The heart is a powerful example of the nature of evolutionary development. The progress of life forms from single-celled structures to multiple-organ individuals demands the emergence of a system to supply nutrition and remove effluent from all parts of the body. For the first days of human embryonic development, whilst embedded in the wall of the mother’s uterus, oxygen and nutrients can reach all of the cells by simple diffusion from areas of high concentration to those of low concentration (along concentration gradients). Soon, however, the embryo reaches a size that requires a formal circulation, as the distances across which diffusion would be necessary become too great. Capillary networks form, through which the vital nutrients can reach the rapidly dividing cells. From the fourth to the sixth week of development, a group of cells form into a tube, which begins to differentiate into the chambers of the heart, beginning with the atria.⁴ These cells possess the special property of contractility, and from a very early stage, cells can be seen to contract with a spiral wave beginning at the atrial end, in an action that resembles the wringing out of a towel. Interestingly, before the arteries and veins link together through the capillary networks, the first flow of blood within this tubular system is Galenic, with an ebb and flow pattern. It is only as the foetus grows and the capillary connections form that the formal circulation begins. As the tube of muscle cells grows, it twists and rotates in an extremely complex way, eventually establishing the relationships that we recognise in the adult heart.

The fully formed heart consists of four chambers arranged in two pairs. There are two weakly contractile receiving chambers called atria (from the Latin atrium, which means a hallway) and two powerful pumping chambers called ventricles. They are connected in series and they communicate through one-way, non-return valves. Between the right atrium and right ventricle is the tricuspid valve, and between the left atrium and left ventricle is the mitral valve. These valves allow blood to pass from the atria into the ventricles but oppose return flow into the atria. Similarly, two unidirectional valves control the outflow from the ventricles. The pulmonary valve prevents reflux of blood in the outflow from the right ventricle to the lungs, and the aortic valve performs the same function in the outflow from the left ventricle to the rest of the body.

The pairs of chambers are described as being either right or left. The concept of sidedness of the heart relates to their relationship to the lungs and not to their mature position within the chest. In the natural state within the body, the right atrium and right ventricle lie as much on the front of the heart as they do on the right side. The left atrium and ventricle are more towards the back of the chest than they are to the left side.

Blood leaves the right side of the heart to enter the lungs, from which it returns to the left-sided chambers, before departing for the rest of the body. Blood returning via the great veins from the many organs of the body to the right side of the heart is low in oxygen content and high in carbon dioxide. The reverse is true of the blood that returns to the left side of the heart from the lungs. The passage of blood through the lungs removes carbon dioxide and enriches it with oxygen.

A general rule states that arteries always carry blood away from the heart, and veins towards the heart. With the exception of the pulmonary artery, all arteries contain oxygenated blood. The pulmonary artery, which leaves the heart to carry blood to the lungs, carries the deoxygenated blood that has returned from the tissues of the body. Similarly, all veins other than the pulmonary vein contain blood rich in carbon dioxide.

Arteries branch like the branches of a tree, and veins receive tributaries, as does a major river. As we shall see, the cross-sectional area of the parent vessel is equal to the sum of the cross-sectional areas of its branches. Leonardo defined this relationship as a natural law.

All of the structures of the heart exist to provide the impetus, direction, and rhythmic flow of the blood. Therefore I will describe the structures of the heart in the order that the blood flowing through it will encounter them. Blood returns from the head and upper body via converging tributaries of veins that culminate in one large vein, the superior vena cava (SVC). This expands to form the upper part of the right atrium. Blood returning from the lower body via a myriad of converging venous tributaries enters another large venous channel called the inferior vena cava (IVC). The IVC then enters the inferior part of the right atrium. This chamber is in two parts: The first is formed by the confluence of the superior and inferior vena cavae; it has a smooth internal lining and thus is called the sinus venosus part. The rest of the right atrium is formed as part of the heart tube and has a lining that is elevated in a mass of interwoven bars of muscle called trabeculae. This is where the true heart muscle begins. The junction between these two areas forms a ridge called the crista terminalis (terminal crest).

The blood then flows across the first of the atrioventricular valves within the heart, the tricuspid valve. This valve has a complicated relationship with the right ventricular muscle below. Fine, tendinous cords (which look rather like the strings on a parachute connecting the sheet to the person suspended below) connect the tricuspid valve to outcrops of muscle in the ventricle, called papillary muscles (Fig. 4.1). This valve, as with the other three valves, opens and closes under the force and direction of the flowing blood in response to the cyclical contraction and relaxation of the heart muscle. Other influences, such as vortex formation in the flowing blood, play an important part in the closure of the valves in such a way that no backward leakage of blood can occur. As we shall see, this idea was one of Leonardo’s most accomplished pieces of work, not appreciated until the twentieth century.

