Introduction

In textbooks and articles that narrate the history of neurosurgery, there seems to be a blank space between the cerebellum and the spinal cord. Even leading historians of neuroanatomy appear to have forgotten a chapter about the brainstem. On closer observation, it is not so much that scholars have forgotten to chronicle the discovery of the brainstem and its components, but rather that these components are hidden between the lines in chapters devoted to other regions of the brain. However, deeper consideration of the medical history of the seemingly forgotten brainstem makes it clear why authors might exclude a chapter explicitly on the brainstem; it was because the brainstem was once regarded as part of its surrounding structures—the cerebellum, diencephalon, and spinal cord. In Cerebri Anatome, Thomas Willis (1621–1675) described the gross structures of the midbrain, pons, and medulla, although he identified these structures collectively as the “cerebellum.” Willis is credited with being the first to accurately describe the general functions of the brainstem and to recognize the individual areas of this brain structure, although he did so under the impression that they were part of the cerebellum. For this reason, Willis is considered to be the first to identify the brainstem.

The purpose of this chapter is to serve as a guide to the history of brainstem surgery in what we believe to be the first dedicated history of the brainstem itself. In this chapter, we will chronicle the history of the brainstem as the earliest pioneers of neuroanatomy saw it. After we stretch a canvas across the centuries, from Galen to modern-day surgery, we will paint a picture of the evolving identity of the brainstem as it unfolded in accordance with associated techniques and technology. In this way, we will compartmentalize the epochs of neuroscience, starting with macroanatomy in the 2nd century AD and concluding with microanatomy leading into the 21st century. In a similar manner, this chapter will discuss the development of brainstem surgery.

The history of brainstem surgery can be understood most clearly by placing it within the historical context of neuroanatomy and surgery as a whole, which can be divided into three distinct phases: premodern (before 1879), gestational (1879–1919), and modern (after 1919). The transition from premodern to gestational was marked by the introduction of clinical cerebral localization, the antiseptic or aseptic technique, and the use of anesthesia. The pioneers and the driving force behind the birth of neurosurgery include the famous surgeons William Macewen (1848–1924) and Joseph Lister (1827–1912). ¹ When operating within the brain parenchyma became more common, leaders such as Victor Horsley (1857–1916), Fedor Krause (1857–1937), Harvey Cushing (1869–1939), Charles Elsberg (1871–1948), Wilder Penfield (1891–1976), Walter Dandy (1886–1946), Ernest Sachs (1879–1958), and Charles Frazier (1870–1936), foremost among others, worked tirelessly to establish neurosurgery as its own specialty. ² After Cushing’s presentation at the 1919 meeting of the American College of Surgeons, William J. Mayo declared, “Gentleman, we have this day witnessed the birth of a new specialty—neurosurgery.” ³ This momentous occasion marks the transition from the gestational period into the modern era of neurosurgery. Several pivotal discoveries during the development of modern neurosurgery have revolutionized its practice, including the introduction of computed tomography (CT), magnetic resonance imaging (MRI), and the surgical microscope. ⁴ The scope of this chapter will focus on the birth and progression of brainstem surgery until the introduction of the surgical microscope in the 1970s.

