Humphrey Ridley (1653–1708): 17th century evolution in

Neurosurg Focus / Volume 33 / August 2012

Neurosurg Focus 33 (2):E3, 2012

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In the modern microneurosurgical era, cadaveric dis-section holds an important position for aspiring and practicing neurosurgeons, not only to improve the

understanding of the surgical anatomy but also as a tool of advancing neurosurgical operative techniques. Pro-fessors Yaşargil25 and Rhoton15 led the evolution in the understanding of the surgical neuroanatomy in the late 20th century. Although the use of selective colored cere-brovascular injections in cadaver dissections has recently become a routine practice to enhance the cerebral vascu-lature, it is an old art that was introduced more than 300 years ago.1,8,13,14,16,18

Humphrey Ridley, M.D. (1653–1708), a British phy-sician in the 17th century, injected mercury and tinged wax into the cerebral veins of freshly executed criminals, taking advantage of the considerable venous engorge-ment to demonstrate the anatomy of the venous plexus

of the skull base. His work is reflected in the naming of the circular sinus the “Ridley sinus.” Despite this, Ridley remains a relatively unknown figure of the postmedieval era of neuroanatomical discovery.4,9,11,16

We reviewed the published works on neuroanatomy authored by Ridley to understand his novel contributions to skull base anatomy and to understand the innovative cadaveric dissections and research methods that he used. This historical vignette attempts to describe Ridley’s aca-demic work and his influence on neuroscience and neuro-surgical understanding.

Personal LifeStandard search term strategies including PubMed/

Medline or Cochrane databases failed to return articles on the historical contributions of Ridley. The biographical

Humphrey Ridley (1653–1708): 17th century evolution in neuroanatomy and selective cerebrovascular injections for cadaver dissection

Jai Deep Thakur, M.D., ashish sonig, M.D., M.s., M.Ch., prashanT ChiTTiboina, M.D., M.p.h., iMaD saeeD khan, M.D., rishi WaDhWa, M.D., anD anil nanDa, M.D., M.p.h.Department of Neurosurgery, LSU Health Shreveport, Louisiana

Humphrey Ridley, M.D. (1653–1708), is a relatively unknown historical figure, belonging to the postmedieval era of neuroanatomical discovery. He was born in the market town of Mansfield, 14 miles from the county of Not-tinghamshire, England. After studying at Merton College, Oxford, he pursued medicine at Leiden University in the Netherlands. In 1688, he was incorporated as an M.D. at Cambridge. Ridley authored the first original treatise in English language on neuroanatomy, The Anatomy of the Brain Containing its Mechanisms and Physiology: Together with Some New Discoveries and Corrections of Ancient and Modern Authors upon that Subject.

Ridley described the venous anatomy of the eponymous circular sinus in connection with the parasellar com-partment. His methods were novel, unique, and effective. To appreciate the venous anatomy, he preferred to perform his anatomical dissections on recently executed criminals who had been hanged. These cadavers had considerable venous engorgement, which made the skull base venous anatomy clearer. To enhance the appearance of the cerebral vasculature further, he used tinged wax and quicksilver in the injections. He set up experimental models to answer questions definitively, in proving that the arachnoid mater is a separate meningeal layer. The first description of the subarachnoid cisterns, blood-brain barrier, and the fifth cranial nerve ganglion with its branches are also attributed to Ridley.

This historical vignette revisits Ridley’s life and academic work that influenced neuroscience and neurosurgical understanding in its infancy. It is unfortunate that most of his novel contributions have gone unnoticed and uncited. The authors hope that this article will inform the neurosurgical community of Ridley’s contributions to the field of neurosurgery.(http://thejns.org/doi/abs/10.3171/2012.6.FOCUS12139)

key WorDs • Humphrey Ridley • circular sinus • cadaver dissection • history of medicine • arachnoid mater • cistern • neuroanatomy

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details of Ridley’s life were collated from the historical information obtained from the Archives and Rare Books Departments at Cambridge University, and University of Oxford, England. Additionally, the Oxford Dictionary of National Biography (http://www.oxforddnb.com/) and Joseph Foster’s Alumni Oxonienses provided important details about his life and work.3

Humphrey Ridley was born in 1653 (the exact date is unknown), in the market town of Mansfield, 14 miles from the county of Nottinghamshire, England. His father Thomas Ridley was also a native of Mansfield. In 1661, he was admitted to Merton College, which was one of the colleges of University of Oxford, England. Founded in 1264, it was widely regarded as a premier educational institution. At Merton College, Ridley was a regular fee-paying student with no records of scholarships to supple-ment the tuition. He finished his studies at the college on July 14, 1671, but did not obtain a degree and later joined the Merton College School of Medicine. Before finishing the coursework at the school, Ridley moved to the Neth-erlands to train at Leiden University in medicine. The last entry of Ridley’s purchase recorded in the university’s local cafeteria register, known as “Buttery Books,” was on September 25, 1674. He graduated with a doctorate in medicine from Leiden University in September 1679 with a thesis in sexually transmitted diseases titled “De lue venerea.”