When the ventricle is nearly full, the right atrium contracts to send the final portion of blood into the ventricle, whose cavity is then stretched open even further. Heart muscle has an elastic component, and this final stretch imparts extra energy to the ventricle. This elasticity was also recognised by Leonardo in relationship to the normal function of the aortic valve and root. The elastic recoil then powers the beginning of the ventricular contraction phase, which drives some of the blood up against the tricuspid valve, forcing it to close. The remainder of the blood forces open the pulmonary valve and flows onwards to the lungs. Whilst the ventricle is contracting and thereby emptying, blood continues to return to the atrium, filling it so that it is ready for the next phase of ventricular filling.

The main pulmonary artery divides into two, one part for each lung. These then divide again and again as the branches pass into the depths of the lung tissue. At their terminations, they become very small and are termed arterioles. At their ends, they merge into the capillaries. The capillaries are tubes that are only a single cell thick, forming a very weak barrier across which gas and nutrition can easily diffuse into the surrounding tissues.

The red cells of the blood pass through the fine capillary network of the lungs. They contain complex molecules called haemoglobin, whose special property is to loosely bind molecules of oxygen to themselves. Thus oxygen is carried onward to the tissues of the rest of the body. Oxygen is the active ingredient of the mystical vital spirit “pneuma”. The transfer of oxygen, carbon dioxide, and the nutritious elements of the blood is passive. That is, the transfer occurs with the expenditure of minimal energy, along concentration gradients. In other words, the blood entering the lungs has given up a lot of its oxygen and is entering an environment where the oxygen concentration is high. Oxygen molecules therefore move from the lung air spaces into the blood. The reverse is true for carbon dioxide.

At the end of the capillary networks, the tiny vessels coalesce to form small veins (venules), which themselves coalesce into the larger veins, and ultimately these all merge into the pulmonary veins, which leave the lungs and enter the left atrium of the heart. There are two pairs of pulmonary veins (one pair from each lung), which enter either side of the left atrium. From the left atrium, blood then flows across the second atrioventricular valve, the mitral valve, located at the mouth of the left ventricle, the powerhouse of the systemic circulation. When this ventricle contracts, the blood is ejected under high pressure out through the aortic valve and onwards throughout the body via the aorta. The contraction of the left ventricle simultaneously causes the mitral valve to close.

The closure mechanism of the aortic and pulmonary valves is sophisticated and involves complex hydrodynamics that were first described by Leonardo. We will explore this discovery in detail later.

The contraction of the chambers of the heart is instigated by nerve impulses, which arise first in a specialised group of heart cells collectively called the sinoatrial node. Found at the junction between the superior vena cava and the right atrium, this is a concentration of electrically active cells that fire together at a rate that is controlled by the demands of the level of exercise or stress of the individual; increased demands for oxygen and energy cause chemicals in the blood to change, making the cells fire more rapidly. The nerve impulses then flow throughout the atrial muscle, causing a wave of contraction. They then coalesce at another clump of specialised heart muscle cells called the atrioventricular node. This lies in the crest of the septum (the dividing wall) between the two ventricles. The electrical wave then spreads throughout the two ventricles, causing them to contract. The two atria and the two ventricles contract together and in sequence, atrial contraction preceding contraction of the ventricles. As the atria contract at the end of the ventricular filling phase, the ventricles are relaxed. When the ventricular filling phase is ended, the ventricles contract, and at this time the atria relax to refill. The phase of ventricular contraction in the cardiac cycle is referred to as systole and the phase of relaxation as diastole. As we shall see, Leonardo commented upon the neuromuscular activity of the heart.

The internal structures of the heart are complex both in their overall form and in their content. The right ventricle operates under lower pressures than the left, and consequently it has a thinner muscular wall. Its external wall is relatively unsupported, and as a result in some animal species it has a muscular bar stretching from the internal (septal) wall to the external wall. This is the structure first recorded and named by Leonardo as the “moderator band.” Its presence prevents over-distension of the thin ventricular free wall (Fig. 4.2). The internal aspect of the ventricular walls is characterised by the protrusion of muscular bands, which form many nooks and crannies between them. These protrusions coalesce into large muscular projections called papillary muscles. From these arise the tendinous cords that support the ventricular side of the tricuspid and mitral valves. These cords are arranged in three layers, referred to as primary, secondary, and tertiary cords. The primary cords insert into the leading edges of the leaflets and act as guy ropes bringing the leaflet edges into line as they close in systole. The secondary cords, which are sturdier, reach up to the underside of the valve, where they fan out to spread like the fan vaulting of a church roof (Fig. 4.3). This arrangement allows allow the load placed upon them at maximum pressure of the ventricular contraction to be equally dissipated. The third group of tendinous cords, the tertiary cords, extend to the base of the valve just next to its origin from the ventricular wall. These cords are under tension throughout the cardiac cycle and act like “tie-rods” in an engineering sense, supporting the ventricular wall. These architectural relationships are important for the normal functioning of the ventricles. If it is not respected and preserved during valve replacement surgery, the heart will progressively distort from its normal conical shape to a sphere. This altered geometry causes the heart to fail over a number of years. An example of this type of cord is drawn by Leonardo, but without commentary.