From Galen to Gall

To properly analyze the discoveries pertaining to neuroanatomy, and specifically the brainstem, we must extend the scope of our subject to events that subsequently advanced the development of neuroanatomy. Such landmark events range from huge societal events such as the lifting of the ban on dissection in the 15th century, ⁵ to the simple 16th-century suggestion to flip the brain upside down. Although the earliest “neurosurgical” procedures took place in ancient Egypt during the second millennium BC, one might argue that these practices were merely a means of embalming rather than a curative or diagnostic form of medicine. ⁶ It was not until Herophilus (c. 335–280 BC) entered Alexandria in 300 BC that the development of neuroanatomical medicine truly took flight. Described by Elhadi et al ⁶ as “fully equipped to support medical education,” Alexandria was the birthplace of anatomical study, with Herophilus and Erasistratus (c. 310–250 BC) as two of its most significant contributors. The Egyptian pharaohs Ptolemy I (c. 366–282 BC) and Ptolemy II (c. 308–246 BC) decreed that vivisection of sentenced criminals was allowed, and Herophilus apparently did so publicly, in large numbers. Although Herophilus and Erasistratus had amassed considerable knowledge of cerebral and cerebellar anatomy, and they had discussed the importance of the ventricular system, the history of the brainstem begins with the macroanatomical studies of Galen of Pergamum (c. 129–216) in the 2nd century AD, as he was the first to write about explorations around the brain-stem. ^{7 , 8 , 9 , 10} Galen lived in Alexandria during a time when human dissection was no longer allowed, and he was therefore forced to explore the neuroanatomy of animals. ⁸ According to early texts, Galen, and many others until the 1600s, believed that the brain was the seat of the human soul and that the ventricles were the conduit of the animal spirit. Galen supported this claim through experiments showing that compression of the fourth ventricle caused marked depression of animal behavior. ¹¹

For the purposes of this chapter, it is important to understand this point: How these early anatomists approached the brain reveals what they considered important and how they used it to construct their theories. To understand why Galen was so fascinated with the ventricles, one must know how he viewed the brain, which was from a top-down approach. In the translation of his personal account, titled Anatomical Procedures, he explained that his dissection starts by separating the brain into its natural divisions, the two cerebral hemispheres and the cerebellum. ¹² Naturally, one can infer from a dissection of this nature that the first structures he came across were the septum pellucidum and the ventricular system. Notably, this approach allowed Galen to explore other parts of the brain, such as the corpora quadrigemina, the cerebellar vermis, and the cerebellar peduncles. ^{11 , 12 , 13} As part of Galen’s quest to prove his theory of a ventricular conduit, he postulated that animal spirits could be made within the linings of the ventricles and stored within the ventricles until called into action in the brain. ^{8 , 14} Galen used his dissection methods to reveal other deep structures of the brain beyond the ventricular system, including the basal ganglia. ¹¹ Notable scholars such as Avicenna (980–1037) and Mondino de Luzzi (1275–1326) worked diligently to preserve and uphold the status of Galen’s work until the Renaissance. ¹³ Thus, Galen’s findings lasted 1500 years until 1664 when Thomas Willis eventually penned the most accurate reasoning on the function of the ventricular system. ¹³

After Galen’s death, developments in neuroanatomy remained largely dormant throughout the Dark Ages when the church still forbade human dissection. However, Pope Sixtus IV (1414–1484) lifted the ban on human dissection in 1482, allowing the bodies of executed prisoners to be dissected. ⁵ One of the first to take advantage of the Pope’s decree was Leonardo da Vinci (1452–1519), who had previously been dissecting corpses in secrecy. ⁹ It is believed that Leonardo was among the first persons since Galen to perform his own dissections, as anatomists at the time hired untrained barber surgeons or butchers. Leonardo diverged from Galenic teachings, albeit not immediately nor in all aspects. ¹⁵ He maintained the belief that the ventricular system carried the vital spirits of “humanness,” and he relied on Galen’s texts for much of his work. ^{15 , 16} However, because Leonardo did not know that Galen’s work was based on animals, some of his earlier illustrations were inaccurate because Galen’s work depicted animals rather than humans. ^{15 , 16} It was not until Leonardo began illustrating what he learned through firsthand experience with cadaver dissections, and not by reference, that his work more accurately portrayed human subjects. His abiding interests in physics and mechanics helped him to illustrate cranial sinuses. Yet even more impressive was his use of molten wax injections to create a cast of the ventricular system, for the first accurate depiction of these structures in humans. ^{15 , 16} Although much of Leonardo’s work was not realized because of the untimely death of his partner and probable publisher, it did pave the way for the beginning of the “Renaissance of Anatomy,” which began with the work of the Flemish anatomist and physician Andreas Vesalius (1514–1564).

