Fritjof Capra (34 page)

Figure 9-2: Model of the eye and diagram of visual pathways, Ms. D, folio 3v

FROM THE OPTIC NERVE TO THE SEAT OF THE SOUL

From his earliest studies of sensory perception, Leonardo did not limit his investigations of vision to the optics of the eye, but followed the paths of sensory impressions through the nerves into the brain. Indeed, even his early “onion drawing” of the scalp and eyeball (Fig. 9-1), which represents the medieval conception of the eye, shows the optic nerve leading to the center of the brain, where the vague outlines of three cavities can be seen. According to Aristotelian and medieval philosophy, these were the areas in the brain where different stages of perception took place. The first cavity, named
sensus communis
(common sense) by Aristotle, was the place where all the senses came together to produce an integrated perception of the world, which was then interpreted and partly committed to memory in the other two cavities.

These hollow spaces do exist in the central portion of the brain, but their shapes and functions are quite different from those imagined by medieval natural philosophers. They are called cerebral ventricles by today’s neuroscientists; there are actually four of them, all interconnected. They support and cushion the brain and produce a clear, colorless fluid that circulates over the surfaces of the brain and spinal cord, transporting hormones and removing metabolic waste products.

Leonardo embraced the Aristotelian idea of the ventricles as centers of sensory perception, expanded it, and, by employing his skills as an anatomist and empirical scientist, integrated it with his ideas about the nature of light and the physiology of vision. To begin with, he determined the exact shape of the cerebral ventricles by carefully injecting wax into them.
²²

He recorded his results in several drawings, for example, the one shown in Figure 9-3, which also exhibits the pathways of several sensory nerves to the brain. Comparison of this drawing (which is based on the dissection of the brain of an ox) with those in a modern medical textbook makes it evident that Leonardo reproduced the shapes and locations of the cerebral ventricles with tremendous accuracy. The two anterior, so-called lateral ventricles, the third (central) ventricle, and the fourth (posterior) ventricle can easily be recognized.

Leonardo’s neurological theory of visual perception must be ranked as one of his greatest scientific achievements. It has been analyzed in admirable detail by the eminent Leonardo scholar and physician Kenneth Keele.
²³

In Leonardo’s anatomy, the optic nerve is pictured as expanding gradually where it enters the eyeball and attaching itself directly to the back of the spherical lens, forming a kind of restricted retina. This is where the visual images are transformed into nerve impulses. He saw this process as a percussion of the optic nerve by the light rays, which triggers sensory impulses
(sentimenti)
that travel through the nerves in the form of waves, just as the “tremors” triggered by stones thrown into a pond propagate in the form of water waves.
²⁴
However, Leonardo specified that the sensory, or nervous, impulses are not material. He called them “spiritual,” by which he simply meant that they were incorporeal and invisible. Following Galen, he thought that the optic nerve, like all nerves, was hollow, “perforated” by a small central tube through which the wave fronts formed by sensory impulses travel toward the center of the brain.

Figure 9-3: Cerebral ventricles and pathways of cranial nerves, “Weimar Blatt,” in Anatomical Studies, between folios 54 and 55

Kenneth Keele concludes that Leonardo’s physiology of sensory perception is “thoroughly mechanistic,” because it prominently features movement and percussion.
²⁵
I disagree with this assessment in view of Leonardo’s explicit emphasis on the nonmaterial nature of the nervous impulses. According to modern neuroscience, the nerve impulses are of electromagnetic nature, wave fronts of ions moving along the nerves—and, as Leonardo stated, invisible to the naked eye. The neurons form long, thin fibers (called axons), surrounded by cell membranes, for which Leonardo’s term “perforated tubes” does not seem a bad description. Inside these tubes, the wave fronts of ions move in the fluid of the nerve cells. These are phenomena in the realms of microbiology and biochemistry, which were inaccessible to Leonardo. As a good empiricist, he simply stated that the sensory impulses are invisible and did not further speculate about their nature. No scientist could have done better before the development of the microscope and the theory of electromagnetism several centuries later.

From his very first anatomical studies, Leonardo paid special attention to the pathways of the sensory nerves in the human skull, in particular the optic nerve. Indeed, as Keele points out, “Leonardo’s personal investigations of the anatomy of the eye and optic nerves…formed the central motive for his beautiful perspectival demonstrations of the structure of the human skull.”
²⁶
These stunning pictures of the skull are famous for their delicate renderings of light and shade and their masterful application of visual perspective (see Fig. 8-2 on Chapter 8). In addition, the trained eye of the physician sees in them amazingly accurate depictions of the skull’s cavities and nerve openings—the eye socket, its neighboring sinuses, the tear ducts, and the openings (foramina) for the optic and auditory nerves.
²⁷

When Leonardo followed the optic nerves from each eyeball into the brain, he noticed that they intersect in an area now known as the optic chiasma (“crossing”).
²⁸
He documented this discovery in all his drawings of the optic and cranial nerves (see Fig. 9-3). Leonardo speculated that the crossing of the optic nerves served to facilitate “the equal movement of the eyes” in the process of visual perception.
²⁹
He was on the right track, but he did not know that the process of synchronizing the visual perception of the two eyes is much more complex, involving the subtle interplay of several sets of muscles and nerves.

By the time Leonardo drew the so-called Weimar Blatt (Fig. 9-3), around 1508, his knowledge of the nature and course of the cranial nerves had reached its peak. He still maintained that all the nerves carrying the sensory impressions converge in the anterior ventricle,
³⁰
but he departed from Aristotle by shifting the location of the
senso comune
to the central cavity of the brain.
³¹
In the anterior ventricle, Leonardo located a special organ not mentioned by anyone before him, which he called the receptor of impressions
(impressiva)
.
³²
He saw it as a relay station that collects the wave patterns of sensory impressions, makes selections by some process of resonance, and organizes them into harmonious rhythmic forms that are then passed on to the
senso comune
, where they enter consciousness.