The two arterial valves, pulmonary and aortic, are made up of three semilunar leaflets (so called because of their resemblance to a half moon). These “lunules” (valve leaflets) must support each other at their closure line, as no tendinous cords are attached to them (Fig. 4.4). They close under a lower pressure than the tricuspid and mitral valves, which must close in the powerful contraction phase of the ventricles as they generate their maximal pressure. The left ventricle generates a much higher pressure than the right ventricle, and thus the mitral valve has to close under the highest pressure of all of the valves.

The heart has its own blood supply. This is delivered to the heart by two main coronary arteries, the right and the left. They are the first branches of the aorta (the large artery that leaves the left side of the heart to supply blood to the whole body other than the lungs). The coronary arteries arise from hemispherical expansions in the root of the aorta, (the sinuses of Valsalva), which begin immediately above the leaflets of the aortic valve.⁵ The coronary arteries give their names to the particular aortic sinuses from which they arise. Hence, there is a right, a left, and a noncoronary sinus. The latter (as its name implies) is the sinus of the aorta, which does not give rise to a coronary artery. As the blood exiting the ventricle enters the sinuses, vortices form within them as the blood is channelled back upon itself towards the upper side of the base of the leaflets. This motion begins to make the semilunar leaflets drift towards each other, initiating the first phase of valve closure. It also drives blood into the coronary arteries, enhancing perfusion of the heart.

The coronary arteries divide repeatedly and delve down into the heart muscle, providing its nourishment. Maximum flow in these vessels occurs during the relaxation phase of the cardiac cycle. Blood returns from the heart muscle via venous tributaries to enter the large vein, the coronary sinus, which ultimately flows back into the right atrium of the heart.

All of the structures of the heart are forged by the dynamic function that they are called upon to carry out. Therefore a teleological approach to their understanding is a laudable way to proceed. Thus Leonardo’s method of using the observed effect to define its cause yielded fascinating results. In his own words: “O marvellous necessity, thou with supreme reason constrainest all effects to be the direct result of their causes, and by a supreme and irrevocable law every natural action obeys thee by the shortest possible process”.⁶

The special nature of the heart has been recognised throughout the history of anatomical exploration. The spiritual, emotional, and physical roles of the heart have always been interwoven. Although there is a predominance of discussion around the more subjective aspects, the physical (scientific) properties have been considered from earliest times. Aristotle, Hippocrates, and Galen all declared their beliefs with regard to the role of the heart. Its physical and spiritual place in the body was bound up together; its motion and its corporeal centrality vested it with importance.

Leonardo’s comment upon the central position of the heart in the body is sparse and terse. He placed it midway between the seat of the soul and the source of life, between the brain and the testis. He wrote, “The heart is placed exactly in the middle between the brain and the testicles.”³⁷

He also spent time considering its position within the chest, describing it as lying obliquely, towards the left of centre. This he explained from the point of view of balance within the body. He stated that together with the spleen beneath, it counterbalanced the mass of the liver on the opposite side of the body.³⁸ Elsewhere he ascribed the obliquity of the heart to the difference in weight between the right and left ventricles: “The right ventricle was made heavier than the left ventricle in order that the heart might be placed obliquely.” ³⁹

He rhetorically discussed this concept in another passage, in which he argued against the comparatively heavier weight of the thicker-walled left ventricle acting as a counterweight to the right, with its greater volume of blood. In this passage he introduced the concept of the support of the heart by its great vessels. He wrote,

And if you say that the left exterior wall was made thick in order that it should acquire greater weight to form a counter weight to the right ventricle which has a greater weight of blood, you have not considered that such a balancing was unnecessary inasmuch all terrestrial animals except man carry their hearts lying horizontally. And likewise the heart of man lies horizontally when he lies in bed. But you would not be a good balancer because the heart has two suspensoria descending from the pit of the throat, and according to the fourth [proposition] of On Weights, the heart cannot be balanced unless it has above it one suspensorium only. These two suspensoria are the aortic artery and the vena cava. Furthermore, when the heart is deprived of the weight of blood on its contraction and gives it in deposit to the upper ventricles, the centre of gravity of the heart would then be on the right side of the heart and so the left side would be made lighter. But such weighting is not true, because as stated above, animals which lie down or stand on 4 ft have their hearts lying down [horizontally], and no such weighting of the heart is to be looked for. And if you say that the testicles are made only to open or close the spermatic ducts, you are mistaken, because they would only be necessary to rams or bulls which have very big [testicles]. And when these testicles had re-entered the body on account of the cold then coitus could not be performed. And the bat which sleeps and always places itself upside-down, how does it balance the right and left ventricles of its heart? ⁴⁰

The concept of balance is illustrated by Leonardo in the small sketch of a man holding a long object in front of himself, with his feet carefully placed to maintain balance and his head and back also positioned as to maintain a balanced posture.⁴¹ This drawing is accompanied by a series of other small ones showing the heart attached to the great vessels and one in which the ventricles are dilated and contracted (Fig. 4.19a). The small diagrams at the top of RL 19084 recto also show the heart in relation to the great vessels in a similar way (Fig. 4.19b). It would appear from these diagrams that he correctly concluded that the heart is maintained in its position by its attachment to the great vessels and that the proposed balancing effects of the respective weights of the two ventricles were not a factor. The importance of the anchoring effect of the great vessels can be seen in victims of rapid deceleration injuries (such as head-on collisions), in whom the weight of the heart and its contained blood is thrown forward within the chest. If the force is great enough, the aorta will tear just before the point where it is anchored to the posterior chest wall. This injury, which always occurs just beyond where the vessels to the head and neck arise, is commonly fatal.