Ridley then returned to England and was incorpo-rated as a physician at Cambridge University in 1688. He later moved to London and became a potential candidate of the Royal College of Physicians on September 30, 1691. He was elected a fellow of the College after a year. Rid-ley was a presenter at the prestigious annual Goulstonian lecture series in 1694, a tradition of the Royal College of Physicians. Although the details of his family life are not available, he was recognized as a prominent member of the society. Gideon Tibbetts Ridlon17 discussed the gene-alogical origins of his family in 1884 in the History of the Ancient Ryedales: and their Descendants in Normandy, Great Britain, Ireland, and America, from 860 to 1884. Ridlon speculated on the genealogical descent from the Riddell, Ridley, Ridlon, Ridler, and Redlon lines. While discussing Humphrey Ridley, he stated, “I do not know the pedigree of this man, and place this notice here be-cause of contemporary dates.”17 Ridley died in April 1708, and he is buried at St. Andrew’s Church, Holborn.

Ridley and His Treatise on NeuroanatomyPredilection for the Brain

The Renaissance Period holds a vital position in the history of evolution of the study of human anatomy.4,11,20 During this period, many anatomical discoveries were described and were meticulously documented in ana-tomical drawings.2,4,11,22 Although most anatomists of the era were interested in global human anatomy and patho-physiology, a few held distinct interests in the neurologi-cal system. Humphrey Ridley authored the first treatise focused on neuroanatomy published in English language in 1695, titled The Anatomy of the Brain Containing its

Mechanisms and Physiology: Together with Some New Discoveries and Corrections of Ancient and Modern Au-thors upon that Subject (Fig. 1).16 It was illustrated by the famed surgeon, William Cooper (Figs. 2–5).

In the introduction to the treatise, Ridley described his interest in the human brain: “And this part, I take to be the brain; the delicacy of whole structure is such, that with no little resemblance to its divine author, while it gives us the greatest and the clearest discoveries of other things, lies most concealed itself.”16

Research Motives and PrinciplesRidley was perhaps only the third scientist in history

to publish a comprehensive treatise on neuroscience after Thomas Willis’ Cerebri Anatome (1664) and Raymond Vieussens’ Neurographia universalis (1685).10,14,23 In the preface of his treatise, Ridley recognized that both Willis and Vieussens were the inspirations for his work on the human brain.

The principles of research Ridley used in the 17th century were both innovative and admirable. He stated that the nature of the human body could be understood only if one put emphasis on the theory of “cause and ef-fect.” He trusted the objectivity of his senses as opposed to the philosophers of his era, who sought knowledge with their eyes shut.

Although he recognized the role of his predeces-sors in the work on descriptive neuroanatomy, Ridley performed his dissections and experiments with an open mind. When his observations contradicted earlier reports, he made clear notes in his treatise. Ridley concluded his preface by stating, “I have quoted authors, not out of os-tentation, but both for their truth and errors, to the end that at the same time we may see it reasonable and con-venient to read all they say, we may be rendered cautious how we believe; and to put us in mind that we find some-thing done to our hands by those who have gone before, there is reason we should do something for those who are to come after.”16

Selective Cerebrovascular Cadaveric InjectionsSeventeenth century Europe was discovering selec-

tive cerebrovascular injections, which increased the un-derstanding of the cerebrovascular anatomy and physiolo-gy. The initial work of selectively injecting cerebral blood vasculature in humans and animals was done by Thomas Willis, Richard Lower, and Christopher Wren. They in-vented syringes and connective silver tubings and used a variety of colored injection materials, such as India ink and liquor, leading to the discovery of the circle of Wil-lis.8,11,14 A few years later, Humphrey Ridley started us-ing warm injections. He used mercury and tinged wax to highlight the cerebrovascular anatomy along with liquor. The tinged wax would solidify upon cooling, making the vascular anatomy clearer and manageable during dissec-tions. Eventually in the 19th century, cold injections were introduced that used a catalyst to harden colored liquids.13 Nowadays, colored silicone and latex injections have largely replaced other techniques of injecting cadaveric heads.1,18