HEARING AND THE OTHER SENSES

Although Leonardo considered sight “the best and most noble of the senses,”
³³
he investigated the other senses as well, paying particular attention to the pathways of their cranial nerves. From his earliest drawings of the head, he consistently delineated the auditory and olfactory nerves, as well as the optic nerve, and showed how they all converge toward the
senso comune
.

In his famous drawings of the skull in perspective, Leonardo clearly depicted the auditory canal, but in his known manuscripts there is no detailed description of the anatomy of the ear. He was aware of the eardrum and recognized that its percussion by sound waves produces sensory impulses in the auditory nerve. However, he did not document any of the intermediary processes, having convinced himself, perhaps, that the generation of auditory nervous impulses by means of percussion was analogous to that of the impulses in the optic nerve, and that both of them ended up in the
senso comune
.

Leonardo may or may not have recorded more detailed studies of the human perception of sound in manuscripts that have been lost, but we know for certain that he spent considerable time studying the
production
of sound by the human voice. He not only investigated the anatomy and physiology of the entire vocal apparatus to understand the formation of the voice, but extended his studies to phonetics, musical theory, and the functioning and design of musical instruments.
³⁴

The larynx, or voice box, which contains the vocal cords, is a notoriously complicated organ, and it is not surprising that Leonardo did not fully understand its functioning. However, he produced astonishingly accurate drawings of its detailed anatomy, far beyond anything known in his time, and he also realized that many other parts of the body are involved in the formation of the human voice. In the words of Kenneth Keele, Leonardo realized that

voice production involved the integrated function of structures ranging from the thoracic cage, through lungs, bronchi, trachea, larynx, pharynx, nasal and mouth cavities to the teeth, lips and tongue; and he considers all these structures, producing unprecedentedly accurate drawings of them all.
³⁵

In his studies of the human voice, Leonardo frequently used the mechanisms of sound production in flutes and trumpets as models. In fact, he always used the word
voce
(voice) for the sounds produced by these instruments. His investigations of the variation of pitch in wind instruments naturally led him to study scales and develop elements of musical theory.

Leonardo’s musical talent was well known by his contemporaries and played an important role in his early success at the Sforza court in Milan.
³⁶
We also have contemporary reports that he composed pieces of music for the theatrical performances and other spectacles he produced at court.
³⁷
Unfortunately, no musical score by Leonardo has been preserved. On the other hand, we can find numerous drawings of musical instruments in his Notebooks, most of them with designs for improving existing instruments. These designs include keyboards for wind instruments, tuned drums, glissando flutes (like Swanee whistles), and a
viola organista
(organ violin), a kind of organ with timbre similar to a string instrument.
³⁸

Leonardo’s dissections of the cranial nerves and the central nervous system convinced him that all five senses are associated with special nerves that carry sensory impressions to the brain, where they are selected and organized by the receptor of impressions
(impressiva)
and passed on to the
senso comune
. There, in the central ventricle of the brain, the integrated sensory impressions are judged by the intellect and influenced by the imagination and memory.

In several of his drawings of the human skull, Leonardo indicated the position of the third cerebral ventricle by three intersecting coordinates, with complete spatial accuracy in three dimensions (see Fig. 8-2). This cavity in the center of the brain he identified not only as the location of the
senso comune
, but also as the seat of the soul. “The soul appears to reside in the judicial part,” he concluded, “and the judicial part appears to be in the place where all the senses come together, which is called
senso comune
…. The
senso comune
is the seat of the soul, the memory is its store, and the receptor of impressions is its informant.”
³⁹
With this statement, Leonardo links his elaborate theory of sensory perception to the ancient idea of the soul.

COGNITION AND THE SOUL

In early Greek philosophy, the soul was conceived as the ultimate moving force and source of all life.
⁴⁰
Closely associated with this moving force, which leaves the body at death, was the idea of knowing. From the beginning of Greek philosophy, the concept of the soul had a cognitive dimension. The process of animation was also a process of knowing. Thus Anaxagoras, in the fifth century
B.C.
, called the soul
nous
(reason) and saw it as a world-moving rational substance.

During the period of Hellenistic-Roman philosophy, Alexandrian thought gradually separated the two characteristics that had originally been united in the Greek conception of the soul—that of a vital force and that of the activity of consciousness. Side by side with the soul, which moves the body, now appears “spirit” as an independent principle expressing the essence of the individual, and also of the divine personality. Alexandrian philosophers introduced the triple division of the human being into body, soul, and spirit, but the boundaries between “soul” and “spirit” were fluctuating. The soul was situated somewhere between the two extremes, matter and spirit.

Leonardo adopted the integrated view of the soul that was held by Aristotle and the early Greek philosophers, who saw it both as the agent of perception and knowing and as the force underlying the body’s formation and movements. Unlike the Greek philosophers, however, he did not merely speculate about the nature of the soul, but tested the ancient views empirically. In his delicate dissections of the brain and the nervous system, he traced the sensory perceptions from the initial impressions on the sense organs, especially the eye, through the sensory nerves to the center of the brain. He also followed the nerve impulses for voluntary movement from the brain down the spinal cord, and through the peripheral motor nerves out to the muscles, tendons, and bones; and he illustrated all these pathways in precise anatomical drawings (see, e.g., Fig. 9-4).
⁴¹