Galen declared that the liver was the principal organ of the body, being the first to be formed, and as such was the source of all the blood vessels. Aristotle held another view, suggesting that the heart was the principal organ. More than a millennium later, Leonardo entered into this debate and concluded on the side of Aristotle that the heart had pride of place over the liver as the origin of the blood vessels. This he argued from a wider natural perspective using visual and verbal metaphors. He likened the heart to a seed or a nut, and wrote, “The heart is the nut which generates the tree of the veins; which veins have their roots in the manure, that is the meseraic [mesenteric] veins go on to deposit the blood they have acquired in the liver, whence then the upper veins of the liver [hepatic veins] are nourished.”⁴² Accompanying this text are two simple drawings, one with the word “nut” (noccollo) written at its centre and the other with the word “heart” (core) in the same place (Fig. 4.20). Near to them are the words, “A plant never arises from its branches, for the plant exists before the branches and the heart exists before the veins.”⁴³ In the drawing labeled “heart” can be seen a simple representation of the vessels to the liver joining the inferior vena cava below the heart. In the parallel drawing, the roots and branches are shown directly emanating from the side of the seed. The visual analogy is clear.

Beside these small drawings is a more substantial one illustrating this important relationship of veins and arteries to the heart, the sovereign power. The major text on that sheet affirms in no uncertain terms this relationship:

All the veins and arteries arise from the heart; and the reason is that the biggest veins and arteries are found at their conjunction with the heart. And the more they are removed from the heart the finer they become, and they divide into very small branches. And if you say that the veins arise in the gibbosity of the liver because they have their ramifications in this gibbosity, just as the roots of plants have in the earth, the reply to this comparison is that plants do not have their origin in the roots, but the roots and other ramifications take origin from that lower part of the plant which is situated between the air and the earth. And all the lower and upper parts of a plant are always less than this part which adjoins the earth. Therefore it is evident that the whole plant takes origin from such a size and in consequence the veins take their origin from the heart where they are biggest. Never does one find a plant which has its origin from the tips of its roots or other branches; and this observed by experience in the sprouting of a peach which arises from its nut as is shown above at a b and a c.⁴⁴ (The labels are clearly seen on the accompanying small drawings.)

Thus, in the opinion of Leonardo, the heart has pride of place within the body and is the source of all of the blood vessels.

As the heart is the only continuously moving part of the body, it is natural that investigators should look to it as a vital force in life. Its responsiveness to emotional change led to an extended debate as to its role in generating the emotional state as well as the viability of the whole animal. With the benefit of evolving investigative technologies, we have been able to demystify this crucial organ to the extent that we recognise the heart as a highly responsive, self-regulating ­muscle pump, which is the engine of the circulation. This statement seems obvious to us, but in the time of Leonardo, even the fact that it is made of muscle had not been established. This was Leonardo’s first significant contribution to the practical understanding of the heart.

Modern biology recognizes three distinct forms of muscle: (1) Skeletal muscle, the most abundant form, which is responsible for the mobility of the animal. It has the microscopic characteristic of stripes running horizontally across its fibers, giving it the alternative name of striated muscle. (2) Smooth muscle, so called because it lacks the microscopic stripes of skeletal muscle. This type is found in places such as the eye and the bowel. It responds to chemical and autonomic nerve stimulation with such actions as the focusing of the eye and the movement of the gut. (3) Cardiac muscle, the unique muscle of the heart. It too is striated under the microscope, but the muscle fibers have branched endings, which interdigitate with the surrounding muscle fibers, forming a syncytium.⁴⁵ It has the property of continuous contraction and relaxation throughout life. It responds to chemical and nervous stimuli, driven in turn by exercise and emotional change.

There was no way of distinguishing these forms of muscle until the development of the microscope in the late sixteenth century, when It became possible to discern their different characteristics. Until then, differences could be determined only by examination of gross form and function. Galen recognised that the tissue of the heart was different and described it as having a special “hard” flesh. Whilst recognising that it was constructed of fibres resembling muscle, both he and Avicenna a millennium later said that it was in reality quite a different material.⁴⁶ In contrast to this position, Leonardo clearly stated that the heart was made of muscle. He ­recognised its difference from skeletal muscle in describing it as being of greater density. In support of his claim that it was a muscle, Leonardo pointed out that it possessed its own arterial blood supply and venous drainage: “The heart of itself is not the beginning of life but is a vessel made of dense muscle vivified and nourished by an artery and a vein as are the other muscles. It is true that the blood and the artery which purges itself in it are the life and nourishment of the other muscles.”⁴⁷

In Galenic terms, the heart was perceived as having three specific roles: the movement of blood to the lungs and the periphery; drawing air from the lungs into the heart during the relaxation and filling phase of the ventricles; and generating the heat of the body, through its often-violent movement. Leonardo held to that philosophy, although, as we shall see, he began to question the direct connection between the trachea and the heart. A considerable amount of Leonardo’s writing on the heart is spent in considering its role in the generation of innate heat. He perceived that several of the components and structural relationships in the heart are defined by their contribution to this vital function.