Ridley’s contributions to skull base anatomy and neuroscience

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Ridley developed innovative and effective methods of tissue dissection. In the preface of his book while de-scribing the methods, he stated, “Some of [the cadavers] have been upon subjects in their natural, some in their morbid, some upon those of untimely death; and on those last sometimes whist the natural fluids remained in their proper vessels, though after a preternatural manner oc-casioned by strangulation; sometimes when in the room thereof, other bodies have been introduced by injection, as tinged wax and mercury, the first of which by its con-sistence chiefly, the other by its permanent nature and color, contribute mightily towards bringing to view the most minute ramifications of vessels, and secret recesses of Nature.”16

Ridley and the Evolution in Cerebrovascular Arterial Anatomy

Ridley recognized the work of Thomas Willis on the circle of Willis as a masterpiece. He confirmed Willis’ anastomotic principle by selective injections to the carotid and vertebral arteries. He affirmed, “…if even three of the four great arteries [of the circle of Willis] which furnish this part with blood, were totally obstructed, there would yet be a way left for a competent supply from the other unobstructed fourth.”16 Ridley used the term “retrograde motion” to describe how he saw the filling of posterior

brain circulation through the communicating branches (posterior communicating arteries) despite obstruction to one of the major arteries in the circle of Willis.

Ridley pointed out that Dr. Willis failed to show the origin of meningeal arteries from the intracranial inter-nal carotid artery. While describing Willis’ observation and illustration, he stated, “…the very styliform process, where the carotid artery does indeed enter the long canal, to the place where it perforates the Dura Mater to enter the brain, there is not one branch sent out from it; which error, by injecting with wax, which keeps longer in and shows the vessels much better than small tinged Liquors, had very easily been avoided.”16 He then continued to de-scribe the intracranial origin and distribution of the men-ingeal arteries.

There is a cursory mention about the labyrinthine ar-tery or the internal auditory artery. While describing the arteries to the dura mater, Ridley noted, “…and the eight pair of nerves pass out of the Cranium, which passage of this artery is not hitherto described by any that I know of….”16

Similarly, Ridley described the ophthalmic artery while discussing the precise origins of the branches of the carotid artery. There is no recorded description of this artery before his treatise. He may have been the first to describe the ophthalmic artery. He described, “One more

Fig. 1. The title page of The Anatomy of the Brain Containing its Mechanisms and Physiology: Together with Some New Dis-coveries and Corrections of Ancient and Modern Authors upon that Subject, authored by Humphrey Ridley and published in 1695. From Ridley H: The Anatomy of the Brain Containing its Mechanisms and Physiology: Together with Some New Discoveries and Corrections of Ancient and Modern Authors upon that Subject. London: Samuel Smith & Benjamin Walford, 1695. Image courtesy of Evans Library of Texas A&M University, College Station, Texas.


J. D. Thakur et al.


branch I take leave to mention only upon the score of its never hitherto having been taken notice of by any, and that’s a small artery attended with a vein passing through the lateral part of the Os Cuneiforme, (which constitutes the back part of the orbit of the eye), just under a very little process of that bone, (which either by reason of its size has escaped being seen, or inconsiderable use, was never be-fore, as far as I know, thought worth the mentioning); and this, upon raising the fore lobes of the brain, offers itself to the eye of any needful observer.”16 The ophthalmic artery was later described by Johann Gottfried Zinn (1727–1759), Albrecht von Haller (1708–1777), and F. Meyer (1800s), but unfortunately Ridley’s earlier description of the artery and the vein remains unacknowledged.5–7,21

Ridley’s assessment of interpreting cadaveric obser-vation was brilliant and objective. He observed that the cerebral veins do not run concomitantly with their corre-

sponding arteries, and he recognized this difference from the other parts of the body. He noted, “the carotid [enters] at the fourth hole in the base of the skull and the internal jugular at the eighth.”16 He expressed dissatisfaction with the explanation offered by his contemporaries for this phenomenon: “…that it may receive equal warmth at the top as at the bottom, as being thereby very much assisted in the production of Animal Spirits in an equal propor-tion all over….”16 Ridley then described his understand-ing on this matter as, “…if we consider, that if the veins had ascended with the arteries through the holes in the bottom of the cranium, upon all great ebullitions of the blood, the pulsation of the arteries would in that stricture of the vessels made by the bone, of necessity hinder the freedom of its return by the veins, and consequently oc-casion a stagnation of blood through the whole brain….”16 Interestingly, he also noted an exception to this rule by giving the examples of veins draining the meninges.

