by The Science of Leonardo: Inside the Mind of the Great Genius of the Renaissance
Don’t you see that the eye embraces the beauty of the whole world? It is the master of astronomy, it practices cosmography, it counsels and corrects all human arts; it transports man to different parts of the world. [The eye] is the prince of mathematics; its sciences are most certain. It has measured the heights and sizes of the stars, it has discovered the elements and their locations…. It has created architecture, perspective, and divine painting…. [The eye] is the window of the human body, through which [the soul] contemplates and enjoys the beauty of the world.2
It is not surprising that Leonardo spent more than twenty years investigating the anatomy and physiology of the eye by carefully dissecting the eyeball and associated muscles and nerves. One of his earliest drawings, made around 1487, shows the human head and brain surrounded by several membranes, like layers of an onion (Fig. 9-1). In fact, this onion analogy was widely used by leading medieval anatomists.3 Beneath the layers of the scalp Leonardo shows two membranes (known today as dura mater and pia mater) surrounding the brain and then extending to form the eyeball, which contains a round lens. The pupil is formed by a transparent gap in the membranes in front of the lens, which appears to lie unattached, presumably floating in some clear fluid. This crude drawing is a faithful illustration of the medieval view of the eye, which was based almost entirely on imagination rather than on empirical knowledge.
With his own anatomical dissections, Leonardo soon progressed far beyond these traditional ideas. The “onion drawing” already shows one of his discoveries, the frontal sinus above the eyeball, and in the subsequent years he would gradually add many fine details concerning the anatomy of the eye and the pathways of visual perception.
He was well aware of the novelty of his discoveries. “The eye has until now been defined by countless writers in a certain way,” he noted in the Codex Atlanticus, “but I find through experience that it works in a different manner.”4
LEONARDO’S ANATOMY OF THE EYE
Leonardo’s study of visual perception was an extraordinary program of scientific investigation, combining optics, anatomy of the eye, and neuroscience. He explored these fields without any inhibitions, applying the same meticulous empirical method to them that he used to explore everything else in nature, never fearing that some phenomenon might be beyond his grasp.
Figure 9-1: Leonardo’s illustration of the medieval view of the scalp, brain, and eyeball, Anatomical Studies, folio 32r
One of the first things Leonardo noticed when he studied the structure of the eye in detail was its ability to change the size of the pupil according to its exposure to light. He first observed this phenomenon while painting a portrait, and then tested it in a series of experiments in which he exposed subjects to varying amounts of light. “The pupil of the eye,” he concluded, “changes to as many different sizes as there are differences in the degrees of brightness and darkness of the objects which present themselves before it…. Nature has equipped the visual faculty, when irritated by excessive light, with the contraction of the pupil…, and here nature works like someone who, having too much light in his house, closes half of a window, and more or less according to necessity.” And then he added: “You can observe that in nocturnal animals such as cats, screech owls, tawny owls and others, which have the pupil small at midday and very large at night.”5
When he investigated the mechanism of these contractions and dilations in his dissections of the eyeball, Leonardo discovered the delicate sphincter of the pupil. “I find by experiment,” he recorded, “that the black, or nearly black, crinkled rough color, which appears around the pupil, serves no other function than to increase or decrease the size of that pupil.”6 In another passage, he likened the action of the radial folds of the sphincter to the closing of a purse with a string.7 Leonardo’s detailed description of the “nearly black, crinkled rough color” of the pupillary muscles is amazingly accurate. Indeed, it is almost identical to that of modern medical textbooks, in which the muscles on the central opening of the iris, the so-called “pupillary ruff,” are described as a dark brown, wrinkled rim.8
In the Middle Ages and the Renaissance, most natural philosophers believed that vision involved the emission of “visual rays” by the eye, which were then reflected back by the perceived objects. This view was first proposed by Plato and was supported by Euclid, Ptolemy, and Galen. Only the great experimental philosopher Alhazen expounded the opposite view—that vision was triggered when visual images, carried by light rays, entered the eye.