The presence of bodily heat in an animal implies life, and Leonardo stated that assumption as follows: “Where there is life there is heat, and where vital heat is, there is movement of vapour.” ⁴⁸ The last phrase of this quotation refers to the creation of convection currents within fluids, be they in the liquid or gaseous phase. Hot air and hot water rise, whilst the opposite is true for cold liquids.

In keeping with the beliefs of his time, Leonardo accepted the four elements of the universe—earth, water, air, and fire—and he saw the constitution of man as a reflection of the world around him and as such, as a part of the microcosm/macrocosm continuum:

By the ancients man was termed a lesser world and certainly the use of this name is well bestowed, because, in that man is composed of water, earth, air and fire, his body is an analogue for the world: just as man has in himself bones, the supports and armature of his flesh, the world has the rocks; just as man has in himself the lake of the blood, in which the lungs increase and decrease during breathing, so the body of the earth has its oceanic seas which likewise increase and decrease every 6 h with the breathing of the world; just as in that lake of blood the veins originate, which make ramifications throughout the human body, similarly the oceanic sea fills the body of the earth with infinite veins of water. The nerves are lacking in the body of the earth…. But in all other things they are similar.⁴⁹

Therefore it was natural for him to use examples from the wider world in his explanations of the inner workings of the human body. At work within the four elements were the four powers of nature: weight, force, movement, and percussion. The resistance between bodies, of which at least one was moving, creates friction.⁵⁰ Simple friction creates heat, and this Leonardo reflected in two statements on the production of heat within the heart: “And the revolvings made by the blood whirling round in itself in different eddies, and the friction that it makes with the walls and the percussions in the recesses [the spaces between the pectinate muscle bundles of the right atrium], are the cause of the heating of the blood, and of making the thick viscous blood fine….”⁵¹ and “On The Cause Of The Heat Of The Blood: The heat is generated by the movement of the heart; and this is evident because the more quickly the heart moves the more the heat multiplies, as the pulse of febrile persons moved by the beating of the heart teaches us.” ⁵²

A further use of natural analogy is found in other comments, in which he uses a river and a lake as similes for the blood flow and the heart. Here he is restating his belief that the ebb and flow motion of the blood is essential for the generation of heat:

And such motion of the blood would behave like a lake through which a river flows, which acquires as much water from one side as it loses at the other. However, the only difference is that the movement of the blood is discontinuous and that of the river flowing through the lake continuous and from this lack of flux and reflux the blood would not be heated and consequently the vital spirits could not be generated and for this reason life would be destroyed.⁵³

Leonardo’s concept of the heart as a source of renewable heat is evidenced in the following words: “As natural warmth spread through the human limbs is driven back by the surrounding cold which is its opposite and enemy flowing back to the lake of the heart and the liver fortifies itself there, making of these its fortress and defence.”⁵⁴

Within Leonardo’s interpretation of the necessity of the generation of heat by the movements of the heart was his insistence that the heating of the blood caused it to be less viscous and thereby to pass more easily through the fine pores in the interventricular septum, as described by Galen:

Because the heat of the heart is generated by the swift and continuous movement which the blood makes by its friction within itself through its eddyings, also by the friction it makes with the pitted walls of the right upper ventricle into which it continually enters and escapes with impetus, these frictions made by the velocity of the viscous blood heat it, subtilise it, and make it penetrate through the fine meati [pores], giving life and spirit to all the organs into which it is infused.”⁵⁵

Leonardo used another example at another place in his notes to prove that continuous movement produces heat. In this case, he used the making of butter by the churning of milk as an example of heat production through motion. A difference with this argument is that in this process, the liquid milk becomes more viscous, whereas he argues that blood becomes “thinned”. This observation recognises the difference in response to the motion of different liquids, which is predicted by the content of the liquid. The extreme example is that of a thixotropic liquid, one that is extremely viscous at rest (more like a gel), so that it will not flow. The energy imparted to it on violent agitation makes it much less viscous and allows it to flow freely for a while, but the as the energy is expended, the liquid returns to a gel state⁵⁶:

Observe whether the revolution of milk when butter is made, heats it. And by such means you will be able to test the strength of the auricles of the heart which receive and expel the blood from their cavities and other passages; these being made only in order to heat and refine the blood and make it more quickly penetrate the wall through which it passes from the right into the left ventricle⁵⁷ where through the thickness of its wall, that is, of the left ventricle, it conserves the heat which the blood carries to it.⁵⁸