Circular Sinus, Cavernous Sinus, and the Petrosal SinusesThe skull base venous anatomy was uncharted ter-

ritory until the 17th century. Ridley stated that Fallopius

Fig. 2. As described by Ridley, “Figure exhibits the base of the brain, with part of the Medulla Oblongata, the blood vessels being injected with Wax.” Additionally, Ridley pointed out in this figure the various branches supplying the fourth ventricle and plexus choroideus (existence of lat-ter in the third or the fourth ventricle was not known previously). From Ridley H: The Anatomy of the Brain Containing its Mechanisms and Physiology: Together with Some New Discoveries and Corrections of Ancient and Modern Authors upon that Subject. London: Samuel Smith & Benjamin Walford, 1695. Image courtesy of Evans Library of Texas A&M University, College Station, Texas.

Fig. 3. As described by Ridley, “Figure showing the internal base of the cranium, the Sinus’s being injected with the wax.” The figure is intended to depict in particular the venous anatomy of skull base after selective venous injections, a novel elucidation. Also, the inferior left part of the figure depicts the first accurate description of the fifth cra-nial nerve ganglion and its 3 branches, which several years later was confirmed by Jacques B. Winslow who termed it “nerf trijmeaux.” From Ridley H: The Anatomy of the Brain Containing its Mechanisms and Physiology: Together with Some New Discoveries and Corrections of Ancient and Modern Authors upon that Subject. London: Samuel Smith & Benjamin Walford, 1695. Image courtesy of Evans Library of Texas A&M University, College Station, Texas.




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was probably the first to describe the existence of supe-rior and inferior petrosal sinuses. Furthermore, he noted that Vieussens described the origin of the superior and inferior petrosal sinuses from the venous plexus of “Re-ceptacula Sella Equine,” known today as the cavernous sinus or more correctly, the parasellar compartment.16 This was the first mention of the parasellar compartment in the evolution of the skull base anatomy. Surprisingly, the communication of the 2 cavernous sinuses by the in-tercavernous sinus or the circular sinus was not reported by Vieussens.

Ridley performed selective venous wax injections and found that Vieussens had missed the circular sinus. He stated, “Another I discovered by having first injected the veins with wax running around the Pituitary Gland on its upper side forwardly within a duplicature of the Dura Mater, backwardly within a duplicature of the Dura Mater and Pia Mater, there somewhat loosely; stretched over the sub adjacent gland itself, and laterally, in sort of a canal made up of the Dura Mater above and the carotid artery on each outside of the gland…thereby constituting a cav-ity communicating with the two aforementioned forward and backward ones, from whence the above mentioned four small sinus’s do descend by a viable continuity, on each side from a little beneath the hinder process of Sella Turcica: and this from its figure may not unfitly be called the circular sinus”16 (Fig. 3).

Ridley did not identify the cavernous sinus as a sepa-rate entity comprising the venous plexus, carotid artery, and cranial nerves. The sinus was characterized as a dis-tinct structure by Vieussens in 1684 and later by Jacques B. Winslow (who coined the term “cavernous sinus”) in 1732. Winslow elaborated on the trabeculated appearance of the venous plexus apart from the presence of the carotid artery and the course of cranial nerves within the sinus.12,23,24 Rid-ley considered the venous complex of the cavernous sinus and the intercavernous sinus a single identity of the circular sinus and therefore concluded that the superior and the in-ferior petrosal sinuses drained the venous blood from the circular sinus complex to the transverse sinus or sigmoid sinus. The rationale behind rejecting the term “receptacle” was that despite the selective injections to the vessels of skull base, he did not find evidence of direct communica-

tion between the arterial and the venous anatomy of the “receptacle complex.” Furthermore, he observed that most cranial nerves crossed the posterior fossa to their respec-tive foramina through a duplication of the dura mater and hence could not be described as a part of the receptacle, as described by Vieussens.

