in its own chamber blood content (hemoskeleton). In our research, we have always considered that the myocardial band should have a fixed point of attachment to allow its helical rotation in order to achieve its motions and the essential muscle force of shortening-twisting and lengthening-untwisting. The study of a supporting point in the myocardial band finds correlation with an organic engine, such as the heart, which without a firm attachment to a resistant nucleus would lack the necessary mechanical faculties for its considerable power.

Figure 1.15. Detail of the pulmo-tricuspid cord between the tricuspid orifice and the pulmonary artery where the myocardial band begins. The circle shows the site of the cardiac fulcrum, which is visualized when the pulmonary artery and the pulmo-tricuspid cord are removed from their insertion at the beginning of the myocardial band unfolding (observe figure 1.27). The right coronary artery has been cut to reveal the cord trajectory (bovine heart).

This point of attachment implies, as in any muscle, its ability to achieve the necessary leverage and also to act as a bearing or pad, preventing the force of ventricular rotation, either by torque or torsion strain from transferring to the aorta, thus dissipating the energy produced by the helical muscle motion, and avoiding aortic constriction or bending during systolic ejection. (134)

Young bovine and human hearts (from spontaneous abortion fetuses, explanted adult hearts and cadaveric hearts retrieved from the morgue) were used to study the myocardial band insertion. The dissection was performed as described in Section 4: “Anatomy of the cardiac band. Dissection”, of this same chapter.

In anatomical investigations we have found in all the bovine and human hearts studied a nucleus underlying the right trigone, whose osseus, chondroid or tendinous histological structure depends on the specimen analyzed, and is oriented towards the muscle fibers of the ascending segment which intimately penetrate its structure to attach themselves. This point of attachment would serve to support both the origin and end of the myocardial band, as the fibers that initiate the right segment, origin of the myocardial band, attach to the anterior part of this nucleus as well as to the pulmo-tricuspid cord (Figure 1.16).

Figure 1.16. Descriptive photograph of the muscle bundles emerging from the cardiac fulcrum, which belong to the right segment forming the right ventricle (transverse section of a bovine heart).

This osseus, chondroid or tendinous attachment point is found in the vicinity of the tricuspid valve (right), the aorta (posterior) and the pulmo-tricuspid cord (anterior) (Figure 1.20). To find it, it is necessary to unfold the myocardial band and move the pulmonary artery, the pulmo-tricuspid cord and the right segment to the left of the observer, stripping the root of the aorta (as a scarf that separates from the neck). Next, the muscular plane of the descending segment is separated from that of the ascending segment to follow the latter up to its insertion in the cardiac fulcrum. This maneuver reveals the fulcrum in front of the aorta and below the right trigone and the origin of the right coronary artery (Figures 1.18 and 1.19), detached from the aortic continuity and inserted as a complementary element between the aorta and the myocardium. This structure, origin and end of the myocardial band, has a more rigid consistency than the trajectory between the trigones.

Figure 1.17. Panoramic view of the pulmo-tricuspid cord.

An osseus formation, called os cordis, in bovine and sheep hearts, is reported in the veterinary literature in the same location where we have studied this structure both in bovids and humans. (69, 134) Beyond its mere mention in bovids, no reason for its presence or function was ever assigned to this structure, and it has not been described in humans. In bovids, its consistency is osseus at palpation, which has been confirmed by histological studies, and its size, according to our studies, is of approximately 45 mm × 15 mm, being its triangular shape (Figure 1.20).

Figure 1.18. The figure shows the ascending segment, which is going to insert in the cardiac fulcrum (bovine heart) (see Figure 1.19).

Figure 1.19. Cardiac fulcrum below the right trigone (bovine heart). The insert shows the resected piece.

Figure 1.20. Resected cardiac fulcrum

(bovine heart).

The findings in the human hearts are surprising from the point of view of interpretation, since it is logic to consider its presence in all the evolutionary mammalian chain. It should be assumed that this structure, analyzed in different species, has the common function of providing support to the myocardial band to generate the power needed by any muscle, which is different in diverse mammals. Therefore, its presence is constant in all the hearts studied, both in bovids and humans, but its structural characteristic is different, and this diversity in the intimal analysis of the cardiac fulcrum is undoubtedly associated with the resistance it must oppose to the activity of the myocardial band in hearts of different sizes.

The analysis of a 10-year-old human heart showed in the same place a myxoid-cartilaginous formation with approximately 2 cm diameter (Figure 1.21). A similar finding, both in structure and location, occurred in the heart of a 23-week gestation human fetus (Figures 1.22 and 1.23).

A fact defying logic is having found in the adult human heart a formation presenting consistent characteristics, both to observation and palpation, in the same location and with similar triangular morphology (Figures 1.24 to 1.27). However, the histological analysis revealed a matrix similar to that of a tendon.

The macroscopic and microscopic observation reveal the muscle fiber attachment to this solid, homogeneous nucleus, which is closely related with the aortic wall on the side of the tricuspid valve. Its configuration has been confirmed histologically. We have called his structure, origin and end of the myocardial band, cardiac fulcrum, as a parallelism and tribute to the definition of the supporting point of a lever expressed by Archimedes of Syracuse (Greek, 288 B.C. – 212 B.C.). It is located anterior to the central fibrous ridge (right trigone) and it clearly shows that the myocardial fibers of the right segment originate in its structure (Figure 1.16) same as the ascending segment courses to meet it in order to attach (Figures 1.18 and 1.19). It should be noted that to visualize the cardiac fulcrum it is essential to unfold the myocardial band. This osseous, cartilaginous or tendinous structure was always present and with the same morphology in all the hearts analyzed by us, albeit with different histological texture. No description of its characteristics or function has been reported in the literature, except the mere mention of its presence as os cordis in bovids.