The analogy of the heart as a stove or a furnace is to be found on a page in the Codex Arundel.⁵⁹ There Leonardo illustrates a stove with an inlet and an outlet valve, which he labels “m” and “n”. Writing about this stove, he regards the heart as the furnace and the trachea as the chimney. The air is drawn in through one opening labelled “n” and is blown out in the other direction through an opening labelled “m”. Leonardo likens this to the cardiorespiratory system: “Short and frequent breathing suffocates the person, short because each breath does not change all of the air in the lung that has been heated. But a part of it remains there so greatly heated that it would do great damage to an animal unless big and long breaths were to drive it out of the lung.”⁶⁰

This analogy is carried over into an early drawing in the Windsor series, in which he labelled the two tubular connections to the lung in the same way “n” and “m”. ⁶¹ In one part of the drawing, the labelled connections can be seen intact; another part shows the same view in cross section. Leonardo explains that they are the inlet and outlet for the products of combustion in the heart. The style and substance of this drawing suggest a date earlier than the final heart series of 1513–1514. Clark⁶² and O’Malley and Saunders⁶³ date this and a similar drawing (RL 19112 recto) as 1504–1507.

Having established that the heart was in this way a “furnace”, Leonardo had to adopt another property for heart muscle, namely its ability to withstand heat. He wrote, “The heart is of such density that fire can scarcely damage it. This is seen in the case of men who have been burnt, in whom, after their bones are in ashes, the heart is still bloody internally. Nature has made this great resistance to heat so that it can resist the great heat which is generated in the left side of the heart by means of the arterial blood which is subtelised in this ventricle.”⁶⁴

In reality, contracting muscle does generate heat. The obvious example is skeletal muscle on extreme exercise. Similarly, the heart produces more heat with increasing exercise, and this heat is dissipated in the blood flowing through it via the coronary arteries and the main stream of blood. The heat is then dissipated in the periphery by sweating and through the lungs in breathing. All metabolic processes in the body operate under the control of enzymes (biochemical catalysts), and these operate within limited temperature bands. Heat control is therefore very important for normal muscle function, albeit as part of a very different concept than that imagined by Leonardo.

The actions of the heart when an individual is at rest are barely perceptible. As exercise levels increase, however, its motion becomes increasingly forceful and obvious. At extremes of exercise, its movement can feel almost violent, producing a pounding in the chest and head and a bounding peripheral pulse. These enormous and sustainable reserves of power were recognised by Leonardo in the following words: “The heart is a principal muscle of force, and it is much more powerful than the other muscles.”⁶⁵ As one of the four great powers, Leonardo’s use of the term “force” was specific. His definition was as follows: “a spiritual energy, an invisible power which is created and imparted, through violence from without, by animated bodies to inanimate bodies, giving to these the similarity of life, and this life works in a marvellous way, constraining and transforming in place and shape all created things. It speeds in fury to its undoing and continues to modify according to the occasion.”⁶⁶

Galen’s heavy reliance on the “vital spirits” for the satisfactory explanation of the viability of an animal led to his perceived importance of the relationship between the heart and the lungs for the extraction of the vital spirit, “pneuma” As we shall see later, access of “pneuma” to the heart was supposedly through the trachea. During the ventricular relaxation phase of the cardiac cycle, air is drawn from the trachea into the ventricle and was supposed to mix with the blood, re-entering the right ventricle after contraction of the atria. On the next right ventricular contraction, part of the blood (now enriched with pneuma) would be forced through the imaginary septal pores and depart through the aortic valve on its way to the brain. Unlike Galen, Leonardo’s writing lacks frequent reference to the “vital spirits” and places much heavier emphasis on the four powers of nature,⁶⁷ but he does spend some effort in trying to demonstrate this fictional relationship.

Although much of Leonardo’s text on the function of the heart relates to the flux and efflux of pulmonary blood flow, it is clear from these lines that he was well aware of the importance of the heart in providing the flow of blood to the periphery. He stated as much in a section describing the way that an animal sustains itself by the consumption of nutrition: “And the same thing happens in the bodies of animals by means of the beating of the heart which generates a wave of blood through all the vessels which continually dilate and contract. And dilatation occurs on the reception of superabundant blood and diminution occurs on the departure of the superabundance of the blood received. This, the beating of the pulse teaches us when we touch the aforesaid vessels with our fingers in any part of the living body.”⁶⁸

Therefore, to Leonardo the heart was a muscle pump of considerable power, capable of withstanding great heat. He perceived it as the source of propulsion in the ebb and flow movement of the blood, as well as the generator of heat.⁶⁹