Ridley was the first to accurately describe the gan-glion of the fifth cranial nerve and correctly identify its 3 branches (Fig. 3) as opposed to 2 branches described by Vieussens. This observation was independently confirmed by Jacques B. Winslow who named the fifth cranial nerve the “nerf trijmeaux,” or trigeminal nerve, in 1732.24

Ridley and 17th Century Experimental and Translational Research

Arachnoid Layer and the Cisterns

The 2 neuroanatomical identities left unexplored by the artistic medieval anatomists were the cavernous sinus and the arachnoid layer. Until the 17th century, the brain was known to have 2 coverings, namely the dura mater and the pia mater.19 The first mention about the possibility of a layer separating the 2 meningeal layers was in 1664

Fig. 4. Left: Drawing showing the posterior view of the brainstem divided at the pons superiorly and medullary spinalis inferiorly. Right: Depiction of the sagittal section of the venous sinuses of the brain after their dura mater coverings were opened. From Ridley H: The Anatomy of the Brain Containing its Mechanisms and Physiology: Together with Some New Discoveries and Corrections of Ancient and Modern Authors upon that Subject. London: Samuel Smith & Benjamin Walford, 1695. Images courtesy of Evans Library of Texas A&M University, College Sta-tion, Texas.

Fig. 5. In this figure, Ridley intends to illustrate, through an axial section of the brain, the anatomy of fornix and its relationship with the thalami nervorum opticorum. Medullary tracts between the thalami nervorum opticorum and corpora striata, lateral and third ventricles, and plexus choroideus with its blood supply and confluence of ve-nous sinuses. From Ridley H: The Anatomy of the Brain Containing its Mechanisms and Physiology: Together with Some New Discoveries and Corrections of Ancient and Modern Authors upon that Subject. London: Samuel Smith & Benjamin Walford, 1695. Image courtesy of Evans Li-brary of Texas A&M University, College Station, Texas.


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by a Dutch anatomist, Gerardus Blasius (1626–1692).19,26 Sanan and van Loveren19 reported extensively on the history of the arachnoid membrane. After Blasius, they stated that Frederick Ruysch (1638–1731) was the next to describe the membrane and demonstrate its cobweb ap-pearance by blowing air under it in 1699. In our readings of Ridley’s treatise, it appears that he had empirically de-scribed the existence of the membrane before Ruysch.

Ridley mentioned that most of the anatomists, includ-ing Willis and Vieussens, believed that there were only 2 membranous integuments of the brain. According to him, Govert Bidloo and John Bohn were the 2 anatomists who believed that there was another distinct membrane sepa-rating the dura and the pia maters. While testing vari-ous cadaveric heads for the arachnoid membrane, Ridley gave an example of an extremely hydropic brain, in which he clearly appreciated a distinct membrane spanning the brain. He noted that he could successfully divide the membrane easily, especially over the superficial plica-tures (folds) of the cortical parts of the brain. To describe its constituency, he wrote, “…I have often observed, not in a continuous, but rather retiform contexture, and so, by such as love hard words, or terms of art, may be called after the same name of that membrane investing the crys-talline Humor of the eye, Arachnoeides.”16

He further described that the membrane invests vari-ous cerebral vessels and intracranial nerves. Also, he not-ed, “…there was any larger than ordinary duplicature of this membrane, as there are at the end of Calamus Scrip-torius, between the superincumbent Cerebellum and Me-dulla Spinalis, in the isthmus or space between the Cere-brum and Cerebellum, upon the processes called Nates and Testes…and the first appearance or coming out of the Olfactory nerves…there was found a great deal of water distending this duplicature much beyond its natural lim-its….”16 He clearly pointed out the existence of what we know today as the cerebellomedullary cistern, the quad-rigeminal cistern, and the olfactory cistern. Until now, it was thought that the concept of cistern was described for the first time by Bichat in 1802 and François Magendie in 1822.26

Despite Ridley’s descriptions of the cisterns and arachnoid membrane in the 17th century, we do not find any mention of him in the literature with regard to his pivotal work in this area.

Translational ResearchRengachary et al.,14 while listing remarkable contri-

butions of Thomas Willis, pointed out the nature of some of the experiments done by him on animals, which could be translated to humans. In essence, since his research principles typically demonstrated a “bench to bedside” result–oriented pattern, they advocated that Willis should be considered as a pioneer of translational research.

We found a similar spirit of experimentation and its application to human anatomy in Ridley. He mentioned in chapter 6 of his treatise that numerous anatomists, in-cluding Willis and Vieussens, believed that some cerebral arteries terminated directly into the major venous sinuses of the brain.16 Ridley was eager to put this controversy to an end.