Leonardo debated the merits of both points of view at great length before agreeing with Alhazen.9 His principal argument in favor of the theory of “intromission” was based on his discovery of the pupil’s adaptation to changing illumination. In particular, he saw the fact that sudden bright sunlight produces pain in the eye as decisive proof that light not only enters the eye, but can also cause harm to it and, in extreme cases, even its destruction. An additional argument for the entry of light into the eye was Leonardo’s observation of afterimages. “If you look at the sun or another luminous body and then shut your eyes,” he noted, “you will see it similarly inside your eye for a long space of time. This is evidence that images enter the eye.”10
After a hiatus of almost twenty years, Leonardo returned to the study of vision around 1508 to explore further details of the eye’s anatomy and its visual pathways.11 This time, he also made use of his new technique of embedding the eyeball in egg white during dissections.12 He recognized the cornea as a transparent membrane and noticed its prominent curvature, concluding correctly that this extends the visual field beyond 180 degrees: “Nature made the surface of the cornea in the eye convex in order to allow surrounding objects to imprint their images at greater angles.”13
Leonardo realized that the extension of the visual field by the prominence of the cornea’s curvature is due to the refraction of light rays when they pass from the air into the denser medium of the cornea, and he carefully illustrated this phenomenon in several sketches. In addition, he tested the refractions experimentally by building a crystal model of the cornea.14
Leonardo was quite familiar with lenses from his optical experiments as well as from his own use of spectacles, which he had to wear by the time he studied the lens of the eye.15 Naturally, he applied his knowledge of refraction to his investigations of both the cornea and the lens. However, he always presented the lens, which he called the “crystalline humor,” as spherical and located in the center of the eyeball, suspended in a clear fluid, rather than right behind the pupil. Kenneth Keele has pointed out that Leonardo’s sophisticated technique of dissection of the eyeball, developed around 1509, would certainly have enabled him to recognize the true shape and location of the lens, and Keele has speculated that Leonardo either did not continue his dissections of the eye after that time, or that more accurate drawings have been lost.16
The detailed optics of the light rays inside the eyeball presented great difficulties for Leonardo, as they did for all his contemporaries. Today we know that the rays are refracted by the convex lens in such a way that they cross each other behind the lens and form an inverted image of the perceived object on the retina. How the brain then corrects the inversion to produce normal vision is still not fully understood.
Since Leonardo could not know that a second inversion of the image is performed in the brain, he had to construct two consecutive inversions of the light rays within the eyeball to produce an upright image. He came up with a brilliant though incorrect idea. The first inversion of the rays, he postulated, occurs between the pupil and the lens, caused by the small opening of the pupil, which turns the image upside down like a camera obscura.17
The inverted rays then enter the lens where they are inverted a second time, resulting in an upright image at the end. Leonardo built a simple but very ingenious model of the eye to test this idea and illustrated it clearly with a charming drawing in Manuscript D (Fig. 9-2). In the lower part of the drawing, he has sketched the visual pathways a
ccording to his theory. The light rays, entering the eye from below, are slightly refracted by the cornea (except for the central ray), proceed through the small opening of the pupil and, as in a camera obscura, produce an inverted image on the spherical lens. There, the rays are inverted again before they form a proper image on the back of the lens, from where they would enter the optic nerve.
The upper part of the drawing shows Leonardo’s model. He has filled a transparent globe, representing the eyeball, with water and at the front has fitted a plate with a small hole in the middle, representing the pupil. Suspended in the center of the globe is a “ball of thin glass,” representing the lens, behind which Leonardo places his own eye underwater in the position of the optic nerve. “Such an instrument,” he explains in the accompanying text, “will send the images…to the eye just as the eye sends them to the visual faculty.”18
Leonardo’s construction of the visual pathways was certainly ingenious, but it also had some serious problems. The camera-obscura effect would work only if the size of the pupil were much smaller and its distance from the lens greater than they actually are. And even if that were the case, the images of objects on the retina would be affected by the contractions and dilations of the pupil in response to varying exposures to light. Leonardo considered that possibility and also experimented with alternative visual paths, but he was never able to resolve the inconsistencies inherent in his construction.19 Nevertheless, his discoveries of many fine details of the eye’s anatomy are truly remarkable.
Leonardo was the first to distinguish between central and peripheral vision. “The eye has a single central line,” he observed, “and all the things that come to the eye along this line are seen well. Around this central line, there are an infinite number of other lines, which are of less value the further they are from the central line.”20 He was also the first to explain binocular vision—the way in which we see things stereoscopically by fusing the separate images of the visual field formed in each eye. To explore the details of binocular vision, he placed objects of various sizes at varying distances from the eyes, from very close to very far, and looked at them alternatively with the right and left eye and with both eyes. His conclusion was unequivocal and correct: “One and the same object is clearly comprehended when seen with two concordant eyes. These eyes refer it to one and the same point inside the head…. But if you displace one of those eyes with the finger, you will see one perceived object converted into two.”21
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.