Figure 1.21. Cardiac fulcrum in a 10-year-old human heart (explant).

Figure 1.22. Human embryo heart (23-week gestation) showing sectioning planes.

Figure 1.23. Cardiac fulcrum in a 23-week gestation human embryo hear.

Figure 1.24. Fulcrum in the adult human heart.

Figure 1.25. Fulcrum in

the adult human heart.

Figure 1.26. Fulcrum resected from

an adult human heart.

Figure 1.27. The drawing shows the myocardial band arrangement at the beginning of its unfolding. The pulmonary artery and the right segment have been separated from the fulcrum to show its intermediate location between the right segment (anterior location) and the ascending segment (posterior location). A: macroscopic image of the fulcrum in the adult human heart. B: microscopic image of the human fulcrum. Notice the myocardial fibers inserting into the tendinous fulcrum matrix

Histology. The microscopic analysis of the bovine cardiac fulcrum shows a trabecular osteochondral matrix with segmental lines. Its general structure resembles the metaphyseal growth of the long bones (Figures 1.28 and 1.29) and increased magnification reveals bone trabeculae with osteoblasts and segmental lines secondary to bone apposition (Figure 1.30).

Figure 1.28. Histological section of the cardiac fulcrum showing trabecular bone tissue with osteologic segmental lines (bovine heart). Hematoxylin-eosin stain.

Figure 1.29. Mature trabecular bone forming the cardiac fulcrum tissue (bovine heart). Hematoxylin-eosin stain at low magnification (10×).

Figure 1.30. Cardiac fulcrum trabecular bone with osteoblasts and segmental lines. The structure constitutes the trabecular bone tissue scaffolding similar to the metaphyseal area of growth in long bones. The histological section also shows bone trabeculae with osteoblasts and segmental lines secondary to bone apposition (bovine heart). Hematoxylin-eosin stain at high magnification (40×).

Figure 1.31. Ten year old human heart. Central area of the fulcrum formed by chondroid tissue in a 10-year-old human heart. Hematoxylin-eosin stain.

Figure 1.32. Cardiac fulcrum in a 23-week gestation fetus showing prechondroid bluish areas in a myxoid stroma. Masson’s trichrome staining technique.

In the 10-year-old human heart, the histological findings in the cardiac fulcrum were associated with that early age. Figure 1.31 shows a central zone of the fulcrum formed by chondroid tissue. Given the 10-year-old age, it is logical that the fulcrum is smaller and characterized by more chondroid than bone tissue. This finding was repeated in a 23-week old human fetus with the characteristic prechondroid bluish areas in a myxoid stroma (Figure 1.32).

The osseous structure in the bovine os cordis and its relationship with the myxoid-chondroid cardiac fulcrum texture in human hearts, even in gestational stages, is rational for the interpretation analysis. This disparity is associated with the different age evolution from chondroid to osseous material and by the greater force developed in bovids requiring a more rigid supporting point.

Figure 1.33. Myocardial insertion in

the cardiac fulcrum. Bovine heart.

Figure 1.34. Insertion of myocardial fibers in the fulcrum

chondroid tissue of a bovine heart.

However, the histological analysis of the fulcrum in adult human hearts evidenced a tendinous collagenous matrix, needing an additional clarification. In principle, there is constancy in the detection, site and morphology of the fulcrum in all the hearts analyzed. This means that from a functional point of view, its presence is akin to myocardial band insertion, as established in the histological analysis, becoming a solid point of interpretation to achieve its biomechanical function. And we find this demonstration when the histological examination is directed to the myocardial insertion in the osseous, chondroid or tendinous fulcrum. All the hearts studied presented this myocardial attachment to the rigid structure of the fulcrum, integrating a cardiomyocyte-matrix unit, regardless of its osseous, cartilaginous or tendinous nature (Figures 1.33 to 1.38).

Figure 1.35. Festooned cardiomyocytes penetrating the fibro-colagenous matrix (adult human heart).

Figure 1.36. Fibrocolagenous matrix at higher magnification (adult human heart).

At this point, fundamental questions arise. Why does the human fulcrum have characteristics similar to a tendon, despite it fulfills the same function of attaching the myocardial band? Why does it not have the same structure of the fetal or child human heart?

Figure 1.37. Festooned colagenous fibers integrating the fibrotendinous matrix of the fulcrum (adult human heart).

Figure 1.38. Cardiomyocytes penetrating the fibrocolagenous tissue (adult human heart). The circle details the insertion site.

Our interpretation is that perhaps the osseous fulcrum is a vestigial organ specific of mammalian evolution. A vestigial structure must be understood as the preservation during the evolutionary process of genetically established attributes which have lost all or part of their ancestral function in a certain species. (45) As a result, it is found in the initial process of human gestation, but later loses its osseous character, remaining as a tendinous matrix able to achieve myocardial band insertion to attain a muscle power which is much lower than that of larger mammals. Recall that in bovids the fulcrum found in this investigation is of bone nature (Figures 1.28 to 1.30).