In this era, it was common for men and women to be obsessed with their bodies, even journaling about the appearance of the body and various bodily functions. ¹⁷ It was also the beginning of the search for improved artistic means to truly represent the human body. Explorations of nature were aimed at understanding reality, and the introduction and demonstrations of linear perspective and the communication of its mathematical basis significantly impacted renderings of anatomy, especially the brain. ¹⁷

Vesalius, while adhering to old doctrine, reinvented neuroanatomy ( Fig. 1.1 ). As described in Brain Renaissance, a biography of Vesalius:

Vesalius laid down a new paradigm in medical knowledge: a revolutionary inductive approach that seeks direct evidence to explain the wonders of the human form. Vesalius was not prepared to take for granted what had not been clearly demonstrated for the human body at the dissection table. His anatomy was based only on knowledge derived from direct observation, which led him to identify significant differences between animals and humans. ¹⁸

Although the texts of Vesalius’s works are not recognized for being groundbreaking expressions, his illustrations were immediately held to be revolutionary. ¹¹ His 1543 compilation of drawings in De humani corporis fabrica [On the fabric of the human body] is considered to be one of the most influential graphic works of its time and of all of science, and the drawings remain very close to modern brain illustrations. ¹⁹ In fact, the folio publication of the Fabrica (i.e., Epitome), which contained brief text and concentrated on the illustrations, became even more popular among students. Like Galen, Vesalius devoted much of his work on the brain to the ventricular system, ^{9 , 13 , 16} but that is where the similarities end. In fact, Vesalius was arguably Galen’s first dissenter. While a senior lecturer in surgery at the University of Bologna, Vesalius came to realize Galen’s shortcomings as an anatomist. ^{8 , 9} Perplexed about how such error-filled texts carried such weight for 1300 years, Vesalius finally realized that Galen had not been referring to humans. ^{8 , 10} This realization is what sparked his effort to create the first human-based neuroanatomical masterpiece, Fabrica. ^{8 , 11} The singular importance of Fabrica was that for the first time, science (anatomical examination method and technology) and art were brought together to produce a cohesive, illustrated anatomical publication. For this innovation, Vesalius is recognized as the greatest of the Renaissance anatomists.

Vesalius observed the brain from a top-down perspective, just as Galen had. However, it was the change in Vesalius’ dissection technique that enabled him to better understand the ventricular system and to even better visualize deep structures such as the basal ganglia. For the most part, Vesalius’ technique involved horizontal sectioning from top to bottom with the brain still attached in the cranium. ¹⁸ This attitude allowed him to realistically portray the brain as it had never been—to draw the deeper layers of the brain, specifically the lateral ventricles and basal ganglia, in greater detail than Galen. On the downside, however, this technique limited his ability to understand relationships among various structures. ¹³

Vesalius was not the illustrator of his texts, and identification of the artists remains controversial. ²⁰ He had close association with the Titian’s bottega, and specifically with Jan Stefan van Calcar, a student and nephew of Titian. In keeping with Vesalius’ ground-breaking approach, he combined the intricate approach to science with the very best illustration capability he could find so as to have artists intimately involved with the dissections. ¹⁷

Overall, the extent of anatomy that Vesalius was able to illustrate in his texts was vastly greater than what he could label or describe. ¹¹ In Fabrica, he did mention the brainstem, which he called the “dorsal cord,” and he described its attachments to the cerebellum via the peduncles and the sharing of the fourth ventricle with the cerebellum. ²¹ Vesalius is credited with showing a radically different detailed illustration of the posterior fossae with the cerebellum and the posterior brainstem by lifting and tilting the brain forward and labeling the fourth ventricle, cerebellar peduncles, and three pairs of cranial nerves (CNs). ^{7 , 18} He even provided instructions for properly visualizing the area. ¹⁸ After removing the cerebellum, he described in some detail the corpora quadrigemina (on the basis of their resemblance to male genitalia) and the cerebellar peduncles. ¹⁸ Anteriorly he described less, leaving the dorsal cord without labels, aside from the optic nerve. Further description in Fabrica leads the reader to believe that Vesalius considered the cerebellum and dorsal cord to be two separate brain structures, which is an idea that was contradicted by Thomas Willis in the following century. ¹⁸ It took another 30 years before Costanzo Varolio (1543–1575), a contemporary of Vesalius, further elucidated the brainstem.