“Of the heart: This moves of itself and does not stop unless forever.”⁷⁰

The lifelong continuous contraction and relaxation of the heart sets it apart from all of the other organs in the body. It follows. therefore, that having proved to his satisfaction that it was made of a sturdy and most powerful form of muscle, Leonardo should ponder on the cause of this continuous motion. He asked himself, “And which part of the heart it is which is the cause of its movement; and whether it is inside or outside the heart”.⁷¹ We know that this continuous movement of the heart is activated by the spontaneous generation of a wave of cellular depolarisation that originates in the sinoatrial node and spreads throughout the heart. At rest, this occurs about 70 times a minute. It changes in response to exercise and emotional challenges. This change is moderated in part by a nerve originating in the brain called the vagus nerve (cranial nerve X). Leonardo was aware of this nerve and referred to it as the reversive nerve. It was given this name by earlier anatomists because on the left side of the body, it reverses its direction in the chest. After arising from the brain, it runs from the base of the skull to the abdomen. In the chest, it divides as it passes around the aorta, and part of it returns to supply the vocal cords of the larynx. The rest of the nerve continues along the pericardial sac surrounding the heart and continues into the abdomen, where it provides important stimuli for the normal function of the gut. On the right side of the body, it divides and reverses its direction higher up in the neck, looping around the right subclavian artery. As it passes over the pericardium, branches penetrate the heart and their activation contributes to the control of heart rate. This nerve is clearly shown in Leonardo’s drawing on RL 19050 verso (Fig. 4.21). In a personal record of his dissection of this nerve as part of his attempts to discover more about its anatomy and its role in the control of heart muscle contraction, Leonardo wrote, “The reversive nerves arise at a b, and b f is the reversive nerve descending to the pylorus of the stomach. And the left nerve, its companion, descends to the covering of the heart, and I believe that it may be the nerve which enters the heart.”⁷²

The following quotation forms part of Leonardo’s investigation of the opposing Aristotelian and Galenic positions on the seat of the soul using the role of the vagus as the arbiter:

Do not fail to follow up the reversive nerves [vagi] as far as the heart; and see whether these [nerves] give movement to the heart, or whether the heart moves by itself. And if such ­movement comes from the reversive nerves, which have their origin in the brain, then you will make it clear how the soul has its seat in the ventricles of the brain, and the vital spirits have their origin in the left ventricle of the heart. And if this movement of the heart arises from the heart itself, then you will say that the seat of the soul is in the heart, and likewise to the other nerves because the motion of all muscles arises from these nerves which their ramifications are infused into the muscles”.⁷³

Though couched in the terms of his time, this discourse by Leonardo illustrates the use of rhetorical argument in an attempt to resolve a problem. The activity of the brain does control the activity of the vagus nerve, which in turn has an identifiable effect on the heart. Our heart rate quickens and slows in response to our emotional state. In this approach, Leonardo further allied himself with the view that the seat of the soul is the brain and not the heart. It is interesting to note that the movement of the heart was not understood by Harvey, who claimed that the movement of the heart was not ­spontaneous. He suggested that the vital spirits contained within the blood were the source of the heart’s movement.⁷⁴

In addition to the control of heart movement, Leonardo was interested in the movement of the heart in life and the form that it took after death. His most important description in this respect was that the atria and the ventricles move independently and separately. As the atria contract, the ventricles relax to fill with blood, and then as the ventricles contract, the atria relax. The atrial contraction occurs towards the end of ventricular diastolic filling and is called the “atrial kick” The muscular walls of the ventricles are significantly elastic and obey Hooke’s law of elasticity: Up to a point (the elastic limit), the greater degree of stretch applied, the greater the force of recoil when released. The injection of blood into the ventricles engendered by the late atrial contraction further expands the elastic, muscular heart. This expansion significantly increases the power of recoil of the ventricular muscle and is an important contribution to cardiac efficiency. Leonardo illustrated this effect is his only known heart drawing housed outside the Windsor collection, found in Manuscript G in Paris, on the opening page⁷⁵ (Fig. 4.22). In this drawing, the dynamic nature of this motion is clearly visualised. On Windsor sheet RL 19087 recto are found a further series of sketches of the changes in shape of the atria and ventricles through the cardiac cycle (see Fig. 4.19a). Having established that the atria and the ventricles contract separately and consecutively, it is interesting to hear Leonardo’s full description of the cycle of contraction of the cardiac chambers. First he makes the point that the atria are more distensible than the ventricles:

…And since the upper ventricles [atria] are more suited for the driving out of themselves the blood which dilates them than pulling it into themselves, Nature has so made it that by the closure of the lower ventricles (which close on their own) the blood which escapes from them is that which dilates the upper ventricles. These through being composed of muscles and fleshy membranes are suitable for dilation and for receiving as much blood as is pushed into them; they are also suited by their powerful muscles for contracting with impetus and driving out of themselves the blood into the lower ventricles, of which when one opens the other closes. And the upper ventricles [atria] do the same thing in such a way that when the right lower ventricle dilates the left upper ventricle contracts, and when the left lower ventricle opens, the right upper one closes.”⁷⁶

The sequential nature of atrial and ventricular contraction is correct, but the suggestion that the ventricles contract sequentially and not synchronously is not. The contraction of the right ventricle followed by the left ventricle is the opposite of what actually happens, when both ventricles contract in unison, as do both atria. This statement makes it clear that either at that time Leonardo had not had the opportunity to observe the beating heart in situ, or he had simply misinterpreted the observed facts. I suspect that his keen powers of observation would not have allowed the latter, as so much of the rest of his observations of anatomical fact are so accurate. Interestingly, on another sheet he declares that the ventricles contract in unison: “ON THE CONTRACTION AND DILATATION OF THE TWO VENTRICLES OF THE HEART: Of the two lower ventricles arising from the roots of the heart, their dilatation and contraction are made at one and the same time, following the first by reflux from the upper ventricles arising on top of the roots of the heart.”⁷⁷ Clearly this was work in progress.