He described his experiment: “I took off the upper part of the skull of a dog alive, by which means the Dura Mater with its third Longitudinal Sinus lay bare to the eye and touch, to neither of which senses, at first, either any beating of the membrane in general, or of the Sinus, was the least discernible. After some pause, by chance the Sinus itself, which I designed to have opened with a Lan-cet, being touched with a cauterizing Iron, poured out the blood very violently and at first without any remarkable pulsation….I cut this Sinus through almost the length of it, to see whether any arteries would discover themselves by throwing out their salient blood, but no such sign ap-peared.”16

Concept of Blood-Brain BarrierRidley’s contribution toward our understanding of

blood-brain barrier was recently discussed by Shane A. Liddelow.9 Until Ridley, it was considered that the choroid plexus responsible for production and secretion of CSF was restricted to the lateral ventricles. Ridley added to this understanding by describing the presence of the cho-roid plexus in the third and the fourth ventricles. More-over, he was the first to point out the differential perme-ability of the cerebral blood vessels toward a substance (wax/mercury) injected into the bloodstream.9,16

Other Contributions to NeuroscienceRidley was the first to describe a pineal tumor. In

chapter 10 of his treatise where he described the “Glan-dula Pinealis,” he stated, “In an hydropical Brain of a stumous Boy, I have seen it swelled to a size of three times its ordinary magnitude, and by reason of the abundance of stagnate gelatinous Lympha contained in it, perfectly transparent.”16

Ridley expressed great understanding of the various deep nuclei of the brain in his treatise. Even his descrip-tion of the fornix and its pathways are regarded to be the first accurate descriptions (Fig. 5).

Ridley made many advances in the art and science of neuroanatomy and neuroscience. These advances are clearly illustrated in his treatises and in his extensive body of work on these topics. Some instances, such as his discovery and descriptions of the ophthalmic artery, the trigeminal ganglion, the arachnoid membranes, and arachnoid cisterns, are very significant in retrospect. De-spite these discoveries, his contributions have remained in relative obscurity. We speculate that the reason for his relative obscurity was the nature of clinical and experi-mental practice. There are no records that reveal his leg-acy in training a successful generation of leading neuro-anatomists or physicians. Additionally, Ridley remained at one institution with minimal contact with outside aca-demicians. Unlike him, his better-known contemporaries, such as Joseph Guichard Duverney, William Cheselden, and Johann Friedrich Meckel, traveled extensively and were keenly interested in training generations of success-ful physicians and scientists. We can only guess at the stoic nature of his personality. Very little is known about his personal life, and by extrapolation one may assume that he was as private and humble about his work as well.




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ConclusionsEven today, the selective cadaveric injection is con-

sidered an art. Humphrey Ridley mastered the art in the 17th century. Through his meticulous cadaveric dissec-tions and experimental research methodologies, he fueled a better understanding of the skull base. His contributions to neuroscience have laid the foundation for modern neu-rosurgery. Ridley was clearly a pioneer in describing the venous anatomy of the skull base and in providing an ac-curate description of the fifth cranial nerve ganglion and its branches. He also proved the existence of the arachnoid mater as a separate layer, and he introduced the concept of the subarachnoid cisterns and that of blood-brain bar-rier. It is unfortunate that most of his novel contributions have gone unnoticed and uncited, but we hope that this article will bring to notice his novel work on brain and inspire the future generation of neurosurgeons to bring innovations to our field.

Disclosure

The authors report no conflict of interest concerning the mate-rials or methods used in this study or the findings specified in this paper.

Author contributions to the study and manuscript preparation include the following. Conception and design: Thakur. Acquisition of data: Thakur, Sonig, Wadhwa. Analysis and interpretation of data: Thakur, Sonig, Chittiboina. Drafting the article: Thakur, Sonig, Khan. Critically revising the article: Nanda, Chittiboina, Khan, Wad-hwa. Reviewed submitted version of manuscript: Nanda. Approved the final version of the manuscript on behalf of all authors: Nanda. Study supervision: Nanda.

Acknowledgments

The authors wish to thank Ms. Nidhi Setya for her assistance in providing the digitalized copy of Ridley’s treatise (expired copy-right) from the Evans Library of Texas A&M University, College Station, Texas. The compilation of bibliographical details was pos-sible only with the help of staff at Cambridge, UK: Ms. Margaret Wilson, Ms. Emily Dourish, Ms. Dawn Moutrey, and Ms. Jacqueline Cox; and the staff at Oxford, UK: Ms. Jayne Ringrose, Ms. Michelle Conway, and Ms. Julian Reid.

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ment of Neurosurgery, LSU Health Shreveport, 1501 Kings High-way, Shreveport, Louisiana. email: [email protected].


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