The name Costanzo Varolio, better known as Constantius Varolius, should be familiar when discussing the pons varolii. Varolio identified the first brainstem segment when he decided upon a simple change in view: flipping the brain upside down to examine it from its base upward instead of from the top downward. ^{13 , 22 , 23} In his short life, Varolio accomplished much; as a student of anatomist Giulio Cesare Aranzio (1529–1589), who was himself a student of Vesalius, he became a professor of surgical anatomy at the University of Bologna and Sapienza University of Rome, and he later became the personal physician to the Pope. ²³ Believing that the important parts of the brain resided at its base ( Fig. 1.2a ), Varolio decided to remove the brain to examine it using a bottom-up approach. Although doing so would allow for a greater appreciation of the anterior brainstem and the origins of the CNs, Vesalius did not achieve this feat. ²⁴

In his 1573 text, De nervis opticis nonnullisque aliis, praeter communem opinionem in humano capite observatis, epistolae, Varolio states, “When the membrane has been removed, it will be seen at once that the spinal marrow [brainstem] does not take origin thence where it was first attached, but ascends further upwards and anteriorly. . . .” ²⁵ The “origin thence where it was first attached” most likely refers to the site that Vesalius illustrated, which is at the base of the occipital lobe where the cerebral and cerebellar peduncles meet the brainstem. Varolio may also have included the inferior-most surfaces of the hypothalamus since he noted that the infundibulum and pituitary are attached to the same membrane as the brainstem, or “spinal marrow,” as he called it. Upon removing the vessels and membrane overlaying the spinal marrow, Varolio described it as “a series of swelling transverse fibers,” which is part of the marrow he called the bridge from which the auditory nerve originates. ²⁵ Given this new anatomical description, Varolio accurately stated that the cerebellum must play some role in movement, as Galen once suggested 1300 years earlier. ¹⁸ Varolio then explained the ease with which the optic nerves and the other CNs can be traced into the brainstem. ²⁵ Lastly, Varolio presented one of the first attempts to describe tracts running through the brainstem as two anterior tracts for sensation and two posterior tracts for cerebellar function. ²⁵ These tracts would be corrected and further detailed 300 years later, but Varolio’s idea set the stage for descriptions by Willis in the 1600s and by the German “Naturphilosophen” in the early 1800s.

Between Varolio and Willis, there were relatively few developments made regarding the brainstem or the cerebellum. The career of Willis marks the first in “medical physiology,” for those who were more interested in clinical experiences than in referencing old texts. ⁸ Willis believed that a correlation existed between location and the evolutionary status of the brain, such that the superior regions of the brain were the most recently evolved and the most “human,” and the inferior structures were those governing primitive or instinctual functions. ^{8 , 13 , 26 , 27} Such was the explanation behind his somewhat accurate claim that the cerebellum governs involuntary function. However, today his claim would be considered more incorrect than correct without the context of his understanding of the cerebellum. In his texts, Willis showed that he understood the cerebellum to include the midbrain and pons because of their connection via the cerebellar peduncles. ^{9 , 18 , 28} The medulla or “oblong marrow” was a different structure than the rest of the brainstem, and Willis recognized it to be a continuation of the spinal cord, which traveled under the pons and terminated in the deep cerebrum. ^{28 , 29} Willis also considered the medulla to be a part of the involuntary function control center. ²⁷ Willis was not actually the first to discover the circle of Willis, but he was the first to correctly document the nature of its redundancies and its purpose of providing auxiliary flow in the event of obstruction. ^{18 , 30}

Willis is regarded as the “father of neurology” and was the first to present a systematic approach to the functional anatomy of the brain with his Cerebri Anatome. He advanced brain anatomy exploration with a carefully engineered approach to assembling personnel with exquisite specialty training, in fact establishing the model for a medical research and teaching institute at the University of Oxford. The illustrations, done largely by Christopher Wren (1632–1723), especially of the sheep and human brainstems, are incomparable for their accuracy, perspective, line, and objectivity. For Willis to have his friend and partner Wren, an expert in architecture, physiology, and anatomy, render the images of the brain was ingenious. ¹⁷