Leonardo’s correct, albeit inconsistent, assumption about the cyclical motion of the heart gave rise to his error in calculation of the number of contractions of the heart per minute. From the normal resting heart rate of 70 beats per minute, it is easy to calculate the number of beats per hour. The correct solution is of course 70  ×  60  =  4,200. Below is Leonardo’s solution. The main purpose of this calculation is to estimate the amount of new blood that is needed from the liver to maintain the total volume: “How much blood is the liver able to deliver owing to the opening [diastolic relaxation] of the heart? It restores to it as much as it consumes, that is a very small part. Since in 1 h about 2,000 openings of the heart occur, it is a great weight.⁷⁸

Since Leonardo assumed that the ventricles opened sequentially, he divided the normal heart rate by two, giving the total of about 2,000 openings per hour. This is followed by a small calculation “25  ×  12/300” followed by “7 oz”.⁷⁹ It is likely that this small amount (7 oz) represents the amount of blood derived from the liver, although where this sum came from is impossible to be sure. Multiplying 25 by 12 does indeed amount to 300, and if this is divided by 60 for the number of minutes in an hour, 1 arrives at the total of 5; in other words 5 oz/h to be replenished by the liver. We cannot say whether this actually equates to Leonardo’s workings.

Leonardo continued his discussion of the movement of the heart in musical terms of tempo: “…The time interposed between two percussions of the pulse is half one musical temp.”⁸⁰ And later in the same passage, “Therefore in every harmonic or, as you might say, musical tempo, the heart has three movements which are contained below. Of these tempi, one hour contains one thousand and eighty. Therefore the heart moves three thousand five hundred and forty times during each hour in opening and shutting.”⁸¹

What is more interesting is Leonardo’s statement that the amount of blood expelled from the ventricles per hour is a “great weight”. It was this very observation that led William Harvey to derive his theory of the circulation of the blood. One cannot help but wonder where Leonardo’s mind was taken with this thought. Sadly there is no more commentary on it. We can only assume that he was satisfied with the idea that much of the blood continuously ebbed and flowed between the atria and the ventricles acting as a store for the endogenous heat of the body. This idea appears to be reinforced by the following words, in which he considers the capacity of the ventricles: “I say that the greatest receptive capacity of the ventricles is the quantity of blood which escapes from them when they contract. This is received on deposit by the upper ventricles called the auricles of the heart, and it is kept in these until it is returned on the succeeding dilatation of the heart, but with so much less quantity as was contributed to the nourishers of life [i.e., the veins]. This deficiency is compensated by the liver, generator of such blood.⁸² From Leonardo’s calculation this amount would appear to be 7 oz.

As mentioned earlier, the other important function of the heart’s action was to deliver the vital spirits (pneuma) to the brain and the rest of the body. Leonardo’s description of how this came about involved a novel but incorrect function for the pericardium (the fibrous sac around the heart):

Air which takes the place occupied by the upper ventricle when it holds the blood deposited in it by the contraction of the lower ventricle, flows in to replace the upper ventricle when it contracts, and it comes from that in the capsule of the heart [pericardium]; and this capsule having given away its air to the place left by the contraction of the upper ventricle is restored by drawing more air into the lung through the trachea than usual; therefore the air drawn into the lung cannot always be equal.⁸³

In other words, when the blood in the ventricle has emptied backward into the atrium, it is mixed with air from the lungs that has passed via the pericardium. That which has been lost from the pericardium to the heart is restored by air drawn in through the trachea (in other words, a breath). As we shall see, Leonardo is troubled by this assumption.

At that time, the ventricles were regarded as being formed from three different fibre groups: transverse, oblique, and longitudinal.⁸⁴ Whilst Leonardo used these terms,⁸⁵ he also simplified the subdivision of the muscle mass into intrinsic and extrinsic sets using the more customary classification: “And the intrinsic muscles suitable for contracting are of one and the same nature in the four ventricles. But the extrinsic muscle exists only on the intrinsic ventricles [ventricles] of the heart. On the extrinsic ventricles [atria] there is only a continuous dilatable and contractible skin.”⁸⁶

This difference can be appreciated in the ox heart, in which the atrial wall is very much thinner than that of the ventricle. Although this classification seems very naive today, it must be remembered that Leonardo had no means of observation other than the naked eye.