Both Willis ( Fig. 1.2b, c ) and his contemporary, Raymond de Vieussens (1641–1715), studied the medulla ( Fig. 1.3a ). Vieussens labeled the pyramids and olivary nuclei of the medulla, ²⁹ and his Neurographia universalis contains a detailed compilation of his illustrations of brainstem tracts. ³¹ At the end of the 17th century, Domenico Mistichelli (1675–1715) was the first to accurately describe the decussation of the brainstem ( Fig. 1.3b ). ^{22 , 32 , 33} In his words, the medulla is like a woman’s plaited tresses because the woven nature of the pyramidal decussation is like a braid of hair. ³² However, one might question how many times Mistichelli thought these fibers were crossed, for unlike a braided hair, which may cross several times over, the pyramids cross only once. Thus, it could be inferred that his analogy was not to be taken literally, but rather as a way to understand the concept of crossing sides in the anterior medulla. At the start of the 18th century a few years later, François Pourfour du Petit (1664–1741) presented a clinical correlate to complement Mistichelli’s untested theory. Having noted contralateral motor paralysis in French soldiers with brain abscesses, Pourfour du Petit validated the idea of decussating pyramids traveling underneath the pons on their way from the cerebrum to the rest of the body ( Fig. 1.3c ). ^{22 , 33 , 34}

The 18th century would prove to be an uneventful time in the history of brainstem discoveries, aside from the ideas of the two aforementioned scientists. This stagnant period occurred because fresh brain and spinal tissue was simply too difficult to analyze at the level at which these anatomists were attempting to view it. ¹³ In the mid 1600s, Willis came up with one of the first methods of fixation, using alcoholic spirits and india ink. ^{13 , 17 , 19 , 35} It may have been his illustrator, Christopher Wren, who introduced Willis and Richard Lower (1631–1691) to both the fixative and the intravenous method, respectively, which they used to preserve brain tissue. ^{17 , 35} It was with this innovation that Willis could propose the redundant and circular nature of the cerebral vasculature. ¹ Marcello Marpighi (1628–1694) boiled his specimens in water, whereas Vieussens boiled his in oil. ^{13 , 19 , 29} However, none of these techniques was adequate enough to visualize the brainstem at the level of tracts and fibers, which was the direction in which neuroanatomy was heading in the late 17th century. More than 100 years passed before the next landmark development occurred that would launch neuroanatomy into microscopic proportions.

The Neurosciences of the 1800s

In the 1700s, after the work of Willis and his contemporaries, the rate of discovery in neuroanatomy of the brainstem outpaced the rate of technological development. Thus, anatomists like Pourfour du Petit and Mistichelli could only speculate with their theories, which were later proven to be true. The following century would bring a flood of technical, technological, and theoretical developments, making the 19th century a fertile era for neurologic study.

The 19th century marks the shift of neuroanatomical expertise from Italy to Germany, where a group of scientists called the Naturphilosophen pioneered neuroanatomical research. This group included Franz Joseph Gall (1758–1828) and Johann Gaspar Spurzheim (1776–1832) ( Fig. 1.4 ), Johann Christian Reil (1759–1813), and Karl Friedrich Burdach (1776–1847). ^{8 , 11 , 13} Using the technique of fine dissection, these anatomists collectively made great strides in understanding the role of the brain-stem by analyzing the fibers that traveled within it. During the 1600s, Marpighi and Vieussens initially pointed out that white matter was made of fibers traceable from the brainstem to the cortex. ^{7 , 29} This idea was certainly ahead of its time, as it would not be until the 1800s when the same idea would be summoned again and elucidated. In 1809, Reil developed the first consistent method of fixation, which marked the beginning of the reinvigoration of neuroanatomy. Reil found that serial washes of alcohol, potash, and ammonia made tissue far more suitable for fine dissection and for analysis of the tracts within the structures. ^{7 , 36 , 37}

Anatomists of this time were aware of Pourfour du Petit’s theories of decussation and wanted to further clarify the pathways in the brainstem. In 1809, Gall and Spurzheim became best known for the development of what came to be called phrenology, which was a detailed account of the locality of character and mental abilities within the brain. ^{7 , 11 , 13} Furthermore, they contributed a great deal to the collection of information regarding the origin of CNs. ³⁸ They were among the first to begin fine dissection by teasing out tracts and CNs from parenchyma, and they were pioneers in finding a pathway from the olivary nucleus to the midbrain. Yet it was Reil who, in the same year, presented the first realistic idea of this pathway. ^{11 , 38 , 39} Having already described the pes pedunculi and tegmentum divisions of the brainstem, Reil recognized the olivary tracts as a continuation of the gray spinal cord. ³⁶ He traced the pathways from the olivary nuclei to the thalamus in two divisions: one traveling over the external sources of the inferior corpora quadrigemina and below the corpus geniculatum into the thalamus, and probably into the corona radiata; the other bending medially to form the roof of the aqueduct, crossing to its contralateral partner, and possibly also contributing to the posterior commissures. This newly classified Reil’s ribbon most likely contained fibers from what is known today as the medial and lateral lemniscus pathway ³⁶ ; thus, Reil is credited with the original discovery of the pathway.

In 1812, Burdach endorsed Reil’s work and emphasized that this pathway was a continuation of the gray spinal cord, as myelinated tracts were not yet delineated. Between 1819 and 1826, Burdach traced the tractus [fasciculus] cuneatus from the spinal cord into the medulla and pons. ^{40 , 41} He also recognized a portion of the pyramidal tracts that went uncrossed, thus anticipating the pathway of the lateral corticospinal tract. It would be another 60 years before further improvements were made on these pathways, when Theodor Meynert (1833–1892) revamped Reil’s idea by naming a “superior” and an “inferior” lemniscus, only to be outdone by Wladimir von Bechterew (1857–1927). Bechterew is credited with the correct tracing of the lemnisci, which he properly labeled medial and lateral, respective to Meynert’s nomenclature. ^{42 , 43} Meynert and Auguste Forel (1848– 1931) believed that the lateral lemniscus could not be traced superior to the corpora quadrigemina. However, Bechterew insisted that tracing was not necessary. He suggested instead that the inferior colliculi could be traced myelinogenetically down to the superior olivary complex, the trapezoidal body, and thus the vestibulocochlear nerve (CN VIII), via the lateral lemniscus, which would then propagate the idea that the lateral lemniscus pathway carries cochlear signals to the inferior colliculus, which may lead into the cortex by way of the thalamus. ⁴³

An analysis of this era of discovery raises a significant question. What caused such a vast discrepancy between the perspectives of anatomists of the first half of the 19th century and those of the latter half? Furthermore, what led the anatomists of the latter half to posit opposing ideas? The answer lies in the context of parallel developments during this time. Although the early 1800s marked the dawn of tract-based neuroanatomy, it was still 20 to 30 years before the beginning of microscopic neuroanatomy. Although the Naturphilosophen were limited in what they could examine, they nevertheless came up with brilliant ideas that were not far from the truth. With the revamping of the compound microscope in the 1820s, followed by Benedict Stilling’s (1810–1879) invention of the serial microtome 20 years later, we see the shift from macroscopic anatomy, with Burdach as its last disciple, to the advent of microscopic neuroanatomy. ^{8 , 11 , 13} The epoch of microneuroanatomy was layered with technological advancements that were followed by new techniques. During the 1840s, Stilling’s microtome allowed anatomists to analyze the brainstem both inside and out with serial sectioning. ⁴⁴ Also in the 1840s, Reil’s alcohol-and-potash fixative was dethroned by Adolph Hannover’s (1814–1894) chromic acid, which was subsequently supplanted by Ferdinand Blum’s (1865–1959) use of formaldehyde in 1895 (Hannover 1840, ⁴⁵ Blum 1893 ⁴⁶ ). Vladimir Betz (1834–1894) recognized that the earlier methods of fixation did not penetrate the deepest parts of the brain. He devised a solution of iodine in alcohol, followed by potassium bichromate, that allowed him to better examine the white matter tracts of the brain and to cut sections thinner than those of any of his predecessors. ^{47 , 48} His technique also led him to the discovery of pyramidal cells in the deep cortex and initiated the nexus of histology, brain function, and cerebral localization. ⁴⁸

Despite these improvements, anatomists remained uncertain about the validity of many of their observations. In 1877, Forel brilliantly stated:

It must be frankly admitted that almost nothing is known of origin and termination of these fibres. The general direction of these fibres is best studied in serial sagittal sections…but no certain information is obtainable on the real course of individual fibre bundles…since the direct connection with nerve cells can be traced for only short distances. ¹¹

Forel also described how fibers in the brainstem rarely travel in straight longitudinal paths, such that they “seem to join each other in acute angles, forming a network…, they appear to be mainly continuations of the anterior and lateral [spinal] tracts, which become loosened through interposition of grey matter.” The framework had been laid, but the canvas could not be painted, quite literally, until the development of histologic staining.

Histologic staining began in the 1850s with Joseph von Gerlach (1820–1896), who used carmine dye that stained cerebellar gray matter red while sparing white matter. However, it did not allow microscopists to see axons and dendrites. ⁴⁹ In 1873, Camillo Golgi (1843–1926) developed the silver nitrate stain, which made beautifully contrasted pictures of neuron structures. ⁵⁰ At the turn of the 20th century, Santiago Ramón y Cajal (1852–1934) perfected Golgi’s stain, which would garner them both a Nobel prize for Cajal’s neuron theory. ¹³ Tract-tracing techniques were also developing during the latter half of the 19th century. Using Vittorio Marchi’s (1851–1908) degenerating myelin stain, Ludwig Türck (1810–1868) and Bernhard von Gudden (1824–1886) were able to trace the spinal tract and the intracerebral tract, respectively, by inducing degeneration and observing the stained footprint. ^{51 , 52 , 53} Paul Flechsig (1847–1929) performed a similar feat, but developed a method to study staining in myelogenesis instead of degeneration. ^{18 , 54}

If one were to superimpose the timeline of these developments in histology onto the timeline of the neuroanatomical discoveries in the 1800s, one could see why it took until 1885 for Bechterew to correctly map the lemnisci. Furthermore, from this perspective a strong correlation emerges between these histologic developments and Bechterew’s experimental findings of other brainstem tracts and nuclei. In fact, Bechterew and Flechsig were corresponding during their major discoveries of the 1880s. ¹¹ The work of Türck and Flechsig applied not only to the lemnisci, but also to the pyramids and the other tracts. In the 1850s, Türck was the first to examine and confirm the crossed and uncrossed pyramidal tracts on a microscopic level after Burdach macroscopically speculated their existence in the 1820s. ⁵² In his 1877 work, Flechsig described his elucidation of the pyramidal tracts from cortex to medulla, but only after stating, “Until now, there has existed no absolutely exact reports as into how large a segment of the pons seen cross section, or of the cerebral peduncle, or of the internal capsule, etc., the pyramidal tracts extend.” ⁵⁵ He described the struggle that every anatomist had prior to his technique, leading up to Bechterew’s finding. For this work, Flechsig is credited with providing the first accurate depiction of the pyramidal tracts. ⁵⁵ His techniques sparked a gold rush in fiber identification, including the cerebellar peduncles and the trigeminal lemniscus. ¹¹

The turn of the 20th century led to even greater advancements in fiber-tracing techniques. One contributor to this era of development was Josef Klingler (1888–1963), who developed methods of preservation, dissection, and three-dimensional modeling that would become the basis for future stereotactic neurosurgery ⁵⁶ ( Fig. 1.5 ). Although his work included brain-stem fiber tracing, the vast majority of it pertained to the limbic system, insula, thalamus, and basal ganglia. His method of fixation included 5% formalin for 2 to 3 months, followed by freezing at –10°C for 8 to 10 days, and then thawing at room temperature in 5% formalin. He modeled his dissections in wax and plaster casts, and his meticulous tracing techniques received national attention. In 1949, Klingler began to teach M. Gazi Yaşargil (1925-), who went on to become a prominent neurosurgeon.